I don't understand sizing panels to batteries.

Lefty Wright

I am upgrading my off grid system. But I'm unsure how to match my array with a battery bank.

I've read of a 5% and a 10% method. What do these numbers mean? 5 0r 10% of what? Do you use PV watts or amps when figuring battery bank size?

icarus

Re: I don't understand sizing panels to batteries.

With flooded lead acid batteries, the charge current should be ~5-13% of battery amp/hour capacity.

So a 100 ah battery would like to see between 5 and 13 amps. Remember, Pv won't generally put out 100% much of the time, a derating of ~20% is in the ball park. Also, in an off grid situation, charge current needs to be over and above any standing loads. So if you consistently draw say 5 amps on the said 100 ah battery, you might should for ~10-18% for example. I know 'Coot likes to suggest 10% as a starting point.

T

Cariboocoot

Re: I don't understand sizing panels to batteries.

There's a couple of things to consider in getting the ratio right.

First is having an array large enough to produce the kind of peak charge current that will remix the electrolyte and keep the batteries going for years. That's where the 5%-13% rule comes in: it's the charging rate expressed as a percentage of the battery banks' total Amp hour capacity (at the 20 hour rate). You'll also see it expressed as C/10 for example. In other words, a 200 Amp hour battery wants 10-26 Amps of charge current. A pretty broad spectrum, but some batteries need more and some batteries can take more than the 5% minimum. And that is net charging: the current going in to the batteries after taking away whatever is being used to supply loads. For instance if the charge controller is putting out 10 Amps and you're using 2 Amps to run inverter & loads the net charge current is 8 Amps - enough for 160 Amp hours of battery at most.

The other issue is being able to harvest enough Watt hours during the day's charge window to replace what's been used. Usually if you get the peak current right, this number will take care of itself providing your depth of discharge is not too severe. An example using rough calculations:

200 Amp hours of battery @ 12 Volts. Use C/10 as a target: 20 Amps net charging. 20 Amps at charging Voltage of 14.2 = 284 Watts. Divide by typical 77% derating factor (varies with location and atmospheric conditions) = 368 Watts "nameplate rating".

Again, derate the panels and calculate for "harvest":
284 * (typically) 4 "equivalent good sun" hours = 1136 Watt hours per day. Divide by 12 Volts = 94.6 Amp hours. That's slightly less than the 100 Amp hours of battery available at 50% DOD, but it's pretty close. You'd probably go "up in size" on the array to get something that would match available panels (two 200 Watt units for instance). So as long as you keep the margins for error in the right direction (expected usage rounded up, expected harvest rounded down, expected battery capacity rounded down) you're all right.

Lefty Wright

Re: I don't understand sizing panels to batteries.

Thanks, guys. That clears it up.