AGM Battery Exercise

stmar
stmar Solar Expert Posts: 373 ✭✭✭
in Grid Tie and Grid Interactive Systems #1
What should you do with a new AGM battery bank? I have had mine in place for a few months now, we have had a few grid outages from a few minutes to a few hours. System is functioning properly as far as I can tell. During a sunny day the bank gets up to the 28 volt range and at night it is in Float at 26.8. Is the fluctuation in charging enough or should I be doing additional exercises?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: AGM Battery Exercise

    it needs to go to float during the daytime as there's no sun at night. should i ask?
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  • stmar
    stmar Solar Expert Posts: 373 ✭✭✭
    #3
    Re: AGM Battery Exercise
    niel wrote: »
    it needs to go to float during the daytime as there's no sun at night. should i ask?

    I guess I don't understand. It goes to Float at night because it is on grid and the grid charges it as needed to keep it to 26.8 volts. It is functioning properly as far as I can tell. My question is should I be doing anything different or is the solar array charging it during the day to a higher voltage and then having it drop to Float at night good enough exercise for the new battery bank to get it to it's full potential?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,664 admin
    #4
    Re: AGM Battery Exercise

    Exercise the battery bank with a 25% or so discharge once per month... If it is a critical application, run it once a month (or at least several times a year) as much as you expect it to supply in a true power outage. If it cannot support your needs, it needs replacing.

    Floating a battery for several years (unless it was designed for float service)--It is (probably) not going to last as long, and if/when you need it, you might find it only has 1/4 or less of its capacity.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • stmar
    stmar Solar Expert Posts: 373 ✭✭✭
    #5
    Re: AGM Battery Exercise

    Thanks, that is what I was looking for. I think with our grid outages it discharged and recovered a few cycles since I installed them. How do you know when you get to 25% discharge? Is there a formula that you use or a value displayed on the inverter's meter? These are Lifeline AGMs and my Xantrex has readouts for most critical elements.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,664 admin
    #6
    Re: AGM Battery Exercise

    Assuming 4x6 volt @ 400 Amp*Hour capacity for a 24 volt @ 400 AH battery bank...

    If you can measure current, then you want to discharge ~100 Amp*Hours for 25% discharge...

    That can be:
    • 100 AH / 5 hours = 20 amp rate
    • 100 AH / 10 hours = 10 amp rate
    Or for AC loads:
    • 100 AH * 24 volts = ~2,400 Watt*Hours capacity
    • 2,400 WH * 0.85 inverter eff = 2,040 WH for 1/4 or 25% battery discharge of 120 VAC loading
    • 2,040 WH / 5 hours = 425 Watt AC load for ~5 hours
    • 2,040 WH / 10 hours = 204 Watt AC load for ~10 hours
    Some examples....

    The above are very gentle discharge rates. AGMs can take higher discharge rates--But I would avoid going much more than C/5 just to keep things from getting crazy. They should be able to discharge (if I recall correctly) as fast as C*4 rate (discharge bank 100% to 0% in 15 minutes)--But that would be very stressful and could severely damage the battery bank.

    In the end, if you have a common set of loads you want to support--Measure those and then plug them back into one of the equations above.

    I.e., if you need 500 Watt average load:
    • 2,040 WH (25% of battery capacity to discharge) / 500 watt AC load = 4.08 hour of run time minimum
    The backup system is only good for you--If it supports your needed loads (plus a bit for aging/emergency use).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #7
    Re: AGM Battery Exercise

    I would check with your battery manufacturer. For my Odyssey AGMs- they are designed for a 15 yr float life. That said - me thinks there is no harm in occasional exercise!
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  • stmar
    stmar Solar Expert Posts: 373 ✭✭✭
    #8
    Re: AGM Battery Exercise

    So if I get my load to about 4 amps AC, that would be 480 watts give or take. On the DC side I have a 24 volt frige so will have to take that into consideration. Of course the 4 amps AC load will not be constant. I think I can do that with no problem so thanks again for the input.
    I do think the Lifelines (hence their name) are designed for float so that they will be ready for emergencies, that being said I agree with you that exercise is good and should make them even better.
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