Mixing Old and New batteries

I currently have a grid-intertie 48v PV system using sun extender AGM batteries that are now 9 years old. The batteries are arranged as 4 parallel strings of 4 PVX-12255 8D type. My signature contains more specs. Recent testing shows 4 of the batteries seem to have died and sit at 8-9 volts.

My guess is that the others may fail before too long as well but it's hard to tell since they all test at 12.9-13v after charging and in a rest state. Concorde says some well designed systems have gone 10 years or a little more. I haven't done a full load test yet to determine capacity of the remaining banks since I have taken my system completely offline (using grid now). A recent heavy electrical storm and close lightning strike seems to have blown out some trace inverter components and they aren't functioning properly. My wind generator had lighting protection. My guess is the strike cause a severe overvoltage on the grid-tie line.

I know it's not recommended to mix new and old batteries but does anyone have experience in knowing if any real damage can be done to a new string if I remove the 4 bad ones and rearrange the batteries ending up with 3 old and 1 new string? Should I buy a new set of 4 and trickle charge them until the remaining 3 strings go bad then remove the 3 bad strings and install the single new string?

I don't want to wait until they all die before replacing the whole system due to the way the economy is going.



  • BB.BB. Posts: 26,623Super Moderators admin
    Re: Mixing Old and New batteries


    Your best bang for the buck is probably to just configure it down to 3 strings...

    You have been doing it "right" given that your batteries have lasted so long.

    Placing a new string of 4 in parallel will usually end up with the new batteries carrying a larger portion of the load--since they are new and have a lower internal resistance. So--they will tend to cycle more and deeper than the existing batteries and will tend to age quicker.

    If you place the old and new batteries in series--with AGM (I am guessing here based on another mfg who said don't run old/new AGM's in series)--that you cannot do large equalization currents to get all of the batteries in a string to full charge--since the old and new batteries are not matched, the string will tend to go out of balance and cause you more problems (and if you try heavy equalization--you would boil one or more batteries in the string).

    And, if you buy a new set of 4 and set aside trickle charging--when you buy the rest 1-2 years later, then you end up with 1 "old" string and unmatched lot with 3 strings and a matched lot--not a great thing either...

    Perhaps, if you can find some "old but functional" batteries (very cheaply) from somebody else who just replaced a bank--you can use those for a few years.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TelcoTelco Posts: 201Solar Expert ✭✭✭✭✭
    Re: Mixing Old and New batteries

    Another option, pick up some cheap new batteries that will last a year or two, that you won't mind dumping when the whole pack goes.

    And yet another option, go ahead and replace all the batteries now, and sell the old ones. You can sell the existing strings cheap, and tell the buyer that they won't last too long. Perhaps someone here that's near you and wanting to get into solar can buy them to practice on before buying themselves a good set.

    However you go, try to figure out exactly where the failure was, and get it corrected.. If lightning got in once, it'll get in again.
  • icarusicarus Posts: 5,090Solar Expert ✭✭✭✭
    Re: Mixing Old and New batteries

    I agree with both Bill and Telco.

    Either run the string short 'till they all croak, or buy a couple of of cheap batteries that combine equal one or more of you originals and hope for the best.

    In one installation, I am running 3 l-16's that are ten years old, with 2 t-105's seriesed so that I essentially have 4 batteries in the string, but one battery is the 2 t-105s' (Their combined ah capacity is roughly similar to the L-16's). They are also better than ten years old.

  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Mixing Old and New batteries

    be sure that the battery wires/connections didn't cause your batteries in that string to prematurely die, but the damage is done and your options were listed pretty well by the others for the batteries. you just don't want to possibly repeat damaging any future uses with those wires/connections if they are bad.
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