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DC-to-DC converter and MPPT efficiency

Perelho

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Joined
Dec 28, 2021
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So, I measured the Voc and Isc of my 6 panels today. They pretty much matched Voc rating before they heated up, and exceeded the Isc after they warmed up (although voltage had dropped). Voc was around 21.2v before they heated up, and around 20.2v after the Isc tests. The Isc slowly built up to around 9.4A when it started oscillating by about 10mV, that's when I stopped.

I know MPPT works using a DC-DC Buck conversion process, and uses effectively PWM to track the maximum power point (before the Buck Converter)

Now, I want to pose a question. The Buck conversion efficiency is the best when the difference between in and out volts is small. Efficiency drops as the difference increases. Is that a fair understanding?

If I have 12v class panels, with a 17.5v output at maximum power level and I have a "12v" storage system, I would be best served to have all panels connected to the charge controller in parallel, wouldn't I? That would mean that the current rating of the charge controller would need to be at least 60A (as the Isc measured at around 9.4A for each panel) and the voltage rating would need to be 25v or higher. Storage would need to be capable of accumulating 60A total.

If I upped the storage to "24v", I'd be best served to have 3 strings (each made of 2 panels in series) connected to the charge controller in parallel. The charge controller would need to handle 45v or higher and 30A at minimum. Storage would need to be able to accumulate 30A total. This seems to also fit in a 36v storage system, so maybe that would be a better fit (as DC-to-DC would be very very close, so most efficient) Maybe this would actually suck, when the panels are running low (overcast weather). What varies with generation from a panel in low light conditions? Volts are the same but current is low? Volts get low but current is about the same? Volts and current both suffer?

If I go for 48v storage, then 2 strings (each string is 3 panels in series) in parallel to the charge controller. This would require a charge controller with max PV voltage of 65v or higher, and a max current rating of only 20A (or higher). And the storage would have to be able to accumulate at least 20A total.

The 12v storage system would need some stonking great cables to handle the 60A, and there would probably be measurable, noticeable power loss. 24v, 36v and 48v would each realise a reduction of cable guage and would be less subject to power loss.

Does this sound about right?
 
A real MPPT solar charge controller doesn't need the PV input voltage to be close to the battery voltage. My panels provide about 80 volts to the solar charge controller that then charges my 12 volt LiFePO4 battery bank.
 
My charge controller offers efficiency curves that say it's 1 or 2 percent more efficient with slightly lower voltages, say. 80 volts versus 120 volts input, but no big difference to worry about.
 
Yes. you are correct. The converter is more efficient when the input and output voltages are closer. The total efficiency of the system is probably more meaningful to you. If your transmission distance between your panels and charge controller is great then you may be better to put panels in series and run higher voltage to reduce transmission losses. Your charger losses will then be higher, but total system losses may be less.
 
I based the voltage of my system on how the system is going to be used. I have quite a few 12 volt appliances in my RV, so 12 volt makes sense there. If this is a system that has no need for DC other than the inverter, then implementing a 48 volt system is the way to go.

Basing the voltage of the system on the solar panels seems to me to a less effective way to design the system.
 
So Bvillebob and Roswell Bob have made it clear that the MPPT conversion losses are less of a concern than losses in transmission from PV panels to charge controller... and that of course also takes into account the transmission cable gauge. Thanks lads. I'll look into optimising those lines rather than the conversion efficiency.
 
One other point to mull over. If you have excess power then efficiency becomes less of a concern. I have a very large excess of power. I have 4kW of panels, 6 hours of sunlight, and use about 6kWh/day. On the very cloudy days I still have enough juice to keep me happy. Efficiency is not a high priority at this point.
 
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