I’m trying to fully understand the requirements on a solar panel Voc and Vmp when using a Victron Mppt.
I understand that you need to be 5V above the battery voltage to start (Voc), what do you need after that (Vmp)?
I am asking because I will have an 2P8S LiFePo4 bank (29.2V when charged), so will need a Voc greater than 34.2 to start, but after?
I seen some people who say Vmp = Bat voltage + 15 → so 24+15=39V
vBat +5v minimum to start, vBat +1v thereafter.
The higher your PV voltage, the more efficiently the MPPT can operate though… higher differential is better. I have never seen anyone say you need vBat +15v, that’s… oddly specific, while at the same time not based in reality at all… but to the general point, yes, the higher your differential between PV voltage and battery voltage, the better an MPPT will work.
I guess I’m wondering what the rule of thumb would be…
(I’m trying to select a panel that fits in a specific location and there aren’t many that work for the higher voltage when only using one)
Okay.. I think I’ve figure it out while playing with the Victron calculator ( https://www.victronenergy.com/mppt-calculator )
The +15V I think is given because when you take the temperature co-efficents into account you need a good bit higher voltage.
For example one panel I’m looking at:
@-10C Vmpp = 37.44V, but @60C 30.97V. (a 7V drop!)
(and I’m in the tropics so 60C on a panel is not unexpected)
Yes operating temperatures matter. Glad you found that early and taken the time to understand it.
I have had many a (very close to an argument) with people about that. The sticker is not gospel - real world conditions apply. And often there is disappointment.