question

robby avatar image
robby asked

Calculate battery charging in a 12V solar system

Hi,

I have a 400Wp solar panel.
Impp: 11.97A
Vmpp: 34.4V

And 2 107Ah 12V AGM batteries.

Where i live in Belgium we have a means of 5 hours of full sun @ 1000w/m² per day and 0.5 in the winter.

So in the summer my solar panel will yield 2000W and in the winter 200W.

Now to charge the battery i need to choose a charge controller.

The batteries i can charge with max 1/3 of the Ah so that's 2x107Ah = 214Ah/3 = 71.3A

So i need a really big charger.

But i have only 400Wp solar panel, is it correct that i then have maximum 27A of charge current? 400Wp/14.4V = 27A?

So for me i would choose the MPPT100/20

But then i only get 20A of loading, so i will not use the full 400Wp but only 288W? P=U*I = 14.4V*20A=288W? Is this thinking correct?

So with the AGM batteries, i can discharge them 50%. So that's 214Ah/2 = 107Ah that i can use.

107Ah*14.4V=1540W


So with 5 hours of full sun with 288W that's 1440W solar yield in the summer.

So maybe the battery will not be fully charged on a sunny summer day?



Can someone help me figure out my thinking? :)

Kind regards







MPPT Controllersbattery charging
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

3 Answers
kevgermany avatar image
kevgermany answered ·

You'll never get an accurate answer. Too many variables, including the batteries.

There are online tables giving average yields. Be aware that the chargers work from dawn to dusk, you're only considering periods of full sun. And you're assuming full power, most setups don't deliver this. In Belgium the sun will never be directly overhead and you have a lot of summer days with a lot of cloud. Winter max power may be 10% of theoretical, if you're lucky.

Suggest you use the Victron MPPT calculator, there's a link in the MPPT product page linked here.

2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

niklas-schauberg avatar image
niklas-schauberg answered ·

yes your thinking is correct. but you mix up W and Wh!

you should also consider losses of 30-35% within the battery due to lead chemistry is quite inefficient.

plus some minor losses in the cables and the MPPT.

plus of cause the losses in your inverter - if you use one on discharge side.


2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

klim8skeptic avatar image
klim8skeptic answered ·

@Robby Good advice so far.

The most important thing to consider is, how much energy is needed daily to live on, and keep your batteries happy.

Know your energy needs, and factor in the losses.

2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.