Battery mixing question

Steven LakeSteven Lake Solar Expert Posts: 402 ✭✭
Ok, I have a really dumb question, and I figured you guys would be the best ones to answer this. Which is worse: Mixing old Ibut still good) and new batteries together in a system, or two batteries with different amp hour ratings? (same voltage, just different AH's, run in series for 12v*2=24v)


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery mixing question

    It's actually sort of the same thing.
    As a battery ages its Amp hour capacity goes down. So you would in essence be mixing new, higher Amp hour batteries with old, lower Amp hour batteries.
    How effective the mix is depends on the actual difference, and I can only give you the vague answer that a greater difference in Amp hours is likely to cause more problems (chiefly shortening the battery life of the higher capacity batteries if the charging is set for the lower ones, or vice versa if the other way 'round).

    Batteries that are only a year or so old and haven't been heavily cycled or otherwise abused can usually blend in with new ones of the same type without difficulty. Likewise new batteries with only a few Amp hours difference in rating won't be problematic either.

    Not much of an answer, I know.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Battery mixing question

    I would mix the old with the new before I would mix those of significantly different sizes, the normal caveats about mixing batteries not with standing.

  • RCinFLARCinFLA Solar Expert Posts: 1,397 ✭✭✭✭
    Re: Battery mixing question

    I have somewhat a contrary view of mixing batteries.

    A lead acid batteries is a lead acid battery with similar chemical characteristics.

    I have been running 10 parallel strings for over 10 years successfully. I change out a string or two every year to soften the cost and logistics of replacements. I don't like dealing with a >100 lbs battery.

    Main no-no is don't mix batteries of different capacity in a series string. This includes like batteries sizes where some are showing capacity loss with age. I do a 20% discharge capacity test every year or two. I carefully watch float voltage variation to detect leakage that puts stress on other batteries in the series string.

    There will be charge and discharge current variation between strings. If you have enough overall cumulative capacity you can avoid excess discharge and recharge current on any single string which is something that needs to be watched. I find discharge and recharge currents even out over battery cycling within the upper 50% SOC range. A string may start out pushing 2x the current of another string but later on it drops off and the initially lower current string begins taking up more of the load/recharge current.

    I do not cross trap batteries between different strings as it makes it harder to detect a battery going bad before it damages good batteries.

    I have a 100 amp shunt in each of the ten series strings and voltage monitor capable of measuring each of the 40 batteries individually. Each series rack has a RJ45 jack and I use Cat6 cables to bring the sense readings to central monitor unit. All the voltage sense connections have 100 ohm series resistor just in case a downstream short occurs on the cat6 interconnects.
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: Battery mixing question

    The above responses are good and true if solar is a primary source of your power.

    If this is a first time setup and there is grid power to fall back to. Then use what you have and learn as much as you can. Best to make mistakes on old/weak batteries.

    I broke most of the above, First two batts where 3 years old, brought two new ones, then brought 3 used ones.

    I would not mix different AH in the same string.
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