Simple MPPT solar panel charge controllers

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/...=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

 

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

 

Boost with the LTC555

Hello,

I need to build a boost circuit that boosts 3.7V to 5V and 12V. I was thinking of using the low power version of the LM555, the LTC555. It can operate at only 2V, and can work in the astable mode. However, I'm concerned about the output current bringing down the boosted voltage. The last time I made a boost circuit with the 555, it boosted 4.8V to 100V. However, when I applied a load, the current drawn reduced the boosted voltage to 14V. How do I ensure a constant voltage output irrespective of the current being drawn, or how can I limit the output current, so that the output voltage is unaffected?

 

The biggest problem with using transformers is that the wattage stays the same (in therory), the formula is T1/T2 = V1/V2 = I2/I1, so let say you want 10 amps at 100V so your wattage is 1000 w, if your input voltage is 4.8 v then you will need 208.4 amps at the input.

So to address your problem, to stop the output being loaded down, you have to make sure the input current is high enough. Useing a constant current regulator at the output may not help you, if your load reguires a higher load than the regulator can provide, the regulator may always be in overload mode. But is a good feature to include in your CCT as it provide overload protection.

Use your 555 timer to drive a mosfet that feeds the input to your transformer, some of them have a current capacity of 60 amps and more.

Dave,

 

My current and voltage requirements are not that high. The input source is a lithium ion cell rated at 3.7V, 3000mAh. Using the boost, I want to be able to provide 5V at 1A or 12V at 0.5-0.65A. If the output impedance of my circuit is low, will I be able to pull out a constant 5V or 12V even if different loads draw different current?

 

your battery only pumps out 11.1 watts, and your requirements are 5-7 watts, so your battery will last a maximum 2 hours minus any losses due to convertion. If thats all you want, thats fine. You will be replacing your battery most likely every 1 1/2 of useage.

As long as the wattage used on the output does not exceed the wattage provided on the input you will be fine. You can only at best get out what you put in, the wattage stays the same.

 

Hi BillyDoc,

I have been reading this thread with interest as I think it is exactly what I am looking for. Have you managed to test it out yet? I'm really interested to hear how you got on... it's like reading a book only to find the last chapters missing

Cheers
Shaun

 

Hi Shaun,

I'm very sorry about the incredibly long delay, I took a contract to build a rather complex machine . . . and I'm still working on it. I won't have the opportunity to get back to this for at least three more months.

I can hardly wait to actually have a weekend off!

Bill

 

Hi Bill,

You don't have to apologise! I know how it goes... my own project gets put on the back burner at times and months can go by.

My project is a petrol electric 7.25" gauge loco.. a picture is here if you're interested

The reason I'm interested in the charging is because the output from my (adapted) alternator varies (I use PWM on the field and have the motor connected to the output) and would like to charge my batteries from this output... so a buck/boost circuit seems to be exactly what I am after, coupled with the part that checks the battery state. I need to see how I can adapt it, which may take me a while, but I'll let you know how I get on if it's of any use to anybody

Thanks for replying so quickly! I hope you get your weekend off soon!

Cheers
Shaun

 

Hello Bill,

Although I have just registered, I have read/followed this discussion for several months now and would like to study your ideas more closely.

Your message #51 includes two "thumbnails" circuits which unfortunately I cannot read clearly enough to be able to match their reference numbers to your text.

Is there any possibility that you could recreate them as pdfs and post here please?

Kind regards,

Ross

 

Hi Bill/Ross

Ross: the images Bill posted are fine for me... if you put your cursor over them does a magnifying glass appear? if so it might expand it... otherwise I saved the image and printed it out filling an A4 piece of paper.

Bill and/or anyone:

I have been going through the circuit and designing my own... but I have hit a slight problem with the inductor in the buck/boost circuit.

First of all I'll describe my situation... I have an alternator that will produce an output of 0-24v (or there abouts) and I want to use that to charge two lead acid batteries in series. The batteries are 12V 15AH each and have a max charge current of initially 6A... I have some other circuitry attached to them which in total draws a continuous 3.65A so I am guessing that my charging circuit will have to supply at least that just to maintain the batteries.

I was thinking of having a transformer on one of the phase outputs and then a rectifier... (I'm still struggling to work out what power rating I'll need for a transformer)

My problem is that going by the equations in the LM5118 datasheet, I need an inductor somewhere in the range of 60-100uH and of at least 68A... I can't find anything like that on RS or Farnell...

Do you have any ideas? how were you planning on it dealing with a 24 or 48v system?

Cheers
Shaun

 

Thanks for the reply Shaun. I have tried IE and Firefox (my usual browser). Neither produces the magnifier/increased sized view that you described. If I right click and view the information/properties of the image, I get a 200 by 119 pixel report. Clearly, if you can print it, you have an improved resolution. As Bill is obviously too busy to help me at the moment, could I ask you to "print" the two images to a pdf for me ... and anyone else who is facing this issue but hasn't "spoken" up yet.

Much appreciated.

Ross

 

Hi Ross,

I believe I can create a PDF for you... but I am a little puzzled... did you click on the image to open up the full resolution version? you can right click on the thumbnail and select "open in new.." and choose either window or tab.. either is fine...

Cheers
Shaun

 

Shoot! Why on earth didn't I try that? Must be old age. Many thanks Shaun ... now I am "back on the team".

Many thanks,

Ross

 

Bill, When you get back on this project, I would be very interested in discussing the applications with you. At your leisure....