5 y/o AGM batteries

mnsunniemnsunnie Registered Users Posts: 2

I have  a simple setup of 2x 12V AGM [email protected] batteries in parallel for emergency power.  I have only discharged them 2 to 3 times in 5 years (lowest drain was 12.2V).  They sit always fully charge at 13V, I recently tried to use them for a power outage and they went from 13V to 12.4V in 20 minutes just running a radio and a few light bulbs 100watts. 

By not using them and keeping them always charged on float kill them?  How can I prevent this from happening when I buy my next set?  These were $179 a piece, costly mistake.





  • MrM1MrM1 Registered Users Posts: 476 ✭✭✭
    edited May 2018 #2
    Buy 2 6v 225Ah wet lead acid batteries at a warehouse store for about $80-130 dollars (I have a local source that sells Trojan T105s for $115 out the door) and run them in series.   This will eliminate any charge resistance issues and will allow you to keep an eye on SG (specific gravity) and fluid levels to confirm the batteries are indeed 100% charged.
    REC TwinPeak 2 285W 3S-3P 2.6kW-STC / 1.9kW-NMOT Array / MN Solar Classic 150 / 2017 Conext SW 4024 Inverter latest firmware / OB PSX-240 Autotransfomer for load balancing / Trojan L16H-AC 435Ah bank 4S connected to Inverter with 7' of 4/0 cable / 24 volt system / Grid-Assist or Backup Solar Generator System Powering 3200Whs Daily / System went Online Oct 2017 / System, Pics and Discussion
  • mnsunniemnsunnie Registered Users Posts: 2
    Ok, it is in my basement office, do I need ventilation? Also, will not using them kill them just the same?
  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    5 years isn't a terrible lifespan for a typical AGM battery. All batteries eventually die of old age, even with little or no use.

    Also, going from 13v to 12.4 quickly isn't as bad as going from 12.7 to 12.1 in the same time. Resting voltage fully charged will be ~12.7ish, and without the float current, the battery would fall to resting voltage even with zero load.

    AGMs generally have relatively low rates of self-discharge (which is useful in a standby application like this), so keeping them on constant float shouldn't be necessary or even wise. A normal charge cycle every month or two of inactivity would be better IMHO. Keeping them in a cool place can help too. Room temp and above accelerates reactions.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • softdownsoftdown Solar Expert Posts: 3,228 ✭✭✭✭
    edited May 2018 #5
    mnsunnie said:
    Ok, it is in my basement office, do I need ventilation? Also, will not using them kill them just the same?
    I don't believe that AGM's need ventilation. Basements are usually pretty cool, that helps a lot. 

    Yes....batteries go bad in storage. I suspect that 10 years may be the most that one can realistically hope for. 

    Your lights and radio test is not very definitive. The batteries may bounce back up to 12.6 within minutes of having the drain turned off.

    Anything over 12.8 volts is a meaningless surface charge with almost no meaningful energy. 

    Yes....your batteries are weaker after five years. They may not be as weak as you fear however. 

    Not a good idea to keep batteries float charged 24/7 with no load whatsoever. The float charger is highly likely to be set up for flooded lead acid which takes a higher voltage charge than AGMs.

    Just a few thoughts. Plus room temperature will also affect battery voltage.

    Batteries are a bit like food. Properly stored, 2-3 years is probably acceptable. Best to use/eat by five years for best results. I drank too much wine today...
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
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