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?
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?