Charge rate for two different battery groups with different ah rating

90cummins Solar Expert Posts: 57 ✭✭

I have a Xantrex XW4024 off grid backup system with 4 Trojan L16RE-b’s rated at 370ah each.

2 weeks ago I purchased 4 Trojan T105 plus batteries rated at 225ah for scrap with hope of reviving them.

When I received them I checked the Sg and it didn’t register so I proceeded to charge them at 32 volts and discharging them several times.

They recovered and now have approximately 80% capacity.

My question is what rate should I program my charger for with the battery banks connected parallel? The total ah rating for both battery banks is 595ah.

Should I program my charger at 595ah or leave it set at the current setting of 375ah.





  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    You can program for 595 AH battery bank--But you should purchase a DC Current Clamp Meter (this one from Sears is not expensive and "good enough" for our needs).

    Measure the charging and discharging current for each battery string... Ideally they should share the current flow in proportion to their AH Capacity (~62% of current for the 370 AH string and ~38% for the 225 AH string--Of course, if you are within 10-20% or the "ideal numbers", you are doing OK).

    You should also ensure the battery wiring is "balanced" between the two strings (equal wire resistance):

    Monitor the charging/discharging/no load-charging current closely. With a string that "went dead"--It can very easily start discharging from your "good string".

    And watch the "float current" on each string--More or less, if your float current never drops below ~2% rate of charge (assuming you have proper charging set points programmed), it is possible that the old string is not going to work out for you.

    Also monitor the battery strings' temperatures... Hot batteries can be an indication of problems too.

    If your "good string" is really good--You might try running just on the old string for a while (I would suggest 2-3 weeks) and make sure they work OK. Mixing Battery parallel strings can be asking for problems ("bad batteries" kill your good string).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,744 ✭✭✭✭✭✭
    Do what Bill says!
     Be careful with the 32V !!!
    If you are charging with an XW you should do a normal bulk absorb charge and go to float for a minimum  3 hour absorb time.
    Then you do an EQ and watch temp.
    I have seen people set the bulk/absorb to 32 volts and start a fire. It was not pretty. It burned half of the home down. I got paid to tell the insurance company that it was negligence. The XW still worked BTW!

    Bill is telling you that this is not a good idea, I agree, as usual.
    "we go where power lines don't" Sierra Nevada mountain area