Parelling old and new

powersurgepowersurge Registered Users Posts: 27 ✭✭
Even after reading never add different age batteries I still had to try it. I bought 2 new of the same size and brand  GC batteries. Added to my 1 year old batteries.  I Equilized them and put them all 4 in service together. 10 days after S.G. was already needing eq  .030 between cells. (I DID NOT EQ AGAIN) Not sure if it was due to adding  new to older or from parelling. So I separated the batteries into 2 groups  of  2-6 volts 1 system with 4 100 watt panels on pwm. This runs all inverter loads. System  2 is 2x190 gt panels on mppt. This runs all DC 12 volt RV loads. 10 days into running it this way S.G. is only .010 between all 4 batteries. Much better now.


  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Welcome to the forum PS.

    It is nice to hear from somebody that actually experimented with "different" configurations and report on the results. Love to share information here.

    1 year old batteries should not be a "huge issue"--And FLA batteries actually perform better after 20-100 cycles (forming plates).

    So, it is possible that your old vs new batteries may "play together better" after the new bank gets some cycles on it.

    Very roughly, a 0.030 difference between high/low SG reading (cells/batteries) is ~30% capacity difference (one at 1.265 being 100% full and another at 1.230 is ~70% full).

    Another possibility for "differential SG between strings" is "poor wiring" for the battery strings. Here is a nice website that shows several battery wiring configurations:

    Again, very roughly, a 0.10 volt difference or greater between batteries/strings under charging is less than ideal (for a 12 volt system).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • powersurgepowersurge Registered Users Posts: 27 ✭✭
    edited July 2019 #3
    I had the wiring as recommended from smart gauge positive and negative from opposite corners.  (over kill) 1/0 12" cables. Under charge 44 amps  volts were no more than .05 between batteries when they were 2s2p. The new batteries  had the lowest S.G?? I Hadn't thought  of the time it takes for the batteries to form. My DC power needs per day are ~ 750wh. And  my ac loads ~500wh after .85 inverter eff. So 25% of  420 ah. Separated in an AC DC configuration takes 2 of the batteries lower daily than I would  like. I'm gonna cycle the new ones some and try parelling again.
  • BB.BB. Super Moderators, Administrators Posts: 32,027 admin
    Sounds like you are doing OK... I would check with a DMM and see what the voltage is on each string (under charge, under discharge) and see that they are close (less than 0.05 volts would be ideal).

    And use a DC Current Clamp meter on each string to see how well they are current sharing. If they are sharing within 10-20%, you are probably doing the best you can.

    And watch the distilled water usage (assuming flooded cell lead acid)... If you have to add water every 1-3 months you are usually charging fine. If you have to add water more than 1x per month, probably over charging (cut back on voltage and/or setpoint). If you are adding water every 6 months or more between fills, probably not charging enough.

    With a current clamp meter (and/or your charge controller), generally FLA batteries are "full enough" as the current falls to 1% or less of 20 Hour rating (i.e., 200 AH bank, charging current falls to 2 amps or less).

    A DC Current clamp meter ($50 to $300+ USD) is really nice to have--Great for monitoring your system and diagnosing problems... A nice "mid-range" meter (as an example):

    Just be careful with descriptions... There are AC only current clamp meters, and AC/DC current clamp meters... But both types can measure AC/DC voltages--So things can get confusing (you do not want an AC only current clamp meter for DC power systems).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • powersurgepowersurge Registered Users Posts: 27 ✭✭
    edited September 2019 #5
    After 2 months  of cycling my new batteries.
    I put them in parallel again. 10 days in  on each of the new batteries im seeing 1.260. my older ones show 1.265 on 1 and 1.275. On the other . Seems odd that the older ones are higher than the new??? Waiting to see if the .015 spead grows again.
    I know  it's not sun hrs but  Panels  see sun light from 10 to 530
    Im charging to 1% almost daily. With an hour or 2 of floating.

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