Charge rate for new Agm batteries

loggerlogger Posts: 2Registered Users
Hi all, 

I have just upgraded my battery bank, the old flooded Trojan t-105 (225AH) have given up the ghost after 8.5 years.  

They were wIred in a 24 volt system.

My PV array is 6 x 190W panels hooked up to an Outback FM60 MPPT Charge controller.  I had limited the max charge current to 34A. Giving a max charge rate of 15% of the C20 capacity. Slighter higher than the max. Trojan recommendation of 13%.

The new batteries are 4 generic Chinese AGM 310 AH capacity.  I couldn’t find any specific charge rates for these batteries, but from what I have seen for other similar AGM batteries,  it looks like I should be targeting a charging current of 30-40% of the C20 capacity.  After resetting the Outback back to 60 amp max. The panels are maxing out at around 28A in the middle of winter.  Only one more week and the days will be getting longer 👍

Question - do I need to invest in more panels to get the max charge current up? Or is 28 amps in the middle of winter and I’m guessing around 40-42 in summer ok?

New batteries are charging fine and I haven’t seen them drop below 25 volts in the first 8 days of operation.  

thanks Log


  • mcgivormcgivor Posts: 1,746Registered Users ✭✭✭✭
    AGM need to be charged at a lower voltage than flooded with lower float and no equalization, perhaps you already know this. Panel wise, if they were meeting your needs before with the flooded batteries there's no reason why they wouldn't be sufficient with the new. Being slightly larger capacity, they will be cycled to a lower depth of discharge, there would be little need to aim for higher current if what is available presently, is sufficient, in fact it's probably better to be at a lower current over a longer time, as long as full charge is achieved, less stress.

    Life expectancy will not be as good as the flooded, just be aware of that.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • loggerlogger Posts: 2Registered Users
    Thanks McGivor, I had already reduced the absorb and float voltages and done away with the equalisation phase.

    The battery supplier offers a 5 year warranty (albeit pro rata after 2 years) as long as you have a battery monitor and can produce records showing the batteries weren’t thrashed.  I’ll be happy with 5 years, as battery technology looks like it is making some promising leaps.  My next bank will hopefully be Supercaps made from wood fibre.

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