Simple MPPT solar panel charge controllers

[BillyDoc]

I spent some time today trying to find circuits for implementing simple Maximum Power Point Tracking charge controllers. The commercial ones I have seen are all overpriced, in my opinion, or way too capable for my needs. So, I thought I would try to design one. At this time I only want to keep a single marine battery charged with what I think is about a 20 watt solar panel. It's for a cabin I have deep in the woods.

So far I have found two interesting circuits from the same man, W. Stephen Woodward (pdfs attached). The first (MPPT battery charger) describes pretty much what I am looking for, but uses a buck-only Switch Mode Power Controller (SMPC), thus limiting charging to those conditions where the voltage output from the solar panel exceeds the battery voltage by some amount. It also has a feature for dropping the charge current back to "trickle" when the battery voltage reaches fully charged, which I think is a necessity.

The second circuit (Solar array controller) is more sophisticated and uses a boost-buck SMPC, however the one chosen is a surface-mount device with 141 tiny pins!!! It's also damn expensive ($45). I really, really don't want to solder that thing up. The circuit as a whole also seems to lack a "fully charged" mode.

With a lot of study I think I can extract the best features of the two designs, but as I've mentioned before on this forum I'm lazy on principle and would love to get some help here. The MPPT approach seems to me to be the only way to go for this problem. So, has anyone else found good circuit ideas that implement MPPT for small systems? Or, have any good ideas of your own?

BillyDoc

[marshmat]

Hey BillyDoc,

I see what you mean about the regulator chosen for the second circuit... who the hell uses LGA141 for a power device, anyway? (OK, Linear Technology does, but still....) Do note, though, that only 14 of those pins have to be precisely soldered. The rest are soldered up in six banks, where you're allowed to be sloppy. Still, it means buying a bunch of $45 chips and LGA sockets to match. Annoying.

The MPPTs I've used (for solar cars) have all been very sophisticated units, made by AERL in Australia and with prices around $1000, give or take a few hundred. A typical car array would be a kilowatt or two and would have at least two, usually six or eight separate MPPTs. These things worked anywhere from twenty to a couple hundred volts, great units, very compact. I don't have any experience with cheaper and/or simpler ones.

The easy way to go would be some variation of the first circuit you posted, and make sure your solar panel has a higher operating voltage than your battery. Full buck-boost capability is complex, thus expensive.

I'll give it some more thought, though.... there has got to be a cheaper way....

[BillyDoc]

Hi Matt,

From what I've read this morning AERL originated the concept several years ago. They and Outback seem to be the leaders in the field still.

Most of the "big guys" seem to be using microcontrollers in their design, which certainly makes sense if you are going to sell a bunch of them and charge for "features." But I think that Woodward's analog approach is just as good if you can do without digital displays and the like, and I can.

Probably the sensible way to approach this is to look at buck-boost controllers first and work from there. I think I'll give Maxim a visit.

BillyDoc

[Rick Willoughby]

Have you worked out what benefit you will get from peak power tracking on your system. If the panel is a good match to the system voltage does it make much difference. You will need a regulator to protect the battery but is anything more worth the effort and expense.

I can certainly understand the benefit of peak power tracking with wind turbines. Also if the solar panel voltage does not match the battery system then you need some form of voltage switching.

Rick W

[BillyDoc]

Hi Rick,

The solar panel I have is 12" x 36" and seems to put out roughly 16 volts in full sun with no load. Beyond that I don't have any data. It didn't even have any leads (it was a gift), I just attached some with conductive epoxy this morning. I think that I could put a series diode on it and attach it directly to my battery and not go too far wrong . . . but I hate the thought of boiling off the battery juice, and if it isn't too expensive or troublesome . . . why not go all out? So, I've been exploring the MPPT concept today.

And I'm beginning to think this might not be so difficult after all. I found an application note on the Maxim site (App. note 484) that had some very interesting info in it. The thing that really grabbed my attention was their contention that since in almost all cases the maximum power point was at a voltage of 0.484 volts per cell you could connect the solar panel to a small capacitor, monitor the voltage on the capacitor with a comparator until it exceeded 0.484 volts times the number of cells, then switch on a plain-vanilla switching controller to the battery until such time as that voltage dropped below the threshold. Then wait until it built back above threshold, and so on. The rate at which the voltage builds up will be entirely dependent on insolation, so just this procedure effectively tracks the MPP while providing enough stored current to supply a switcher further down the line for a few cycles.

Many switching controllers have "enable" inputs, so this could be a means to a very simple charge controller. Obviously, I need to study it more though. The Maxim app note is attached below, if you're interested. I also found a switcher (boost/buck) that might work and is cheap ($4.18), National's LM5118, data sheet also attached.

So at this point I'm thinking, solar panel connected to capacitor with comparator looking at the voltage and enabling the LM5118, the output voltage of which is set by some flip-flops to go through two or three stages to trickle.

Or something like that.

BillyDoc

[Rick Willoughby]

I am interested in what you are doing because I am looking at panels outputing 60V (peak power at 50V in full light) feeding a 44V system. I also do not have much idea how the voltage-current changes with light level.

In your case I just wonder if you will get much from it by the time you allow for the losses in the controller.

The attached curve is typical of what I have seen for cells. You can see that shifting the operating point around the optimum does not change the power output a great deal.

Rick W

[BillyDoc]

You can see that shifting the operating point around the optimum does not change the power output a great deal.

Rick W

I think that you are absolutely right on this point, but only if the shift magnitude is pretty minimal. To put some numbers to the graph you provide, the peak voltage on the left with no load will be about 0.55 volts (I'm talking about a single cell) and according to the Maxim app note I cited above the peak power point will be at 0.484 volts. My panel appears to have 29 cell "stripes," so the peak voltage is 15.95 volts and the power optimum is 14.036 volts. Let's assume that this panel will put out a current of 2 amps under optimal conditions.

Now, if you look at the graph you see that increasing the load so that more current is drawn quickly moves the operating point of the cells onto a steep downward slope and thus reduces the power available. The question is, by how much.

In my case, my battery will sometimes get discharged to 10.5 volts, so if I attach the solar panel directly to it at this time the battery load will shift the power point to the right until the voltage at that point is 1/29th of 10.5, or 0.362 volts per cell. This will clearly have a bad effect on the power output because of the steep slope of the curve at that voltage.

We don't have numbers on this graph, but from what I've read so far the claim is that the power will be reduced roughly 30% by this effect (neglecting controller losses). So, if my solar panel will put out 2 amps at 14.036 volts optimally, that works out to 28.072 watts. If I reduce that by 30%, then I'm left with only 19.650 watts with which to start recharging my battery. I think its worth it to try and avoid this loss, at least as much as I can.

Figure one in the Maxim app note is the same graph you provided, but with additional light levels. They show the slope of the drop off with increased current to be almost vertical! But more importantly, they show that the mid point of the "knee" on each data line is more or less at 0.484 volts from 0.1 sun to full sun. So, if we have a capacitor attached to a solar cell that is already charged up to say, 0.48 volts, then whatever the light conditions above that solar cell it will be trying to increase the charge on that capacitor with maximum power. If there is only 0.1 sun it is going to take longer than if there is a full sun available, but in each case the maximum power available will be utilized. Capacitors, moreover, give back almost all of the energy stored within, they are close enough to 100% efficient not to worry about the difference.

If we now use a comparator to detect when the voltage on the capacitor gets to, say, 0.488 volts and turn "on," and also build in a little hysteresis so it turns "off" at, say, 0.480 volts, then we can use the output from the comparator to switch on a switch-mode regulator connected to the battery we want to charge.

As an aside, I have a large ancient, battery operated, fork-lift truck. The battery on this thing is huge, and when I got it you could see yellow crust all over the plates. I had read that hitting such a battery with high current pulses with enough time between pulses for the released gases to be re-absorbed could "de-sulfate" the plates and put the battery back in business. So, I rigged a device to do this. I used 70 amp pulses of 1 millisecond duration every half second ... for a couple of weeks. It worked beautifully! The yellow crust went away, and strangely enough the volume of the liquid in the cells increased considerably. I had to suck out the excess several times, and I still haven't gotten around to adjusting the acid density. But the battery now works very well, and it sure didn't before this treatment. The reason I mention this is that I think that bursts of current are a very good way to charge a battery!

So, when a cloud passes overhead and the battery is only getting a shot of current every so often, it is just cleaning up the mess in there, nothing to worry about at all!

Rick, the LM5118 is good to 75 volts input, so I will try to make the entire device capable of this so you can use it as well. You don't happen to know of a comparator that can handle this voltage do you?

BillyDoc

[gonzo]

I don't think a 20W panel needs any regulation. In ideal conditions it will put out 1.7 A. Unless you are charging a cell phone that is barely a trickle charge. I use panels without regulation and never had problems

[BillyDoc]

Hi Gonzo,

I'm sure you are right! But I also think I can get a significant improvement. I know it's probably silly for this application, but I hope to learn enough by working the problem at this level to be able to do the same thing when faced with a much larger problem, like in a boat. I think of it as "education."

It's good to know that I could just directly wire my panel up to the battery though. Thanks!

BillyDoc

[Rick Willoughby]

I have had a look at the actual curve for one of the panels I am considering. The open circuit voltage is less than I thought. I will not charge my batteries directly from them because the voltage is too low.

However if you take the curve and assume it is working from 32V to 42V, similar to your 10.5 to 14V then you get an idea of what is lost by not optimising continuously.

At 32volts they are 5.8A so 186W.

At 42V they put out 5.1A so 214W.

The optimum is 225W. So the losses are not that great considering these values are the range limits.

The worst case will be in the high temperature situation when the batteries are almost charged. Under this condition current is down to 3A. But then it would be going into trickle anyhow.


I do not know what sort of comparator circuit you mean!

Exercising batteries over their range usually restores some capacity but I have never seen heavily sulphated cells.

Rick W

[Rick Willoughby]

Hi Gonzo,

I'm sure you are right! But I also think I can get a significant improvement. I know it's probably silly for this application, but I hope to learn enough by working the problem at this level to be able to do the same thing when faced with a much larger problem, like in a boat. I think of it as "education."

It's good to know that I could just directly wire my panel up to the battery though. Thanks!

BillyDoc

I am certainly interested from the learning aspect as well. I want to buy a big panel to play with. Things like the temperature rise are important factors.

I think the peak power tracking on a turbine will be even more interesting.

Rick

[BillyDoc]

Hi Rick,

Don't give up on those panels just yet, the LM5118 controller will charge your batteries with an input from 3 to 75 volts. And funny you should mention turbines, it's basically the same problem as with solar cells, if you try to suck off too much power you just bring them to a halt from back emf. I did a design for an alternator conversion a couple of years ago that this same circuit could be used for. (http://PoiesisResearch.com/Power.php)

A comparator is an electronic circuit (almost always in an integrated circuit chip) that compares the magnitude of two voltages, and produces a digital output in response. So if one voltage is higher than the other the output is "true" if it is lower the output is "false." You can also design in a "window" between the "true" and "false" responses, called the "hysteresis." Hysteresis is like on a thermostat that turns the furnace on when the temperature drops to 70, but only turns the furnace off when the temperature rises to 73. That thermostat would have a hysteresis of 3 degrees. In this case the comparator would be comparing the voltage on a capacitor being charged up by the solar panel with a reference voltage which would be fixed at 0.484 times the number of cells in series. When the comparator went "true" the controller would turn on and suck the capacitor down as it charged the battery, the comparator would then go "false" and the controller would stop. I'm looking at a comparator from Analog Devices that works from 5 volts to 65 volts that I think will work nicely here (AD8214) but I've yet to read the spec sheet.

I was more than a little shocked when I looked at the plates on my big battery. Which is why I started looking for a way to fix the problem. Those big babies are expensive! It works great now though, and the fix cost me nothing but some time.

BillyDoc

[gonzo]

The improvement given by charge regulators can only be achieved with a power source that surpases the charge in current and voltage that the battery can achieve. Unless you install a huge array of panels, all you are accomplishing is to drop the voltage without any advantage. To learn about regulators of any kind, you are better off using a simple battery charger as a power source.

[BillyDoc]

The improvement given by charge regulators can only be achieved with a power source that surpases the charge in current and voltage that the battery can achieve. Unless you install a huge array of panels, all you are accomplishing is to drop the voltage without any advantage. To learn about regulators of any kind, you are better off using a simple battery charger as a power source.

Hi Gonzo,

Can you elaborate this? I have no idea what you mean! Are you saying that an MPPT charger has to be larger than the battery capability to achieve any benefit? Why?

BillyDoc

[gonzo]

Regulators reduce voltage to control the rate of charge (amperage) . They can adjust to type of battery, state of charge, temperature, etc. To achieve that they start with a higher voltage than necessary and drop it.

[gonzo]

There are some systems that produce a high frequency to "recondition" batteries. I don't know if they work

[BillyDoc]

Gonzo, I believe you are thinking of typical resistive regulators, also known as "linear" regulators. The regulator I posted the data sheet above for (LM5118) can take an input from 3 to 75 volts and produce an output with an even wider range. That is, it can produce voltage way above or below the input voltage on the output. In fact, in the example they give in the data sheet for a 12 volt output the input can be anything from 3 to 75 volts (it does need at least 5 volts to start up). Not only that, but it does this magic way more efficiently than a typical resistive regulator.

It's an extremely complex subject, but if you want to learn more Google on "Switch mode regulator." These things are amazing!

BillyDoc

[mydauphin]

Anyone know of any cheaper Solar controllers that work on my BP panels, they produce over 24 volts dc and would like run serial so that is 50 volts. Outback controllers work great but are expensive.

[BillyDoc]

Hi mydauphin,

Actually, designing something exactly like what you need is what I'm trying to do here. So, keep an eye on progress and please pitch in if pushing electrons around is within your expertise!

To those who love to inconvenience those poor dumb electrons:

I think I have a front-end design worked out now, and have posted the circuit diagram below. The circuit will handle input voltages up to a maximum of 65 volts, and uses roughly 1.25 mA of current when the input voltage is below threshold plus another mA when above threshold to operate the enable output. So, in my case where I have a solar panel with an optimum power point at 14.036 volts, that means that the circuit "costs" a maximum of 0.03 watts to do the MPPT function.

Here's how it works.

The output from the solar panel passes through a "safety" diode to prevent the solar panel from shorting out the works when in the dark. This diode must be rated for the expected current (about two amps in my case) and voltage. I'll be using a 15TQ060 Schottky type myself, which is rated at 15 amps and 60 volts, because at 2 amps the forward voltage drop is only about 0.45 volts. The output from this diode feeds a capacitor, the value of which will be determined later. Lets call the voltage on this capacitor Vin.

Vin goes first to an AD8214 comparator, which is a high voltage comparator rated at 65 volts. When the voltage at pin 2 of this comparator (the positive input) is higher than the voltage at pin 8 (the negative input) the current flowing out of pin 5 jumps from a low of less than 100 nano amps to a high of 1 milliamp. This current output is very handy, because we really won't know at what voltage this pin will rest under various conditions, and we needn't care either. If we find that we need a 5 volt logic level on the other end of the enable output, all we have to do is put a 5 Kohm resistor in series to ground . . . and we've got it. That five volts will be developed across that resistor regardless of the voltage at pin 5 of the AD8214.

A couple of problems now arise: First, the inputs to the AD8214 will only tolerate a differential voltage less than half a volt, but if both inputs do stay within half a volt of each other they can be moved (together!) over the entire voltage span of the device, plus a little. That is, the "common mode" voltage on these inputs can range up and down the input voltage of the device, which can be from 5 to 65 volts. The second problem is that in order to make the MPPT actually track the maximum power point we have to be able to set a fixed voltage for the comparator to trigger on. This is best done with dedicated voltage references, but the widest range device I could find (the TL1431) only has a range of 36 volts. So, we need to make provision to "split the difference" and offset the triggering voltage to somewhere near the mid point of what we actually want. Doing this gives us a functional range of 72 volts for the voltage references.

So, concentrating now on these voltage references, the two TL1431 devices, it should be appreciated first that the "mid-point" voltage needn't be all that precise, because the AD8214 has a nice wide common mode voltage swing. But the overall trip point voltage should be as accurate as possible! The TL1431 devices specified have an accuracy of 0.4%, but this is with precision resistors and calculated values. We are going to do better than that, because we will use sloppy old 5% resistors . . . and an adjustable potentiometer. Then the accuracy will be very strongly related to the multimeter we use to adjust the system with. Don't let this scare you, most decent multimeters are accurate enough.

For now just take my word for the fact that the two resistors, marked R1 and R2 to the left of each TL1431 can be picked or adjusted such that the TL1431 acts just like a Zener diode, except a lot more accurately, at the voltage you want, and with far less temperature drift. So, ignoring the two R1s and R2s for now, Vin initially rises over the tops of a TL1431 connected to Current Regulator Diode - 1 (CRD-1) and over the top of CRD-2 which is, in turn, connected to a second TL1431. Both of the CRDs are designed to deliver 1 mA current (this is what a 1N5297 CRD is). BUT, the TL1431s both look like very high resistances until they reach their zener voltages, and these high resistances prevent much current at all from flowing to the CRDs. The CRDs in turn are trying to get some current going, so they look like low resistances.

But wait! Initially Vin is too low to "turn on" the TL1431s but there's another possible current path. Assume that both TL1431's are simply not there because of their high resistances. Now, trace the current through CRD-2, then through the leftmost 11DQ06, then through CRD-1 to ground. That's a current path that works, with both CRDs in series with a Schottky diode. Even at low voltages this current path will draw the 1 mA the CRDs are designed for, with roughly a 0.35 volt drop over the Schottky diode. This voltage drop over the Schottky will be positive at pin 8 of the AD8214, and negative at pin 2 of the AD8214, thus leaving the "enable" output in a high-impedance state (not asserted).

With the enable not asserted the voltage regulator yet to be designed will not be putting any load on Vin, so if the sun is shining at all Vin will continue to rise.

My panel has 29 "stripes" of photovoltaic material, so I know from the Maxim app note (posted above) that the maximum power point for this solar panel is 29 times 0.484 volts, or 14.036 volts. I've got a diode in series with it that drops about 0.45 volts, so I subtract that and get 13.586 as the optimal voltage that I want Vin to be. I want to divide this 13.586 into two more-or-less equal voltage drops, so, for the lower TL1431 being fed by CRD-2 let's just see how close we can get using common standard value resistors.

Half of 13.586 is 6.793, so the first thing I need to look at is how much resistance I can tolerate with R1 and R1 in series, because I don't want to draw any more than about 0.1 mA here. Using ohm's little law I divide 6.793 by 0.0001 and get: 67,930 ohms. I only need at most 3 microamps at the ref input on the TL1431 so this gives me lots of room to play.

Next I go to the formula on the circuit diagram below, shift it around a little to solve for the R1 to R2 ratio to give me roughly 7 volts and see that it's 1.8 to 1. OK, so now I know that I need to get over 70 K total resistance, and R2 will be a little less than twice what R1 is. Now I go to my resistor trays and look for a resistor that is about 30 Kohms . . . and there it is! A nice 33 Kohm resistor. So far, so good.

Now there is only one unknown in my formula (the one on the diagram below) and that's for R1. So, I plug in 6.793 for my Vzener, and 33,000 for R2 . . . and out pops 56,667.6. Fifty six Kohms is close enough, and it's a standard value as well.

Back to that formula I see that an R1 of 56 Kohms and a R2 of 33 Kohms will give me a voltage drop of 6.742 volts. Close enough! Now I can go to the upper R2 and just guess that 33 Kohms will work nicely, with an adjustable R1 of 100 Kohms nominal. In the discussion that follows let's just call both of these Vzener voltages 7 volts.

Where were we? Oh, yeah . . . we want the AD8214 to go on when Vin gets to about 14 volts.

Remember that Vin is going up all the time we've been playing around with math, so let's just assume that things got a little out of hand and Vin is now up to 14.2 volts. Now the TL1431 associated with CRD-1 has enough voltage to drop 7, and so does the TL1431 associated with CRD-2. Vin will now pass through the top TL1431 with enough current to cause a voltage drop across CRD-1 of 7.2 volts, and at the same time CRD-2 will also develop the same voltage drop for the same reason. Now, however, because CRD-1 is tied to ground the voltage at the junction of CRD-1 and it's associated TL1431 is 7.2 volts, relative to ground, and CRD-2s voltage at the same relative point is 6.8 volts. This will cause a current flow through the rightmost 11DQ06, and also make pin 2 of the AD8214 positive relative to pin 8, thus causing the AD8214 to assert the enable output.

This is what we want. As Vin rises to a pre-determined level the enable is asserted and subsequent machinery will suck current from Vin to charge a battery at a rate high enough to cause Vin to drop below the threshold of the AD8214. If the sun is bright, Vin will soon be back up and the process repeated. If it is dim, it will take longer, but in both cases power will be extracted via Vin at very near the maximum power point for the solar cell.

I'll get to the battery charger next, but meanwhile please look over the above and find those inevitable screw-ups for me!

BillyDoc

[mydauphin]

Does this do basically same thing? http://zahninc.com/sd1a.html

or take a look at this one

http://zahninc.com/optimizer1.html

[BillyDoc]

Does this do basically same thing?


Hi mydauphin,

The short answer is "no, not at all!." Both of your links are to DC to DC converters, but that is only part of the problem. Solar cells are subject to highly variable inputs (full sun, clouds, rain, shade, etc.) and at every "situation" that the solar panel finds itself in there is an optimum current draw that maximizes the usable power (volts times amps). An MPPT charger implements some means of Maximum Power Point Tracking to adjust the circuitry to find and use this maximum power point dynamically. It's a very good thing to do, because doing so can add roughly 30% more useable power to your batteries (other conditions being the same). Google on MPPT or Maximum Power Point Tracking for more information on this.

BillyDoc

[BillyDoc]

I forgot to make a provision for the case where the battery is fully charged and you want to just let the voltage from the solar panel rise to it's maximum with no current being drawn. In this case Vin would cause an excessive current through the two TL1431s and one of the 11DQ06s. The fix is easy, just add two resistors as shown below, as R3s.

To calculate the value of R3, subtract the maximum power point voltage from the solar panel (in my case 13.586 volts) from the solar panel maximum no-load voltage (29 times 0.55 volts, minus the Schottky drop of 0.45 volts in my case, that is, 15.5 volts) and divide this value by 0.002. Then pick the next higher standard value, which in my case is 1K. So, again just in my case, both R3s will be 1K, which will limit the maximum current draw through the 11DQ06 diode to 1 mA when the system is in "idle," but will otherwise have no effect.

BillyDoc

[pistnbroke]

I glanced over all this wonderfull techno stuff but nowhere do I see any mention of the fact that to charge the battery to 80% you need to get it up to 14.4v ....at this voltage it will not gas. 100% but gassing 15.3v .... personally dont see the problem in fitting a shunt regulator to bypass panel output when bat volts reaches 14.4v god its only a 20w panel ..I am with gonzo fit a diode and let it get on with it .....

[BillyDoc]

Yes, pistnbroke, you are quite right . . . and if this was the only charger I need I would do exactly what you say, just hook up the panel and forget about it.

But this isn't the only charger I need. And it serves nicely to learn a bit about MPPT chargers. And grabbing an extra 30% more power isn't such a bad thing either when you are only starting with 20 watts under optimal conditions . . . provided the cost is minimal, and it is.

As for the voltage levels you mention, that part of the circuit isn't implemented yet, so you have to wait a few more days for that. I have a couple of other things I have to do first, but will be back at it by the middle of the week at the latest. As a preview, I will be implementing the LM5118 buck/boost controller with a mechanism to step through the various charging stages you mention as appropriate.

I am also trying to keep everything 60-volt capable so the charger can be adapted to work with 12 volt, 24 volt, 36 and 48 (nominal) volt systems. A "universal" design, if you will, and this does add some complications. So, please be patient a little longer!

BillyDoc

[gonzo]

I think you fail to grasp the basic concept of any regulator: You will lose power. Even if the circuit increases voltage, it will decrease amperage in direct proportion minus a loss. With the panel you have it will still be barely a trickle charge. Also, internal resistance of the circuit will sharply limit what you can do to increase voltage.

[BillyDoc]

Gonzo, Here's a quote from Outback Power:

Innovative solar harvesting and battery charging algorithms allow you to maximize your systems potential and can increase your renewable energy yield by up to 30%. http://www.outbackpower.com/products/charge_controllers/ (emphasis added).

I'm trying to do the same thing here, but with a DIY device for those with electronics experience to save a few bucks (like me!). The Outback Power MX 60 controller (which is extremely nice, by the way) sells for about $500, and is worth every penny; but only if you need that much controller and can't DIY, or just don't want to bother.

BillyDoc

[gonzo]

Controllers are like a faucet. If the water pressure is not enough, not amount of opening it will give you more water.

[BillyDoc]

Well, I guess those guys at Outback are just lying then.

BillyDoc

[gonzo]

Read their specs. The claim is that their controllers are 30% more efficient than other controllers. It is very vague because there is nothing to really compare them to. You still need sufficient power to control. A weewee panel does not produce it.

[BillyDoc]

Actually, Gonzo, I think if you look a little closer you will find that they are claiming to: " increase your renewable energy yield by up to 30%." They do this by tracking the point of maximum power output (watts), as it is produced by the solar panel under varying conditions AND by matching the voltage requirements of the batteries being charged to that available power. If solar panels were linear devices this would not be an issue, but solar panels are definitely not linear. What Outback and others are doing is well beyond mere "efficiency." If you were to use a 100% efficient controller that did not do the tracking part, you would not even get close to what Outback does.

BillyDoc

[BillyDoc]

Hi everyone,

I've been looking at the "proper" voltages for charging, and especially at this web site: http://www.solarnavigator.net/battery_charging.htm. If I'm understanding this info correctly, assuming my solar panel is incapable of exceeding the charge rate limitations of my batteries (wouldn't that be a nice problem to have) then I don't need to worry about current, only voltage to the battery. My "bulK" and "absorption" phases look to be the same, and (in my case) should be at a 15.5 volt level, followed by a "float" phase with the voltage at about 13.11 volts.

Two steps, in other words. Kick the battery as hard as I can at 15.5 volts, then when it reaches that voltage drop the input down to 13.11 volts and maintain it there.

Does that sound right to everyone?

BillyDoc

[mydauphin]

Outback say the Power mx 80 is even better at it. But I really can afford this much efficiency and I hat to buy a 60 when I want to buy an 80... Oh, so I wish I could get a even cheaper one for like $99 that would work with higher voltage system even though I would be buying 80 in future. Any solar controller consolers out there.

[gonzo]

Are you saying that the controlles somehow increases power? That makes no sense. All circuits have power losses. Study some basic high school physics.

[BillyDoc]

Are you saying that the controlles somehow increases power? That makes no sense. All circuits have power losses. Study some basic high school physics.

Gonzo, I'm saying that an MPPT controller "can increase your renewable energy yield by up to 30%," which does indeed have the effect of increasing the available power that actually goes into the battery. In other words, your approach of just hooking the solar panel directly up to the battery will work, but you will be throwing away roughly 30% of the power that solar panel develops, not putting it into the battery.

This is not creating power out of thin air, as you imply, it's a matter of intelligently dealing with the natural characteristics of solar panels. As I pointed out above, they are NOT linear devices. Please take a long hard look at the graph below, which shows typical solar cell current/voltage behavior, which I extracted from the Maxim app note attached above. It might also be useful to go back to the app note itself, and read it. The same goes for the LM5118 controller, which can indeed charge a battery at an output voltage higher than the input voltage. Since you have had that high school physics course it should be easily understandable for you.

As for the high school physics, you have the advantage of me there, I missed it. I was too busy with other things at the time. Sounds like a good idea though!

BillyDoc

[gonzo]

Can you explain what an increase in energy yield is? If it is not an increase in power then how can it charge the battery more? If you start with a solar panel that can only trickle charge the battery, there isn't any power source to charge more than that. I think you are very emotionally attached to the snake oil they sell. I read their website and it is typical of their kind. There are many claims and no real data.
Also, if you charge the battery to 15.5V and then drop the input ot 13.3, whatever the source is will discharge the battery until it gets to 13.3V. You really need to study basic physics and electricity

[BillyDoc]

Can you explain what an increase in energy yield is?

Gonzo, I started to try and explain . . . but after thinking about it a bit and looking over your previous posts here I'm afraid I have to conclude that there is nothing I could possibly say that you would understand. You just don't have the educational background for this sort of thing. So, I'm afraid the answer is, no I can't explain it for you. You might try the web or reading some of the stuff I've already posted, if you are really interested, but my reason for posting here is not to be teaching anyone basic electricity. My reason for posting is simply to provide a handy circuit for those who can understand it, and use it. It's a take it or leave it kind of thing, and you should clearly leave it.

BillyDoc

[gonzo]

In other words your ignorance is my fault. Sorry to do that to you.

[BillyDoc]

In other words your ignorance is my fault. Sorry to do that to you.

Exactly! On at least four occasions above I tried to point you in a direction that would answer your questions, but in each case you couldn't be bothered to do any reading or try to sort it out yourself even though I provided the necessary links. And in most of your replies you also included snide and personal comments. So, yes, my ignorance of how to help you is exactly your fault. And apology accepted. I'll be moving on now, I've a design to work on.

BillyDoc

[Jimbo1490]

Gonzo,

Don't be angry over this; lots of folks don't understand basic electronics, and this subject is just a touch above that. If you like to read books, the Radio Shack books on basic electronics and the one on power supplies will bring you up to speed on all this in as little as one week, maybe less if you grasp all the concepts quickly.

The book "Electronics, 2nd edition, a self-teaching guide (http://www.bookfinder.com/dir/i/Electronics-A_Self_Teaching_Guide/0471009164/)" by professor Harry Kybett, ISBN 0-471-00916-4, also available at Radio Shack, is about as good a self-teach basic electronics course as exists. Though less than 300 pages, if you learn all contained within, you'll know more about electronics than 99% of the population, and be able to design simple and even moderately complex circuits, select component values, understand semiconductor amplification and regulation and power supplies of all types.


I've been following this thread from day one, I just have not had anything of value to contribute thus far. I'll go back into 'lurk' mode now :D

Jimbo

P.S.

Billy has been very tactful and polite about your lack of knowledge of the subject matter. You probably know 10X as much as he does about some subjects, just not this one. Read up!

[marshmat]

Billy,

I haven't had a chance to go over the last schematic you posted in detail, but the general concept of the circuit seems about right. A complete detailed analysis is, admittedly, a bit beyond my field of expertise.

Gonzo,

An MPPT is not the same as a regulator or charge controller. And it's not a "free energy" device. Rather, you can think of it as a sort of variable compensator- much in the same manner as a multi-speed transmission or a controllable pitch prop is a variable compensator of sorts.

In an electrical system, power = voltage * current. Depending on the lighting conditions and the electrical load, a solar panel might operate at higher voltage / lower current, or lower voltage / higher current. The graph in post #34 shows the operating curves for some particular solar panel under different illumination levels.

Now, for any given illumination level, there will be some combination of voltage and current that is optimal- ie, maximizes power output. But it's very rare for the load- the battery, motor, whatever- to demand exactly this combination of voltage and current. A Maximum Power Point Tracker (MPPT) serves as a bridge between load and panel- it loads the panel to its optimum operating point, takes that power (the maximum the panel can deliver), and outputs a different voltage and current that are better matched to the load. In this way, it prevents the load from dragging the solar panel into an operating regime where it would be inefficient- hence the 30% (typical) improvement in performance.

They may be expensive, but they do pay off- all solar cars use them, all solar-powered boats use them, most big solar farms and rooftop installations use them. Only small trickle-chargers, like the kind fitted to many cruising sailboats, do not.

[mydauphin]

Why are Mppt expensive?

[Rick Willoughby]

Why are Mppt expensive?

I guess it depends on what terms you consider expensive. Here is one that costs less than a big panel:
http://www.energymatters.com.au/apollo-turbo-charger-80amp-mppt-batttery-charger-management-syst-p-872.html
When you are looking at household applications with maybe 6 x 200W panels you get a free panel if you have MPPT because tis will be roughly the benefit gained by optimising recovery. A big panel is worth about AUD1800 so the MPPT is good value.

Now when you look at the electronics required you might have a good case to say they are expensive but they are not yet made by the millions so design costs have to be recovered from a small number.

Rick W

[BillyDoc]

Well, here it is Friday morning and I've had "one of those" weeks. Which is to say I didn't get as much done as I wanted to.

But I did manage to formulate a plan of attack in between other work, so let me give you a rough outline here . . . and with a little luck maybe I can get it detailed over the weekend.

The "Front end" circuit will give me a logic signal when there is power stored in a capacitor ready to be harvested, and will automatically take care of the MPPT tracking. What remains is to transfer that power into the battery, with the voltage and current that is appropriate for the battery state, whatever that may be (discharged, charged and floating, etc.).

The LM5118 regulator has a handy "enable" input that shuts the device down to a 10 microAmp standby state, so no problem there, and it also has an input to set the output voltage value. This regulator does boost and buck seamlessly, so my voltage coming in from the solar panel via a capacitor can be thought of as independent of the output voltage and current, for as long as that input power lasts.

So, if I know that my solar panel can put out 2 amps, for example, and I set the regulator to suck off 3 amps, then the capacitor storing power from the solar panel will be sucked down to below the "enable" threshold fairly quickly and the enable will go "false." When the solar panel re-charges the capacitor sufficiently, the enable will go "true" and the process repeats. This sporadic behavior is useful for two reasons: First, bursts of current into the battery are useful for keeping the sulfide (sulfate?) off the plates; and second, the off time is when I want to sample the voltage on the battery to check it's charge condition.

Once I get the battery voltage, I hope to feed it to a LM3914 Dot/bar Display Driver, which is normally used as a "voltmeter" display. I will be using it in the "dot" mode only. Internally this IC has a comparator cascade (10 of them) that trips sequentially in equal steps as the voltage increases at their inputs. So, if I am charging a "12" volt battery, I have a range of roughly 10.5 volts (discharged) to 15.5 volts (fully charged) that I can indicate with half-volt steps . . . which I plan to implement as a battery condition indicator. I will put some circuitry in there to sense when the enable goes false, delay, then sample the voltage briefly. This way the appropriate LED will flash and indicate the battery's voltage state. But in addition, the same outputs that operate the LEDs can be picked off to set or reset a Flip Flop (FF). I think that I only need two voltage settings for my regulator, 15.5 volts and 13.2 volts (or so, I need to check this). The current will take care of itself and be whatever is available. So if the device starts up with the FF calling for 15.5 volts and the actual battery voltage is less than that the comparators will just keep setting the FF to 15.5 volts every time they read the battery voltage. If, however the battery voltage actually gets to 15.5 volts, that comparator output should re-set the FF to the lower, 13.2 volts, and the regulator will self-limit the current to maintain this voltage, i.e., it will be in "float" mode. When a load causes the battery voltage to drop below 13.2 to the next lower step the FF is again set and the process starts over. To summarize, battery voltages less than 13 volts (I need to work on the exact values for these windows) will set the regulator to try and put 15.5 volts into the battery and thus dump maximum current into it. Voltages from 13 to 15 volts will not effect the FF. Voltages above 15 volts will set the FF to deliver 13.2 volts where it will remain until the voltage drops below 13.

Any input on what these voltage values should really be will be appreciated!

And I think that should do it. The "front end" takes care of the MPPT function and also produces a handy pulsing signal used further down the line. The regulator takes care of the battery, and the display driver indicates battery condition. The remaining detail will be how to calculate values for different sized panels and/or batteries, and I think a spreadsheet might be the way to go there. I want to build a prototype before doing that, though. There is a lot of room for errors here.

BillyDoc

[BillyDoc]

Attached below is a circuit diagram for the control part of the circuit (I hope). It's a first draft and I'll have to breadboard it to be sure I haven't screwed up somewhere, so please be patient! The good news is that it does seem to be possible, and at not much cost.

Let me try to walk through the circuit now.

At the left is the MPPT circuitry as above, with the only new part being a tap on pin 5 of the AD8214, the "enable" output. This feeds a 5 K resistor where 5 volts are developed (the enable output is a 1 mA current) which are fed to pin 6 of an ICM7556 "dual one shot" timer. Pin 6 is a trigger input for the first timer, and triggers on the falling edge of the input. So, when the enable is asserted high, the timer does nothing. Then when the enable is released to ground the falling edge of this signal triggers the first timer (pins 1 through 6) which has timing elements to give a roughly 11 mS pulsed output. This output pulse goes out pin 5 and is connected to pin 8, which is the trigger input for the second timer. Again, it is the falling edge that does the triggering. This second timer has components that give it a roughly 1.1 mS output pulse, which goes out pin 9.

The point of the 11 mS delay followed by a 1.1 mS pulse is to allow the battery to settle for 11 mS after being charged by a string of current pulses from the solar panel (not implemented yet), then to initiate a 1.1 mS "sampling" period where the voltage will be determined and displayed, and if the voltage is 15.5 volts a Flip Flop will be set to reset the charging voltage to the lower float voltage of 13.2 volts.

So, the output from the second timer is wired to the gate input of a MOSFET transistor with a very low on resistance (less than 0.1 ohm). The pulse from this timer turns the MOSFET on, which is to say that it's resistance between the Drain and Source drops to less than 0.1 ohm, thus completing a power path from a LT3010-5 high voltage regulator through some LEDs, and a LM3914 display driver. The battery voltage (Vbatt) has been presented to pin 5 of the LM3914 via a voltage divider, where it is compared to a reference voltage derived from an internal voltage reference by a string of 10 comparators. Only one of these comparators can be "true" at any given time, so the LED attached to that comparator has current drawn through it and lights, thus indicating the voltage level of the battery. When a LED has power applied to it, which means it has it's cathode effectively grounded, this ground may also cause current to be drawn through an associated Schottky diode. Any of the voltage indicating LEDs below 13 volts will cause a 10 K resistor to be pulled to near ground, and at the same time will put a negative going pulse on the "clear" input (pin 1) of the 74AC74 Flip Flop, thus causing the "Charge" output to be set high. Power applied to the LEDs between 13.5 and 15 volts will do nothing to the Flip Flop, but power applied to the 15.5 volt LED will pass through an associated Schottky diode to the "Set" input of the Flip Flop, thus causing the "Float" output to be set high. When the Float output is high a pulse from the IRLD024 causes an eleventh LED to pulse as well, thus indicating that the battery is on float charge. This same Float output will be used to toggle the charging voltage back to 13.2 volts when we get to the buck/boost controller, and the Charge output will be used to cause a charging voltage of 15.6 volts.

Two high voltage regulators are used, with the power draw being distributed between them such that there is never more than a few mA being drawn at any given time, and for the pulsed stuff that power is only on briefly.

When the system is in operation the flashing rate of the LEDs will be a good indicator of the rate of power being captured because the input capacitor will be re-charged after every discharge cycle more or less rapidly in direct relation to the solar panel output. The particular LED doing the flashing will indicate the battery voltage, with the Float LED also flashing when in the float phase.

The only thing left now is the buck/boost controller, and for that I will be following the design paradigm given in the data sheet pretty strictly. I think I'm going to take the time to put it into MathCad, then I'll be able to calculate the variables for the four battery voltages anticipated easily (12, 24, 36 and 48 nominal voltages) and can just reduce it to a table of values for you guys.

But not today! The sun is shining and the wife wants to go for a long walk, so I'll get back to it tomorrow. This was the hard part though, the rest is grunt work. Please review and let me know what you think so far!

BillyDoc

[pistnbroke]

I understand your MPPT regulator but have a few points on batteries for you....what is happening at the battery is lost in the explanation of what the electronics are doing ( for me)

First a good battery charged and left for 24 hours will show a little over 12.5v..it does not matter the state of charge as long as its over 20%(no load)
If you charge a run down battery it will pass through 14.4v and start to gas until its fully charges at about 15.3v...so if its unattended for say 1 month + you dont want to go past 14.4v or you will loose water ..acid concentration increases and it buggers the plates . If you want to float it then 13.8 v is the typical figure but you would not want to leave a wet battery at that level or you will sulphate the plates ....perhaps each day when the sun comes up it is brought back to 14.4v before being allowed to drop back to 13.8v....Although the battery technology is 1850 its not simple to keep it happy ,,,,

[BillyDoc]

Hi pistnbroke,

All good points! And very helpful to me as I really don't know much about this part of the problem. Especially the exact values for floating etc.

I didn't discuss it above, but there are two potentiometers (roughly SE from the LM3914) that set the offset and span of the chip input, so you will be able to set it as you please when all is done. So, for now just take the values as tentative.

I did intentionally power the Flip Flop that is holding the memory of whether the device should be charging or floating from the battery so that the state memory would survive periods of no input. As described in the circuit description the system will come on and start charging (if there is an input) in either the Float or Charge mode until the first period when the enable goes low. Then the battery voltage will be sampled, and if it is in the range from 11 to 13 volts the Flip Flop will be set to the Charge state and the controller will try to achieve the full 15.6 volts. Of course the controller is trying to do the impossible for a low battery and will behave the same way, that is dump in all the power that is available, if the controller is set at either 15.6 volts or 13.2 volts. That's why I didn't bother putting circuitry in for lower than 11 volts.

Anyway, eventually the voltage will come up if the sun shines. From what I did read on this subject a small bit of gassing for a short time is a good thing as it stirs the electrolyte and keeps it from stratifying vertically with differing density layers. This is probably a non-issue on a boat where normal wave action will serve the same purpose, but then my understanding (or lack thereof) was that to get to a 100% charge the battery also apparently needs the brief high charging voltage as well. So, the higher charge voltage is set and held until reached (15.5 volts for now), then the next time the voltage is sampled the Flip Flop is re-set back to the float voltage and this voltage is held until the battery voltage drops below 13 volts again. The controller will not output any power at all during this period, because it "sees" a higher voltage on the battery than it is programmed to put out. Eventually the battery will self-discharge down to below 13.2 volts and the controller will turn on and commence to give it small kicks and maintain that voltage.

I was aware of the nasty tendency of an almost discharged battery to read a "good" 12.5 volts with no load no matter what, given enough time to settle out, and that is why I want to set the charge voltage so high. I'm assuming that that is sufficient voltage difference to get the battery to accept something near maximum current.

The design is limited by the fact that the comparators internal to the LM3904 step up by equal increments, but aside from that just about any offset and range could probably be designed or adjusted into the system. The points where the different states are picked off can also be chosen at will (the reset to charge points (diodes), the make no change points (no diodes) and the float point (diode). Also, the actual voltage that the controller attempts to put out can be anything, and if there was a compelling reason could also be more than two different voltages. I really haven't looked more than briefly at these issues and am completely open to suggestions! In fact I would love it if someone out there would do the research and find out how to optimize these variables. Unfortunately, I suspect they will be slightly different for different battery types, but that doesn't mean an individual couldn't program in what's best for his own system . . . if he knows what the parameters are.

BillyDoc

[pistnbroke]

No it does not self dischage down(0.3%/day) to "below 13v"...slighty over 12.5 is the voltage of a fully charged battery that has stood for 24 hrs...
(battery test ..fully charge and stand the battery for 24hrs ..If the voltage is below 12.5v the battery is suspect )

Once you got it up to 15.5v then you dont want to let it drop all the way back to 13v you should keep it at 13.8 v ..thats floating ....alarm and stand by batteries sit at this for years..

Like you say battery types vary but some modern ones are so tightly packed with seperators ( to stop vibration destroying the plates) that there is no real exit between the plates for large volumes of gas so you should be sure your battery can go over 14.4v..

Its not bringing the battery to 15.5v thats the poblem but the time your 2A charger will take to do it and all the gassing involved ..maintanence issue .

If you restict yourself to 14.4 or 14.7 as a max and float at 13.8 with sun up cycles to 14.4 14.7 that would avoid many potential problems ...( jap cars using older battery technology go for 14.7v whilst european go for 14.4v)

[BillyDoc]

Thanks pistnbroke, you sound like you know what you're about and I know I don't.

OK, so assuming a "marine" deep discharge flooded cell type of battery, instead of 15.5 volts, set the upper limit trip-into-float at 14.4 volts, and then float at 13.8 instead of 13.2. Is that about right? It's no problem if that's the best plan.

BillyDoc

[pistnbroke]

go with the max the maker says 14.4 or 14.7 are common and 13.8 is good for floating . Something to bring it up to 14.4 at least once a week would be good as if its been sitting at 13.8 for 3 months it wont have the amount of stored charge that it might have and some of your savings in efficiency will be lost

[BillyDoc]

Hi pistnbroke, sounds like a plan to me. The the voltages used are programmable (just pick a resistor) and could even be put on a switch if required. Same with the span and offset of the "voltmeter" chip. A seven day timer to force a charge cycle would be a bit more difficult to implement at this point, but would be easy if we want to go the extra step and add a microcontroller.

I hope to finish the last part of the circuit today, and then I will order the parts and build it. Then we'll see what we see.

Thanks for your help on this part!

BillyDoc

[BillyDoc]

OK, a complete circuit, but not a final one. There were a couple of things in the buck/boost chip that I'm pretty sure I don't understand completely. At this point I really need to build it and put a scope on it to see what it does, so . . . that's what I'll do.

Meanwhile, here's what I have.

The MPPT and associated logic has already been discussed, so I'll leave that for now. I changed the voltage offset slightly based on pistnbroke's recommendations, and cleaned it up a bit but it is otherwise the same. The resistor values on the buck/boost feedback set 14.4 v and 13.8 v for charge and float.

The Buck Boost controller is a bitch to understand (or I'm too stupid) and there are a couple of things about it I am not at all clear about, namely the current feedback effect on the PWM, and the compensation circuit for the feedback signal. I hope to know more when I play with it.

There really isn't much for me to say about the Buck Boost circuit, because I mostly just copied it from the front page of the LM5118 data sheet. My only contribution was to add a means of digitally changing the feedback set point (charge or float) which I think is self explanatory . . . well it is when you remember that the transistors used have a very low ON resistance less than 0.1 ohm. Oh, oh, speaking of transistors I see I forgot to put labels on the Schottky diodes and the power MOSFETS! They are IRFR3607 and STPS20120D respectively (sorry, I'll correct with the other corrections yet unknown). Anyway, you should be able to see the basic logic of the entire system now. PLEASE let me know if you spot some dumb mistake, or even a smart one!

BillyDoc

[mydauphin]

Billy, you drive me crazy with these diagrams and explanation. But thanks....
Anyway, how much you think something like this cost in parts. And how would it compare with a more expensive unit like Outback. Of course it doesn't have the fancy displays but does it do the same you figure?

[BillyDoc]

Hi mydauphin,

They drive me crazy too! But sometimes it's worth it.

I don't think this circuit will ever be as good as something like the Outback products, because they use a microprocessor to optimize the various variables continuously. I DO think it will get close, however, probably within a few percentage points in terms of efficiency. As for costs, the only way to find out is to go shopping and see. Most of the parts are fairly cheap, for example the LM5118 is $4.16, and I think it is the most expensive IC used. Here's a quick sampling of the major parts from digikey.com:

LM5118 4.16
STPS20120D 1.28
IRFR3607 1.91
IRLD024 1.40
AD8214 1.98
ICM7556 1.33
LM3914 2.60
LT3010-5 3.13
74AC74 0.55
1140-101K-RC 100 uH 9.56

Plus a board to solder it all on, a bunch of capacitors, resistors, etc., then a box to put it into . . . it all adds up! As a very rough guess I would say about $75 to build one.

Other major places to try are newark.com and mouser.com.

THIS IS JUST TO GIVE YOU AN IDEA OF THE COSTS INVOLVED WITH THIS APPROACH. PLEASE DON'T BUY ANYTHING YET!!!! GIVE ME A COUPLE OF WEEKS TO GET THE PARTS AND MAKE SURE THIS THING ACTUALLY WORKS!

BillyDoc

[mydauphin]

Billy, Do you have software to test circuit? I have a cousin in UCF studying EE and he has software to test circuits. Also how are you going to make circuit board. Are you going to breadboard prototype?

[BillyDoc]

Hi mydauphin,

I will actually be using this controller to charge a battery in a cabin my wife and I own in the middle of 160 acres of forest. We go there every weekend, so the usual battery load is only for lights and fans in the summer and an electric furnace blower in winter, which doesn't take much.

So I will be building a "permanent" device by hand-wiring on a circuit board with soldered connections. I should get it done in a couple of weeks and I'll report back then (I just got a bunch of other work I have to do as well, however). I'll take some pictures.

Thanks for the software offer, but I've already tried to use that approach on the National Semi web site (lots of manufacturers have modeling capability on site) and the model they have for their LM5118 is limited to roughly a six amp output. I want to go to higher current pulses into the battery, but make them brief and intermittent, and this is not really how this chip was designed to be used. So, the modeling doesn't really provide the answers I need. I think my old-fashioned oscilloscope will though. The only reason I can see for the six amp limit is the gate charge into the MOSFET transistors being a limiting factor, but I have used much higher current transistors in my design with almost the same gate charge characteristic . . . so we'll see.

It also wouldn't be the first time I did something stupid, so we really aren't going to know until I build the thing and make any necessary corrections.

BillyDoc

[mydauphin]

Hey Billy,
What happen? I miss your totally above my head electronic conversations...

[BillyDoc]

Hi mydauphin,

I am so sorry! I know I promised to get on with this stuff, and I'm seriously late. I hate to do that too. The problem is that I suddenly got a bunch of work . . . and with the economy being what it is I can't afford to turn any work away . . . so it's been on a back burner. I'm in the office working today and will be tomorrow too. Hopefully I can get to it next week. I did order the parts, and they're here. And the actual panel has been put together, so I won't be starting totally from scratch.

Again, my apologies!

BillyDoc

[mydauphin]

Just pulling your chain. I know for the first time in my life I look forward to getting work. Have a good one 4th of July.

[power]

Hello BillyDoc,

Thanks for all that information on solar recharging. A friend of mine and I are working on a school project on designing a solar powered recharger to recharge lithium ion batteries efficiently. We are looking for a low cost solution and were wondering if the voltage regulation is necessary. Can't we simply use a diode in series with the panel and the secondary battery to prevent current flow from battery to panel? The battery voltage can be regulated by a comparator as you suggested, and whenever it is low, the panel can be charging the battery up. Since the battery voltage cuts off the power supply beyond a certain voltage, why do we need a voltage regulator at all?

Thanks

[pistnbroke]

Welcome to the fourum ..We like all types of quetions here and you will find a lot of clever people with a vast knowledge who started like you on projects ......You need to do a lot of research on the specifc requirement of charging lithium Iron batteries which are unlike any other in there charging requirements ...If you wanted to charge a car battery with a low powered say 20w panel yes you could just put in a diode ....but a lithium Iron is not a car battery ..
Like you said its your project so do the research on the internet ..getting someone to tell you how to do it is not research or a project ... Tell us all what you find out and what problems that is giving you and many will help.

What size panel do you have and what is the voltage and capacity of your lithium batteries ?

[Rick Willoughby]

Hello BillyDoc,

Thanks for all that information on solar recharging. A friend of mine and I are working on a school project on designing a solar powered recharger to recharge lithium ion batteries efficiently. We are looking for a low cost solution and were wondering if the voltage regulation is necessary. Can't we simply use a diode in series with the panel and the secondary battery to prevent current flow from battery to panel? The battery voltage can be regulated by a comparator as you suggested, and whenever it is low, the panel can be charging the battery up. Since the battery voltage cuts off the power supply beyond a certain voltage, why do we need a voltage regulator at all?

Thanks

At the stage of development the life of most lithium cells is highly dependent on not taking them outside their voltage range. You can find references to Battery Management Systems (BMS) for charging and ballancing lithium batteries. They monitor each cell individually.

You can get economically priced BMS with buck and boost circuitry from here:
http://www.hobbycity.com/hobbycity/store/uh_listCategoriesAndProducts.asp?catname=Battery+Chargers&idCategory=216&ParentCat=85

In answer to your question the life of the battery will depend on it not being overcharged or or fully drained.

The advantage of a MPPT is that it will get the maximum energy from a given solar cell. This is quite different to a charger that will guarantee good battery life.

If you use a simple regulator even with tight limit on peak voltage then you run the risk of cell imbalance and overcharging some cells while others are completely drained on discharge. Lead acid batteries are not as sensitive to cell differential in the battery.

Rick W

[Prajna]

Wow Bill, exactly what I have been looking for!

I am hoping to build 500w of solar panels for my motorhome and was researching charge controllers. Strange, I had naively imagined that all charge controllers would do mppt and was horrified to find that a. the idea is quite new and b. they are so damned expensive.

Batteries are complex beasts and rather esoteric. I have bought 300ah of elecsol, carbon fiber, batteries. The first thing to know about lead-acid batteries is that there are two distinct types: flooded and sealed, and they both require different charging regimes. Actually, we also need to distinguish between deep cycle (traction) batteries and starter batteries because deep cycle batteries are designed with thick plates so that they can be discharged often, at the expense of being slower to deliver power. Starter batteries have lots of thin plates, so they can deliver a high current, but they really suffer when deeply discharged.

Lead-acid battery charging is generally split into at least 3 stages. The first stage, bulk, delivers as much current as possible (up to the C20 rate generally) until the voltage reaches the 'charged' voltage. In the case of my Elecsol's this is 14.4v. Then there is an absorption stage, where voltage is held constant while the current the battery accepts drops until it reaches a specified level (usually about 1amp). After that there is a float stage where the voltage is dropped. Some chargers also have a pre-charge phase where the battery is pulsed to desulfate the plates and often the float cycle is pulsed slightly too. In addition to that flooded batteries can be 'equalized' to make sure all the cells are producing the same voltage.

For sealed batteries it is bad news to have them gassing because if you lose water from a sealed battery there is no way to replace it. For flooded batteries it is good to overcharge (equalize) them periodically (usually about once a month) which causes lots of gassing for an hour or two and that desulfates the battery and makes sure all the cells have the same voltage. Obviously one doesn't want to do that to a sealed battery.

There is a nice chip that is designed specifically for multi-stage lead-acid battery charging by Microchip. Here is the app note: http://ww1.microchip.com/downloads/en/AppNotes/01015a.pdf

Maxim also have an app note for battery charging here http://www.maxim-ic.com/appnotes.cfm/an_pk/680

So not only is the mppt a concern if the design is to be of general use different battery parameters (including the Ah total of the bank being charged) needs to be configurable too.

Oh, and another thing: the charging voltages change with battery temperature too. Ideal is if a battery temperature sensor is included and voltages adjusted depending on the temperature. Also there is the problem that the voltage reading is the 'surface charge' on the plates, which is why a definitive state of charge can only be read after the battery has rested (after either charging or discharging).

Not sure if this helps but I hope so. I don't know so much about electronics but I have put a lot of effort into researching batteries and would be happy to answer, or find an answer, for anything that would help.

Oh, I am happy with programming too, so if it comes to writing firmware for a microcontroller I'd be happy to help with that.

regards, Prajna