[RE-wrenches] Small battery bank vs too large array

[Larry Crutcher, Starlight Solar Power Systems]

Drake, 

The ability to charge at much higher amperage is typical of all AGM batteries. The recombinant technology does not produce much heat before the internal resistance rises and the current naturally drops. AGM's require up to 20% less energy to recharge as very little is wasted as heat. 

For me, the important aspect of AGM's is that with higher initial current (over sizing the PV system or the ability to use a large charger) and higher efficiency, you reach the absorb set point earlier in the day. For off grid systems, this means you will have more sun hours left to achieve 100% SoC. For these reasons I generally recommend AGM's. 

Larry Crutcher
Starlight Solar Power Systems
(928) 342-9103



On Aug 2, 2012, at 2:06 PM, Drake wrote:


Hello Wrenches,

Thanks for your help with this question.

I've had a breakthrough on the design of this system.  I talked with Tom at Midnight Solar, and he explained that Concord batteries can be charged at C X 5 if necessary, i.e., 5 X AH capacity of the bank.  The batteries like a fast charge.  They are a bit spendy, but will simplify the overall system design. 

From the Manual: 
"The charging current during the Bulk stage should be set as high as practical; higher current mean faster recharge time.  For repetitive deep cycling, chargers should have an output current of at least 0.2C (20 Amps for a 100 Ah battery).  If the output current is less than this value, the cycle life of the battery may be negatively affected.  Due to low impedance design, Sun Xtender batteries can tolerate in-rush current levels as high as 5C (500 A for a 100 Ah battery). " 

It is imperative to keep the system voltage at the required set point.  The CC regulation will prevent the batteries from being harmed, as the voltage will rise quickly.

Drake 

Drake Chamberlin
Athens Electric LLC
OH License 44810
CO License 3773
NABCEP Certified Solar PV Installer
740-448-7328
http://athens-electric.com/ 
 
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