# Solar Panel/ Battery Charging

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Solar Expert Posts: 72 ✭✭✭✭
Hi everyone. I have a couple questions. Lets say we have a 12V 100Ah battery bank, giving us 1200Wh available. Also there is a 200W solar panel charging this bank. Does this mean that with the battery 0% SOC or 100% dod, it would take 6 hours to charge this bank, assuming the panel is putting out 200W (ideally)? Meaning 6hrs x 200W panel = 1200Whrs? Or is this not true due to float and trickle charging of batteries, is it not linear? Thanks

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Re: Solar Panel/ Battery Charging

In theory, the math is correct... In practice it will take much longer...

Normally, for a flooded cell battery, the maximum charge rate is around 13% or ~156 watts for your battery example. And, it will accept the maximum charge rate up to ~80-90% state of charge... Above that point, the charger will hold (for example) 14.4 volts and the current will taper down... That will take another 2-6 hours or so to complete to 100% charge. At that point, the control should cut back to "float voltage" at ~13.6 volts to maintain the battery 100% state of charge.

Also, you do not want to discharge the battery below ~20% state of charge--Because the actual capacity of each cell is not exactly the same, one or more cells will reach 0 (zero) state of charge first before the others. That cell will begin to "reverse charge" and be effectively destroyed at that point.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Solar Panel/ Battery Charging

First off, you don't want to go more than 50% DOD. That means a 100 Amp/hr battery has 50 Amp/hrs available.
Second, replacing the Amp/hrs used is the easy part. A panel's output is more like this:
200 Watts @ 80% derating = 160 Watts * 4 hours of 'good sun equivalent' means 660 Watt hours potential harvest. On a 12 Volt system this is roughly 55 Amp/hrs so it could replenish a 100 Amp/hr battery at 50% DOD providing you are not also drawing loads while recharging. That's the part most people forget about.
Third, there is the peak charge current issue. It's necessary to have current of at least 5% of the battery's Amp/hr rating in order for proper charging to be achieved. 10% is better, and 13% is typically the maximum. You will sometimes see this as "C/#" where C = the battery's Amp/hr rating. In this case, 100/10 = 10 Amps. The 200 Watt panel would provide this: 160 W / 14.2 charging Volts = 11 Amps. Just over the 10% target so there's room for efficiency losses, providing they aren't too much.

This is a good place to start understanding batteries:
http://www.windsun.com/Batteries/Battery_FAQ.htm
• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: Solar Panel/ Battery Charging

basically as was pointed out the math is essentially correct, but even with a battery that can take 100% c rating that as stated charge efficiency comes into play. the final stages of charge do take longer even if you could feed the battery at 100% efficiency in bulk stage as the voltage cutoff in the absorb stage causes a foldback of current and therefor a longer time to top off the battery.
bulk charge efficiencies are typically around 80% give or take and agm batteries around 90%.
for the record concorde's sunxtender can be fed at 100% of its 20hr rating. using this battery type in your example being charged at 100a for a 100ah battery it would have about 90ah in it from 0% soc to start after 1 hour. it would finish off the final 10% in another couple of hours at a reduced rate. again it is not a good idea to go below 50% soc.