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 _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
_______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
