Mixing Batteries

n1st
n1st Solar Expert Posts: 77 ✭✭✭✭
in Off Grid Solar & Battery Systems #1
I understand it's bad to mix batteries in a string, but how about in parallel? I have a battery bank that's a couple of years old. I'd like to add a new set, so I'm thinking of adding them in parallel. The new set may even have a larger ah rating. What are the pros & cons of doing this? What are my alternatives (excluding replacing all the batteries)?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Mixing Batteries
    n1st wrote: »
    The new set may even have a larger ah rating.

    Right there is your answer. Even if they are exactly the same make & model, the new ones will have a higher Amp hour rating because as batteries age their capacity goes down. How much it goes down depends on time, how deeply they are discharged before recharging, how well they are recharged, et cetera. Generally if the batteries are only a year or two old and not deeply cycled (more than 25%) or otherwise abused they can be paralleled with brand new batteries that are the same.

    Why you don't mix dissimilar batteries: the greater the difference in Amp hour rating, the greater the difference in charge/discharge profile. The right rate for one will be too high or too low for the other.

    For instance:
    L16 390 Amp hours: charge rate 39 Amps for 10%
    T105 225 Amp hours: charge rate 22.5 Amps for 10%

    39 Amps would be a 17% rate for the T105's, 22.5 Amps would be 5.7% for the L16's. So the T105's would cook if charged at 39 Amps, and the L16's would charge slowly and would sulphate/stratify at 22.5 Amps.
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #3
    Re: Mixing Batteries

    Have you researched battery switches?

    http://www.solar-electric.com/basw1300amp.html

    Keep the banks separate from each other, but could be manually charged by same PV (provided the new set still meets the recommended rate of charge).
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  • n1st
    n1st Solar Expert Posts: 77 ✭✭✭✭
    #4
    Re: Mixing Batteries

    Cariboocoot,

    Thanks for the explanation.

    Is a battery switch like bmet (thanks) suggested the commonly used method to introduce new batteries to a system? Any other alternatives?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Mixing Batteries

    Battery switches work for splitting banks, for instance if you have only 1/2 as much PV as you need as in two 225 Amp hour 12 Volt banks & 400 Watts of array. But you have to remember to switch them and keep track of which is charged. With one switch you can only select a bank being connected to both charge controller and inverter. With two switches you can charge one bank while drawing from the other. It can get confusing.

    Some charge controllers (notably more expensive ones) have programmable auxiliary functions which can operate a relay to switch when one bank reaches Float stage. Again; more complexity and cost.

    If the two banks are quite different and require different charge parameters than you have to use two different controllers and isolate the (+) sides at least with a battery switch: two independent battery banks & charge set-ups that the inverter can be switched (manually) between. Since greater battery capacity of any type generally requires increased PV for charging, this is perhaps the best way with dissimilar batteries; add a second controller and a switch for the inverter. If you don't get the charge capacity to match the batteries there's no point in getting the batteries.
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