More than three AGM batteries in parallel?

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_OS_
_OS_ Solar Expert Posts: 207 ✭✭✭
in Solar Beginners Corner #1
Hi,

I am considering adding one or two batteries to my bank of two Concorde Sun Xtender AGM batteries. I have e-mailed Concorde and they say it is safe to connect a new battery in parallel with my one-year old batteries. Here is their response:

"Yes, a new battery can be added in parallel with two (or more) older batteries. However, new batteries should never be connected in a series circuit with older batteries."

I did not ask them about how many I could add but my dealer say that I should not connect more than a total of three in parallel. I have also read this on the net. For example see the section "Avoid parallel strings" on this page: http://www.thesolar.biz/Keeping_batteries_alive.htm

Can anybody confirm this or give some input in this matter?

Thanks!

Ole
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
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    Re: More than three AGM batteries in parallel?

    I'll say:
    That after a battery is more than a month old/in use, it has degraded a little bit.
    More time = more degradation.

    Adding new batteries in parallel or serial, will soon degrade all batteries to the level of the WEAKEST battery in the system.

    New batteries will not "cure" a weak battery.

    If you are confident your existing battery is good, go ahead.

    Personally, I would move the old battery to a different system, and use it there, and use a new batch of batteries where it was.

    Upgrading your battery capacity, are you also upgrading your charger ?
    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

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #3
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    Re: More than three AGM batteries in parallel?

    If the other batteries are in good shape, adding a new battery is not the worst thing in the world--especially if you need the extra capacity.

    Basically, it is not recommended if you can help it (i.e., start small and grow over the next few years).

    I cannot figure out why the vendor would be against series addition of the battery--for mixing old and new--that would seem to be the better option.

    Of course, you have to make sure you do not drain your battery bank down towards 0% state of charge (100% discharge)... If the batteries are not matched (usually the same lot, installed together at the same time, and experiencing the same load/charging cycles)--it is possible for a higher capacity battery to reverse charge a lower capacity battery--which will pretty much kill any battery instantly...

    But, since you are supposed to stay about 50% state of charge for flooded cell batteries, and above 20% for AGM (really recommend >50% for AGM too for longer cycle life)--it should not be a problem.

    You can double check by measuring each battery's voltage in the series bank and make sure they are all properly balanced...

    Hmmm that may be the problem--you are not supposed to equalize AGMs (or only slightly equalize) because if they boil, they loose their water. If you put a new battery of unknown charge state into an existing series string--you cannot properly equalize the under charged battery without damaging overcharge of the others in the string...

    It would seem you would need to fully charge all of the AGM batteries (hopefully to the same state of charge) before adding new ones to a series string.

    With a parallel string, adding new batteries with different state of charge levels is not a problem--they will all end up equalizing in short order.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
    #4
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    Re: More than three AGM batteries in parallel?

    Hi,

    Thanks for the replies.

    Yes I am considering upgrading both my battery bank and solar panels.

    I am confident that my batteries are in good shape. I bougth them brand new last September and have discharged them three times down to 60% and maybe four times down to 80%. The rest of the time they have been fully charged.

    Regards,
    Ole
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