Over Paneling MPPT Charge Controller pdf question

[Jonboothx]

Morning All

Wonderful forum, ive learnt so much from the various pdf for beginners

Just checking ive understood the pdf about over paneling solar charge controllers

I have 2 x 385w that will be charging a LifePO4 battery

If the rule of thumb is to divide the output from the panels by charging voltage im thinking 14.5v ? = 53.1 amps

Is the general idea that i would achieve better charging by choosing a 50 Amp controller as oppose to a 60amp controller ? ... or do i need to be a bit more extreme and go for a 40 amp to get the benefit ?

Cheers

 

[FyKnight]

Hi Jonboothx,

Overpaneling is just a way to save money by buying a cheaper controller. When it is a sunny day and the panels are producing at max, you forgo a bit of power... the less capable controller leaves that extra power on the table because it hits its max current limit earlier. But you might not care about that loss of power if you have sized your system to provide enough energy even on a cloudy day, as is often the case.

So which controller you go for depends on how much power you need to get out. If you're counting on say just 3 full sun hours per day from your panels, i.e. 2310 Wh, then you could do that over 6 cloudy hours at a constant half nominal, i.e. 385*2/2 = 385 W.

For calculating the current, it's better to divide by the charging voltage when the battery is fully discharged... that's when you really need the power! If you discharge to 2.5 V per cell that would be 10.0 V for the battery. Charging will bring the voltage up a little higher pretty quickly but not to 14.5 V. Better to use 10 V here so in the scenario above, yes you could get away with a 385/10 = 38.5 A controller.

In my opinion, I always want to get the max power out of the panels. Especially if it's patchy cloud, with some periods of high output and some low (e.g. 100% to 25%), I don't want to be losing out on power during the clear periods. Might not be a big deal for others though