# Max panel wattage per battery

Registered Users Posts: 4
edited June 2017 #1
The batteries I have are sufficient for overnight use. I'd like to increase the number of panels but I don't want to add batteries.

I'm using 8 Crown Batteries rated at 260AH each. The guy at the battery store said the max charge rate is approx 1/10 of the 20 hr AH rating.

I'm estimating the max wattage per battery to be 26a x 13.9v=361watts per battery. 8 batteries x 361 watts = 2888 watts max (actual)
My panels operate at a max of about 80% of the rated power so 2888 / .8 = 3610 watts max (panel rating)

Is there a way to prevent the batteries from being charged to quickly if I add more than 3600 watts in panels?

You have to look up the battery specifications for the exact model/set you got... However, 13% rate of charge should be OK:
• 8x 260 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 5,092 Watt array "cost effective" maximum
If you have a higher rate of charge, your charge controller should have a Remote Battery Temperature Sensor which will reduce charging voltage as the battery bank gets warm/hot.

The Midnite Classic family of MPPT charge controllers does have the "Wizbang Jr." battery shunt, that when connected to the charge controller can limit charging current directly to the battery bank (i.e., you can have other daytime loads consuming power, but be part of the battery charging current).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 4
Thanks for the feedback. Very helpful. I wasn't entirely certain on how to perform the calculation.

I'm using an Outback FM-80 with battery temp sensor.
You should be fine--Assuming those are 12 volt @ 260 AH batteries, the math should be OK...

If you wish to limit charging current, many MPPT type charge controllers (including Outback) can be programmed for less than full output current.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 4
edited July 2017 #5
BB. said:
You should be fine--Assuming those are 12 volt @ 260 AH batteries, the math should be OK...

If you wish to limit charging current, many MPPT type charge controllers (including Outback) can be programmed for less than full output current.

-Bill
You're right. My math is bad. 6 volt batteries. I added the amperage the pair in a series when I shouldn't have. So it's really 1/2 of that.
Thanks.