Help me to understand the usefulness of a MPPT inverter on a yacht

Hi forum members,

I have difficulty to understand how an expensive MPPT inverter is better than a simple circuit with a voltage doubler or tripler, when the voltage drops below the battery or charging voltage.
The MPPT circuit diagrams are not that easy to be reproduced by an amateur. I know there are some MPPT circuits available which are claimed to be simpler.

If a MPPT inverter has the ability with a buck circuit to pump up the voltage when the solar panel voltage has dropped below useful level. That is then fine. It is worth the money. At least you get some energy coming into your battery-bank when overcast or late at night or early in the morning or when the panels are mounted flat horizontal.

But if a battery bank is full. Your MPPT inverter/charger is no longer functioning.

I am busy to add a simple voltage doubler/tripler with a microprocessor which will kick in when the panel voltage has dropped below 25 Volt. (I am using 2 x 12 Volt panels in serial) I am thinking of using at the output a gas discharge lightning protector which has the ideal parameters to short circuit at 35 Volt and if the current has dropped to MilliAmperes, it opens up again or a Diac.

Any experience on MPPT invertors/chargers?
Bert

 

If a battery bank is full, the MPPT is supposed to stop charging. Otherwise, it will damage the battery. There are many simpler, and not so good, circuits you can make. The good quality controllers, calculate the state of charge of the battery and the temperature to provide the maximum safe charge.

 

I'm no expert on MPPT's but, to be honest Bert, your explanation of MPPT made no sense. http://en.m.wikipedia.org/wiki/Maximum_power_point_tracking has some good explanation about them.
BR Teddy

 

Thanks for your response. I do have full understanding for tracking the maximum power point. But lets now assume the battery is half full, and the radiation of photons on the solar cells is so low, that the voltage has dropped below the battery voltage. What happens then with a MPPT charger?.
Bert

 

It's a function of the electro-chemistry of the cells - but you can dump energy into a 'flat' battery quite quickly with minimal side effects, as the battery gets past the 'bulk' charge zone, the current needs to be reduced to avoid boiling off the electrolyte (the acid) and heating up the plates which can cause them to warp and short out, or even disintegrate;-

Modern batteries are much lighter - the lead plates are generally not solid lead but actually sintered - think of sugar cubes - lots of granules all stuck together to give a sponge like structure - it gives a MASSIVE surface area compared to solid plates though they don't tolerate fast charging - it generates too much heat - a modern charger will have a thermocouple to measure the battery temperature and regulate the rate of charge as a calculation based on that.

There is a guy building an Arduino based unit.

http://www.instructables.com/id/ARDUINO-SOLAR-CHARGE-CONTROLLER-Version-30/

 

Thanks Gozo, I have problems with understanding whether the MPPT charger will still provide energy to the batteries when the radiation is fallen below the battery voltage and the battery is half empty. We all agree, as soon the battery is full, whether you have a MPPT or a normal charger. Full is full and it depends totally whether you use your batteries as such, that you need constant to charge the batteries, I cannot see the advantage of a substantial more expensive MPPT charger, if you only top the battery up at intervals. Bert

 

As I understand MPPT charger is reducing the voltage, not increasing it. In practice this means your solar cells should produce way over the voltage needed for charging (let's say 22v), which is then reduced by MPPT to appropriate voltage (like 14,4v). The MPPT charger does this "smartly" so that the excess voltage is not wasted as it would be with "el cheapo" controllers.. When the light and the solar cell voltage decreses it's still well above adequate charging voltage..

 

Hi Teddy. That is the problem. A lot of people, included me, have the wrong idea what a MPPT charger really does and not does. Although I have read all the information included that from Microchip AN1521 which gave me some good idea on how to program a micro controller. But I stopped with programming and experimenting with the PIC18F1423. The MPPT shifts the power-point and increases the Voltage, but reduces slightly the current of a photocell. The product Voltage x current = power (Watt's) is greater than if you do not shift that point. This is due to the PWM (Pulse Width Modulation) But if I look at the internal impedance of a battery, you need the current and the appropriate voltage for that charging. Does that mean by a higher Voltage, the battery charges faster? No, not to my knowledge. Thus all what the MPPT can do for me is Buck boost a lower output of a solar panel to 14.4 Volt (If a Lead Acid Battery) is connected. Should that be the case as the advantage of a MPPT, then my inexpensive voltage doubler will do exactly the same . That is what I am trying to figure out from the forum members.

Look, if I put a MPPT charger directly on a motor , I will get more output power to my prop, as the MPPT charger does give more whooom power out. However I do not think that it makes a difference for charging a battery EXCEPT if it can boost the very low solar panel voltage late at night or early in the morning higher up to more than 14.4 Volt, while the batteries are still half empty.

Then in that case a normal solar panel with normal charger hooked onto a battery will no longer charge the battery as the Voltage has dropped below the charging criteria.

Who can confirm that an expensive MPPT charger does pump up the Voltage.
to a higher Voltage, while the solar panel supplies only 9 Volt. (Overcast, early in the morning etc).
Bert

 

You have to limit the upper voltage somehow or you burn your electronics. The lower limit is the charging voltage for batteries. Other methods vaste the excess voltage, with MPPT it's used to increase amperage. Don't complicate too much just be sure your solar cells produce enough volts most of the day. Dwilight doesn't matter in the big picture..

 

Hi Teddy, Do you have a lot of sun there, in the northern part of Norway and Finland? Do you use solar panels? Interesting remark you made with: "with MPPT it's used to increase amperage". What you thus say is.: The MPPT converts the impedance to a lower impedance. Interesting thought. It means they use a buck or boost or SEPIC topology. (single ended primary inductor converter) Thank you Teddy.
Bert

 

In the summer 24/7, winter none..

 

Thanks all, I have my answers. Easter is over and I went to see the company who install the solar installations. He briefed me:
a) The solar panel directly connected to the battery will drop the solar Voltage to the battery status. i.e. with a lead acid battery , if the battery is 11,5 Volt. The solar panel Voltage will be 11,5 Volt. Just the current is whatever the sun can give and will be less than the MPPT invertor. (Teddy like you said)
b) The MPPT was designed to enable to use 100.000 solar panels with a lower Voltage and less solar cells and the MPPT will correct and regulate this. Thus a pure financial reason. If one can save 100.000 x 4 or 5 Solar cells, with mounting, smaller size etc. that is quite an amount which could be saved.
c) The MPPT will boost the Voltage from less than the battery voltage to an higher Voltage, either by a boost buck circuit or SEPIC circuit.
d) I have now the confidence that my less inexpensive circuit will work

1) First the solar panel is directly connected to the battery.(via a circuit)
2) As soon the voltage has risen to 13 Volt, I switch over to constant Voltage
mode and circuit. It stays there until the Voltage has reached 13.6 Volt (just below the gassing Voltage) and stays there until Voltage is dropped again.
3) As soon the battery gets empty and the solar Voltage drops below the battery Voltage (due to overcast or whatever) , a third circuit kicks in and boost the Voltage higher to the constant Voltage circuit and charge the battery with/in pulse mode.
4) At least I have not the problem of the MPPT topology that oscillating can occur.
Thanks you all for the help
Bert

 

Hi Gonzo, Indeed you are right, in that the MPPT stop charging, however according to the Large solar installations supplier, the MPPT goes over to trickle Voltage charging equal to the leakage current of the battery bank. That certainly make sense. Thanks for your help. Bert

 

Not all or maybe even most MPPT controllers have the boost option and I think boost is not necessary. Say you have a nominal 12 V solar panel. It will have MPPT point at 17-18 V at full power and drop very slowly when the radiation drops. Once the MPPT point is below 14.4 V, the power will be at most only a few % of the maximum. Thus you don't really care about the few minutes or hours you could get 0.1-2% of the maximum power. And if the battery voltage is 14.4 V, it is full and needs no charging or you are using something else to charge. The 0.1-2% power will likely keep the battery well below 14 V.

With MPPT you will typically get 30-40% more power to 12 V and 15% to 14.4 V. That is really important. That is MUCH more Ah/h than boost could give for the low power time.

MPPT can be quite simple. I have designed and made one for up to 10 A. The parts costs about 20 € and it fits into a 80x35x20 mm box. The efficiency is 92-98% depending on voltages and current. Well it's not (yet) a real MPPT device, it just keeps the panel voltage at a set value (17 V) until the current drops to about 0.2 A. After that the panel voltage is 1.5 V above the battery voltage. The only problem with it is the surface mount parts with pads under the device. These are hard to solder at home, but not impossible.

I wrote about it in more detail here: http://www.ruuvipenkki.fi/foorumi/viewtopic.php?f=15&t=1169
Unfortunately in Finnish...

 

Hi Joakim, I agree with you about that not all MPPT inverters are boosting upwards, if the voltage of the solar panel is dropped below an useful Voltage. Hungarian is similar then Finish and I quite like your circuit and explanation. But I think for a yacht, it would be better to have the real MPPT inverter which works on the basis: power in is power out minus efficiency losses. i.e. If a solar panel gives 12 Ampere at 19 Volt, you have 228 watt minus the losses. The quality MPPT drops the voltage to 14.4 Volt, but boost the current to 15+ Ampere. Thus your batteries are quicker charged and if you use energy while charging, you do have a great advantage. Pity that I cannot get your LT2763 here. I would not mind testing your circuit. But whatever, to pay 20 Euro instead of some 140 Euro is certainly worthwhile to explore. Do you have also midnight sun at your place and how does that affect a solar panel. Bert
Bert