How far to run down AGM battery?

Row1
Row1 Registered Users Posts: 10 ✭
in Solar Beginners Corner #1
hi - I am about to get a 35AH AGM battery from Northern Tool. I know rechargeable batteries vary in how they like to be run down and recharged.

What about this AGM?..
"UPG Sealed Lead-Acid Battery — AGM-type, 12V, 35 Amps, Model# UB12350."

I have just received the "Sunforce 50232 30 Watt Folding Amorphous Solar Panel Kit."

My goal is to figure out how to use and recharge this battery, so that I know what I can do with it if a hurricane puts out our power for an indefinite length of time. Run radio, run a box fan, recharge cell phones, recharge digital camera, run laptop for a little while, that kind of stuff.

I plan to post more annoying, newbie questions!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #2
    Re: How far to run down AGM battery?

    For the most part, you can run the battery down to 80% discharge (20% state of charge)... If you run it below that level, you will run the risk of running one or more of the cells flat or even "reverse charge" a weak cell and cause it to fail.

    If you are looking to experiment, RC guys use a DC Amp*Hour / Watt*Hour meter like one of these (I have not used the meter or the company myself).

    AGM and other sealed batteries do not let you measure Specific Gravity (the accurate way to measure state of charge) and measuring battery voltage while charging or discharging is not very accurate. But, if that is all you have--it is what you have.

    A 35 amp*hour battery does not store that much energy:
    • 35 amp*hours * 12 volts * 0,80 = 336 Watt*Hours
    If you had a load to run for 4 hours:
    • 336 WH / 10 hours = 33.6 watt average load
    • 33.6 watts / 12 volts = 2.8 amp average load
    For small loads, may be useful--for larger loads (like an AC box fan)--you may get an hour or so of blowing air.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #3
    Re: How far to run down AGM battery?

    UPS batteries are designed for a few cycles a year, likely less than 100 cycles at full life. You would be OK for a couple of outages.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • Row1
    Row1 Registered Users Posts: 10 ✭
    #4
    Re: How far to run down AGM battery?

    thanks, guys.

    BB - i will look at that site. Figuring out how to measure stuff will be good. All I have now is a kill-a-watt dealio, so I can at least estimate power required to recharge a set of AAs or run a device for an hour.
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  • Renewable Ray
    Renewable Ray Registered Users Posts: 4
    #5
    Re: How far to run down AGM battery?

    I'll second that 80% number, but not on a daily basis.
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