I just went through the same thing, and installed a 2205 Watt system
on 4 Concord 224 Ah (24 hr rate) batteries. I talked with the
Concord tech support, and was told that this was fine. Also, look in
the SunXtender manual. It says a very fast charge is good and allows
for what would be brutal charge rates on a flooded battery.
At 12:43 PM 9/1/2012, you wrote:
Rich,
In your example you have 8kW feeding a 20kWh battery (assuming 400AH
bat). This is about 0.4C charge rate. If these were flooded
batteries, I might consider limiting bulk current to 0.2C because in
standby use they do not need to be fully recharged in one day. Slow
battery charging promotes longer life.
Since you mention that these are AGM I would not worry about the
excess current, especially in a stand by configuration. Some AGM
manufacturers publish that you can provide charge current up to 5C
without harm. For your bank that would be 2000 amps at 57Vdc! I
believe all good quality AGM's can withstand high bulk charge
currents because of the efficient way hydrogen and oxygen are
recombined. Controlling voltage is the more important factor. So I
don't think you are missing anything.
Larry Crutcher
Starlight Solar Power Systems
On Sep 1, 2012, at 6:35 AM, Rich Nicol wrote:
Wrenches,
I'm have a mental block with battery backup systems such as the
Outback Radian. Perhaps someone can help me get over it!
Essentially these systems sell back power off the top of the battery
bank and the array is maintaining the state of charge of the battery
bank through traditional charge controllers. If you have a modest
battery bank sized for limited use on critical circuits and a good
size array for the benefit of net metering I would think it would be
necessary to limit the current the charge controllers are capable of
delivering to the battery bank to prevent damage to the batteries
when the controllers are Bulking or Absorbing, but then it would
seem that we limiting the utility of the larger array. If the
current from the controllers was not limited they will self-limit as
voltage rises, plus I suppose they will mostly be in float mode
except when the batteries are cycled but again it seems that we are
limiting the capability of the array compared to a straight grid
tied system with a traditional grid tied only inverter.
An example would be a 8000 watt array using a couple of flexmax80
controllers with a string of 8 - L16 AGMs feeding a Radian.
What am I missing here?
Thanks
Rich
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