Quick battery question

basewindow
basewindow Solar Expert Posts: 63 ✭✭
in Off Grid Solar & Battery Systems #1
Just a quck query, what are your thoughts in putting a 12v 230ah AGM battery in parallel with a 12v 260ah AGM battery? Assuming the same or very similar charge charateristics etc, for each battery.
Off Grid shack - Victoria Australia. 480W array, 500Ah AGM at 12V. 30A PWM Manison CC. Trimetric 2030. 300W Pure Sine Inverter. 120lt Dometic Gas Fridge. Composting Toilet. 5000lt water tank with 12v 35psi pump. Bosch Hydropower 16 for nice hot water. 4kw Fuji Micro Generator (dead after 7 years) 5kva Subaru Generator.
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  • basewindow
    basewindow Solar Expert Posts: 63 ✭✭
    #2
    Re: Quick battery question

    I assume the larger will become only 230ah? I guess what i want to know is if it is sensible to do and will it damage or shorten the life of either or both batteries?
    Off Grid shack - Victoria Australia. 480W array, 500Ah AGM at 12V. 30A PWM Manison CC. Trimetric 2030. 300W Pure Sine Inverter. 120lt Dometic Gas Fridge. Composting Toilet. 5000lt water tank with 12v 35psi pump. Bosch Hydropower 16 for nice hot water. 4kw Fuji Micro Generator (dead after 7 years) 5kva Subaru Generator.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Quick battery question

    it will downgrade the stronger of the 2 batteries to that of the lesser in capacity. never a good idea to do.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #4
    Re: Quick battery question

    I see it differently. If they were in series, then yes, the lower AH battery would be overcharged while the larger AH battery would be undercharged, leading to problems.
    But if the two batteries, regardless of AH ratings have otherwise equal charging characteristics, then adding a smaller, or larger battery to the existing one wouldn't be all that different than adding more plates to each cell of the first battery. Only thing is those extra plates would be in a separate container. If however they were different types of batteries, or if they had different charging characteristics, then all that would be out the window and there would most certainly be problems, similar to what happens when a new battery is added to an old well used battery. Even if all other things were equal, an old used battery will have different characteristics than the new one, thus problems will result.
    But hey, I often see things differently than others do. Like my Grandfather used to say: "Two heads are better than one - - even if one is a sheep's head".
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #5
    Re: Quick battery question

    I would agree with Wayne. When batteries are in parallel voltage must be the same, but AH will add up 230+260=490AH. The 230AH battery should take 230/490 = 47% of current, and 260AH battery should take 260/490 = 53% of current.

    However, if they're by different manufacturers, or of different age, or otherwise different, they may not share current according to the equation, and one of them will be constantly overcharged and the other one undercharged. Since they're AGM, overcharge may damage the battery.

    I would monitor current to see if it is significantly different from theoretical 47/53 under different conditions (bulk/absorb/discharging).

    Although, frankly I would never do something like that, even though there's no logical explanation.
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #6
    Re: Quick battery question
    basewindow wrote: »
    Just a quck query, what are your thoughts in putting a 12v 230ah AGM battery in parallel with a 12v 260ah AGM battery? Assuming the same or very similar charge charateristics etc, for each battery.

    Quick answer - it will work. The smaller battery will, in general, have a higher ESR and thus you won't see as much current (proportionally) coming from it. As the larger battery discharges the smaller battery will shoulder more of the load. Thus they won't discharge/charge at quite the same rate, but for most purposes that's not significant.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #7
    Re: Quick battery question

    I see many banks of GC-2's mixed with 210-220 amp hr and 235 amp hr Batteries. They don't seem to have any more issues than the others. It's hard to say if they charge or equalize any different because the SG's always seem pretty equal. Seems like anytime you have paralleled batteries the first battery / batteries on the positive side always takes the biggest beating, it'll always use more water and show positive post corrosion first. I always shuffle the bank's battery position's when I service the cables once a year.
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  • basewindow
    basewindow Solar Expert Posts: 63 ✭✭
    #8
    Re: Quick battery question

    Thanks for the replies. Reason I asked I have access to a couple of very cheap batteries. The catch was they were different ah (as above) and different manufacturers, but almost identical charging paramiters. Wantet to know if it was worth replacing my currently working 2 100ah, to get a bit more battery storage.
    Off Grid shack - Victoria Australia. 480W array, 500Ah AGM at 12V. 30A PWM Manison CC. Trimetric 2030. 300W Pure Sine Inverter. 120lt Dometic Gas Fridge. Composting Toilet. 5000lt water tank with 12v 35psi pump. Bosch Hydropower 16 for nice hot water. 4kw Fuji Micro Generator (dead after 7 years) 5kva Subaru Generator.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Quick battery question

    Here's the thing: what's the charge rate going to be on them each?

    You've got 230 Amp hours and 260 Amp hours combines for 490 Amp hours. Charge at 10% of that is 49 Amps. That would "average" to 24.5 Amps each. On the 260 that's 9.4%. On the 230 that's 10.6%. Both well below the maximum either will take and above the minimum they need. So it may not be "ideal" but it's not going to burn up either.
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  • basewindow
    basewindow Solar Expert Posts: 63 ✭✭
    #10
    Re: Quick battery question

    Well that's another thing the charge rate. I've only got 360w or array, putting out about 20Amps on a good day, so not quite in the 5-10% range. Might just let sleeping dogs lie and keep my existing batteries until they actually need to be replaced. I'm pretty conservative with my energy use so my current 200Ah is okay for now. Just didn't want to pass up a bargain. Sounding more like my wife everyday....
    Off Grid shack - Victoria Australia. 480W array, 500Ah AGM at 12V. 30A PWM Manison CC. Trimetric 2030. 300W Pure Sine Inverter. 120lt Dometic Gas Fridge. Composting Toilet. 5000lt water tank with 12v 35psi pump. Bosch Hydropower 16 for nice hot water. 4kw Fuji Micro Generator (dead after 7 years) 5kva Subaru Generator.
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  • 2manytoyz
    2manytoyz Solar Expert Posts: 373 ✭✭✭
    #11
    Re: Quick battery question
    Blackcherry04 wrote: »
    I see many banks of GC-2's mixed with 210-220 amp hr and 235 amp hr Batteries. They don't seem to have any more issues than the others. It's hard to say if they charge or equalize any different because the SG's always seem pretty equal. Seems like anytime you have paralleled batteries the first battery / batteries on the positive side always takes the biggest beating, it'll always use more water and show positive post corrosion first. I always shuffle the bank's battery position's when I service the cables once a year.

    And that was true for my last battery bank. A mix of GC2s. Different ages, slightly different AH ratings, different manufacturers. They all lasted 5-6 years before the last two were pulled out of service last week. My next batch of eight GC2s will be all the same age, size, manufacturer. Not that it seemed to matter much. Other sizes might be more sensitive, but GC batts were built for abuse.

    As for putting them in parallel and having issues with the ones on the end doing all the work, I avoided that issue by connecting them to a bus bar.

    hangerbolts.jpg

    Mine setup is 12V. Two GC batts in series to make 12V, then tied to the bus bar. 4 parallel strings like this all connected together.

    To verify they were equally charging and discharging, I often connected my Extech DC Ammeter to the battery cables to make sure the current was balanced between the strings. It was surprisingly close.

    Hopefully be picking up the new batteries here in a couple of weeks...
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