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AGM batteries and depth of discharge  RSS feed

 
Matt Ohio
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I have an off-grid solar PV system that runs my home office, refrigerators, and freezers for about 25% of each day.  Here is some brief information to establish context for my question:  I have 20 Werker 100aH AGM batteries in 5 strings of 4 batteries (48 volt system) in a climate controlled basement, supplying power to a Samlex 3000 Watt pure sine wave inverter and 120 V distribution system.  The batteries are recharged by 32 - 100W Renogy 100D monocrystalline panels that feed a couple of combiner panels, disconnects, and a Schneider MPPT controller.  My local Batteries Plus dealer told me that to get the best life out of the batteries, I should discharge the battery system approximately 10% every day (and not recharging during discharging), then once I arrive at 10% discharge revert my home office and appliances back to the grid while the solar panels are recharging the batteries by bulk/absorption/float throughout the afternoon until sundown.  The next morning I manually switch back to battery power for approximately 6 hours to achieve the 10% discharge.  I saw a post by Chris Olson stating that FLA batteries should be deeply discharged once/month, but what about AGM batteries?  I would like to get at least 10 years of life from these batteries; most were purchased in 2014 and 2015.  Are there any AGM battery experts that can debunk the 10% DOD suggestion, and suggest a more optimum cycle?
 
Peter Kalokerinos
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Location: Hunter Valley, NSW, Australia
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You'll kill them in no time is you drop them back to zero. 90+% is excellent. Don't do it

I think our system is set for 20% DOD......its still WIP (panels going on this week actually).

 
Matt Ohio
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I apologize, I think that I mis-stated the depth of discharge.  What I meant to say was that I only use 10% of the full charge each day, then recharge it back to 100% during the remaining daylight hours. 
 
Peter Kalokerinos
Posts: 95
Location: Hunter Valley, NSW, Australia
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chicken hugelkultur solar
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Matt Ohio wrote:I apologize, I think that I mis-stated the depth of discharge.  What I meant to say was that I only use 10% of the full charge each day, then recharge it back to 100% during the remaining daylight hours. 


Yep, that's the way to do it. if you drain the battery(ies) to zero you'll kill them in no time. I'd have it set up to use that power from the batteries at night though? and recharge during the day? do you have peak/off peak for your grid supply?

 
Matt Ohio
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Running overnight and recharging during the day is the next step, Peter.  I'm probably going to add another 8 panels to bring the total to 4 kW and then proceed that way.  Days are getting shorter here in Ohio and during the winter it's very cloudy in January and February, so I'll see how this goes.  I may add a wind turbine on a separate controller if I can find one that is 48 V, because if it's cloudy here, chances are it's also windy.  There is no difference in cost for peak or non-peak with the utility, the cost is about USD $0.10 per kWh.
 
frank li
Posts: 207
Location: Michigan
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4000w or 70a@56.2v charging c/7 is a nice charger especially if only needing to recharge 50ah/avg.-day. Not knowing what the daytime load is though...

Either way, dont charge those batteries at too high a voltage or float too high and they will be comfy.

Always charge while discharging if you can and soak up overproduction with diversion/ automated relay control of appliances or mechanical that can be run to do useful work if you are able or want to at all. Samlex makes a nice little transferswitch, more or less plug and play.

Keep them charged as full as recommended and as often as you can, every day is best. Short cycling the battery will get more lifetime amp hours handled by it and enhance efficiency...if you load it good when the sun is shining.
 
frank li
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Location: Michigan
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Nice charger.
 
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