Reasonable array size for 600aH battery bank

Re: Reasonable array size for 600aH battery bank

CoryT wrote: »

I've got 3 banks of UB-GC2 6V 100 aH batteries, wired in series for 48 V and tied to a copper buss bar. 12 SW260 panels are wired as 4 sets of three and are connected to an Outback 8048 Radian with an FM80 charge controller. Each string should produce 780w at 94vdc, for a total of 3210 watts. With a nominal load of .6 to 1.2 kW, does this seem like enough panels to get a proper charge? I'm in Northern AZ, getting some 5.5 hours of sun. I can only add one more string before I need another charge controller, and I wonder if that is worth the install effort.

Let's try some analysis on the description.

You have one battery bank. It consists of three strings in parallel. These strings are 100 Amp hour batteries. That adds up to 300 Amp hours. The nominal Voltage is 48. (I don't know where the 600 Amp hours comes from. That would be six parallel strings of 100 Amp hour batteries.)

If that is correct then the typical array size would be (30 Amps * 48 Volts / 0.77 efficiency) 1870 Watts.

You have twelve 260 Watt panels totaling 3120 Watts. On an FM80 and 48 Volt system that should provide a peak current of (3120 * 0.77 / 48 ) 50 Amps. On a 48 Volt system an 80 Amp controller can take about 5kW of PV. You could add two more strings of three 260 Watt panels in series if need be.

We tend not to worry about load effect on charging if the battery is sized for 24 hour use and the loads are not significant, unless the idea is minimal battery bank with extra PV to offset particular daytime loads such as air conditioning.

But we need to get the actual battery bank configuration ironed out here.

Re: Reasonable array size for 600aH battery bank

That makes more sense.

50 Amps on a 600 Amp hour battery bank is 8% peak charge rate, which ought to do in Arizona with 5 hours of good sun.

I don't know how much derating Outback is using in their sizing tool, but it seems to be too little (especially in hot climate). Fifteen 255 Watt panels would be 3825 Watts * 0.77 / 48 Volts = 61 Amps. If that is five strings of three then the next step up is six strings of three or eighteen panels total which gives 4590 Watts: 73 Amps on a 48 Volt system. Still viable. Keep in mind that current above maximum the controller can pass is just 'clipped' (lost) so over-paneling a bit won't hurt anything.

But you have a 600 Amp hour battery bank and 260 Watt panels. If you add one more string of three that's fifteen total or 3900 Watts which should yield about 62 Amps. You couldn't get closer than that.

Yield comparison: 3900 Watts over 5 hours and 52% over-all efficiency is 10kW hours AC (existing 3120 Watt array yields 8 kW hours), 600 Amp hour 48 Volt battery bank discharged to 25% is 7.2 kW hours. Always good to have more available from the panels than the battery.

Looks like your existing array should work fine.