Hooking up batteries of different capacity

rama
rama Solar Expert Posts: 36 ✭
in Solar Beginners Corner #1
Hi there - I'd like to know if hooking up batteries of different capacity (but same voltage) would result in any detrimental effect on battery life (e.g. batteries not getting charged equally etc). I got this portable mini power station of a 300w inverter with a built-in 15Ah 12V battery, and I use AC to charge it primarily for backup purpose. But now I want to size up the batteries to 45A so I am thinking of hook up the built-in battery with another 30Ah 12V battery.

In this case solar charging is not involved so there's no charge controller - all I got is a AC power charger.

Should I go with 1*30A battery or 2*15A battery? Or does it not even matter?

Thanks.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: Hooking up batteries of different capacity

    Batteries are very sensitive to charging and operational voltage... Ideally when connecting batteries they should be exactly the same mfg, model number, and even date code...

    In practice, you can parallel batteries that are the same type (AGM or Flooded Cell--Do not mix AGM and flooded cell batteries together). And, ideally, the batteries should be the same mfg and type (i.e., both same family of deep cycle batteries--Not one deep cycle and the second a car/motor cycle starting battery).

    For a 12 volt battery, just a 1/10th of a volt difference in chemistry (different electrolyte specific gravity can make a 1/10 volt or more difference)--Is roughly a 10% difference in the state of charge of a bank (12.7 volts = 100% charge, 12.6 volts ~ 90% charge, etc.).

    If you have a choice, my preference is for one 30 AH battery vs two 15 AH batteries in parallel... If you have one 15 AH battery and want to add a second 15 AH battery, why not. Small batteries may only last 1-2 years anyway--And you can change them out with another choice down the road--especially after you have seen how well your first setup works (amount of battery storage, amount of solar panels, etc.).

    You probably are better off starting with a "less expensive setup" first--and get some run-time. Then adjust it afterwords as needed.

    You may find that with a 300 watt inverter, that if you run it at full load that a 30 AH 12 volt battery bank is not really enough capacity...

    A rough rule of thumb is your battery bank should be 2.5x the maximum current (C/2.5 max discharge rate):
    • 300 watts * 1/12 volts * 2.5 max rate = 62.5 AH @ 12 volts minimum battery capacity
    And if this is a 600 watt surge capable inverter (and you expect to use the surge)--then you are looking at a 125 AH battery bank.

    If your power needs are less, you might see if you can find a smaller, more efficient inverter to power your loads (I am not sure you will find a smaller TSW more efficient inverter--would need to look at the manuals/specifications and see).

    Anyway, some quick observations.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Hooking up batteries of different capacity

    I would have posted a one word answer: "don't".
    But there's a minimum size for posts. :p

    So, what Bill said. :D
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  • rama
    rama Solar Expert Posts: 36 ✭
    #4
    Re: Hooking up batteries of different capacity

    lol... thanks Cariboocoot and as always, BB.

    I totally agree that the battery on this mini power station is way too small... but the built-in inverter is 300W and right now I'm just using it for a 60W laptop and I get about 2 hours of use or so... not the best portable solution considering it's already 8kg.

    mmm anyway I think I'll just hold off the upgrade for now. thanks..:p
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  • RRRAAAYYY2
    RRRAAAYYY2 Registered Users Posts: 11 ✭✭
    #5
    Re: Hooking up batteries of different capacity

    I would like to add that when choosing batteries it is important that they also have the same at rest voltage as well as the dates, etc. If you have a battery that is 12.72 at rest and one that is 12.74 at rest, when the charger checks for the batteries voltage, it will read 12.73, the average. It then will bases its settings on that voltage. This means the one battery gets overcharged, and the other under charged. We recommend that you go to at least 2 decimel places when comparing batteries.

    We often seen if one battery was replaced in a group of 4 on big truck applications, it would last about one month. You always had to replace all or nothing, unless the batteries were fairly new, and you could find a battery with a similar at rest voltage.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
    Re: Hooking up batteries of different capacity

    "Should I go with 1*30A battery or 2*15A battery? Or does it not even matter? "

    2 identical 15ah batteries can be paralleled. it would be less connections using 1 30ah battery though making the 30ah battery the better choice. if you were to use a 10ah and a 20ah battery in parallel then that's not good as equality is the goal in using more than 1 battery to keep charges and power drainage equal so as to not cause one battery to do more work than the other and cause the overworked battery to prematurely fail. that also means the interconnecting wires need to be identical when paralleling.
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  • RRRAAAYYY2
    RRRAAAYYY2 Registered Users Posts: 11 ✭✭
    #7
    Re: Hooking up batteries of different capacity

    Something I forgot to add. When you do parallel strings of batteries it is very important to have your power in and out kitty corner to each other. For example if you have batteries A, B, C, D hooked together in parallel.

    At battery A you would want the positive to your devices, and the ground to your charging circuit.

    At battery D you would want the positive to your charging circuit, and the ground to your devices.

    If wired like this your system has to use each battery equally, and makes them act like one large battery.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #8
    Re: Hooking up batteries of different capacity

    To followup on Ray's point:

    Smartgauge's battery wiring writeup

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • RRRAAAYYY2
    RRRAAAYYY2 Registered Users Posts: 11 ✭✭
    #9
    Re: Hooking up batteries of different capacity
    BB. wrote: »
    To followup on Ray's point:

    Smartgauge's battery wiring writeup

    -Bill

    Never considered the last two Methods in that link. Thanks for posting it up. I also had always considered that Method 2 had created a near 100% balance, not realizing it didnt. We had never tested it to that extent, just went by the results as compared to systems using Method 1. So really glad to see real data.
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