Handy solar info for lead acid batteries

Salvo wrote:
Your 24V panels are not "true" 24V panels. A true 24V panel has 2 * 36 cells. You have less. That means the cliff where current drops is more to the left, at a lower voltage. Add the effect of temperature and your 24V pwm controller will have problems at higher temperature.

BFL13 wrote:

Since PWM uses Isc, which does not go down with temperature, you don't care about panel temp with PWM, but it hurts MPPT big time.

The conversation is more about wire gauge but as noted you also get voltage loss from panel heating. There is another issue with the "overhead" reduction using 60 cell 24v panels instead of 72 cell panels so any voltage loss is more critical with 60 cell panels.

I am on limited data so somebody could add my thread on this here as a link. "Hot Solar Panel MPPT vs PWM Results" from last June.

A mystery I never solved was the Isc with 24v as noted in that OP, where Isc was less than rated using 24-24, but as rated using 24-12.

Panel Isc is rated as 9.0a and I was getting just above that, showing that insolation was good at the time of the test.

Disconnected panel Isc was 9.16 to 9.22 and the MPPT controller's "demo" showed that 9.16 as panel amps when doing 24-12.

However, doing 24-24 the demo amps were only 8.02. Meanwhile the PWM controller doing 24-24 also showed only 8.1 amps.

This was a disappointment since I expected the PWM to get Isc as it does with 12-12, so I was expecting to get 9 amps in 24-24( which would be like 18 amps in 12-12) and that would beat the MPPT doing 16 amps. Nope. Both got "16" in 24-24

I thought it would be due to the 60 cell panel vs 72 cell, but the MPPT controller did show Isc at the full 9 amps when in 24-12 mode.

Both controllers showed 8 amps Isc as an upper limit in 24-24. I don't understand how the lower "overhead" of the 60 cell panel would manifest as a lower Isc.

I was able to get the 24v battery bank to 30 volts before it ran out of steam due to the overhead, so it is not true that a 60 cell panel can't fully charge a 24v battery. No way to get over that 30 and equalize though. (it seems 60 cell is the usual way 24v panels come these days. (If you want 72, and get two 12s in series, beware of the total Voc wrt your controller's limit on that)