combining AGM batteries

homerramirezhomerramirez Solar Expert Posts: 102 ✭✭
Hi Guys, I had read a lots about combining diferent batteries is not good but which will be my best option?:

When I first started my system with 9 x 130 w + 1-100w = 1270 w of solar panels(4 months ago), all I could afford was 4 -200ah (NAPA Power Cycler) AGM batteries @ $265.00 ea.:cry: , now the same person that sold me my Sunsei solar panels offer me 10- 170 ah. AGM telecomunication batteries @ $ 125. ea.
I wired my batt. bank @ 24v. those 10-170ah @ 24v = 850 ah. wich is close to what my 1270w of solar panels can support.
I have 2 more 100w (free)solar panels that the glass is broken but still have the rated output 6amps ea.

about how much AH. I will lose if I combine the 2 battery banks? combined will it gave me 1250AH if there is no loses by doing it.
I know that I do not have enough PV to support the 1250AH @ 24v, so my batteries may never float, perhaps I have to wait to a no sun day to run my generator and charge the combined bank with my invercharger Samlex S2024A true sine inverter...

can you guys gave me an advice on what's the best choice....170AH @ 12v for $125.00 no tax or freight is very tempting. what should I do? :confused:

As always thanx in advance.:D

I have a new spare MSW 24v. inverter (that might come to play)


  • hillbillyhillbilly Solar Expert Posts: 334 ✭✭
    Re: combining AGM batteries

    I think that this would fall under the "penny wise, pound foolish" category. I hear ya about trying to score a cheap deal on what is normally very expensive stuff, I've done the same on some solar panels and now have a tricky mix match set of VMP and IMP stats to match up if I ever want to add more PV. Batteries are another creature entirely, as I am sure that you've read/heard about not mixing them up.
    A couple of issues to consider right away:
    First off, you mention that they are 12V? if so, then 10 of them would make up 5 parallel strings plus your existing batteries. That's a recipe that's almost bound to end up with unequal loading, and so is mixing 200ah batteries with 170ah batteries from different manufacturers. They just will not be able to share the load equally, and so you'll likely develop bad cells and your batteries will fail prematurely.

    You might be able to hook up two separate banks, and switch back and forth between them, although this adds to the complexity of charging them. I would look into this option if I thought that I had a set of batteries that was going south in the near future but wanted to continue to use them while they still had some life in them. In this case, I still would not want to get all ten batteries and have so many parallel strings.

    A single string of batteries wired together in series would be the least likely to have issues, and therefore is ideal as far as getting your dollars to go the furthest. Two strings, or possibly three, paralleled together can still be ok (lots of folks do this), but you need to be certain that every connection is solid and clean, with care taken in your wiring to make sure that currents are equally shared throughout the entire bank of batteries.

    I would crunch the numbers, and I think that in the long run buying a good set of batteries and taking proper care of them will end up being much cheaper than buying lots of sets of cheap ones and not taking care of them. When crunching the numbers be sure to factor in what "other" costs may be incurred from premature battery failure; depending on how things fail, those "other" costs can be quite expensive and unpleasant.
    good luck
  • homerramirezhomerramirez Solar Expert Posts: 102 ✭✭
    Re: combining AGM batteries

    Thanks Hilbilly, I was thinking about wire them in pairs to get 24v. then parallel on them (5 strings) to my inverter....then charge my 4-200ah NAPA AGM's for reserve on separate panels and charge controller ....may be better to add 2 more of the same kind and brand 200ah. NAPA AGM batteries then my batt. bank will be 600AH @ 24v. then stop right there, it just was tempting but as you said it coul be worst by combining...

    Thanks Hillbilly for your honest opinion,;)
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