max charging rate fo battery

jtdiesel65jtdiesel65 Solar Expert Posts: 145 ✭✭✭

So I searched for this, but didn't get any specific results.  Posts hinted at 10%. IIRC it's 10-15%.

I have a forklift battery 1576 AH @20 and currently max charge at 180a via generator and 175a via panels. So that is around 11.1 %.  I recently got a new 12KW generator replacing an old 8.5KW unit and am wondering if I can bump up the charge level. I have two XW4024 each with 150A limit. 15% would put it at 236a or 118a per unit. If I did the math right, the new generator should be able to handle this amount.

Anybody know what the rule of thumb on the max rate is?


  • PhotowhitPhotowhit Solar Expert Posts: 5,414 ✭✭✭✭✭
    edited May 2017 #2
    I went looking for GB information on charge rates for forklift batteries, where I had read 15%. I couldn't find the infor but here is an Exide Charger infor showing on page 63 a charge rate of 16%, I could presume that is based on a 6 hour rate so that would be around a 20% charge rate based on a 20 hour configuration. PLEASE LOOK FOR MORE INFO! I had fixed in m mind 15% charge rate was standard for forklift batteries based on a 20 hour rate!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do
  • BB.BB. Super Moderators, Administrators Posts: 31,157 admin
    Roughly 13% or C/8 rate of charge based on 20 hour capacity is common recommendation for flooded cell lead acid batteries. You can go higher--Utility (AC line powered) chargers are sized to 20% to 25% rate of charge. However, when you go above ~13% rate of charge, you need to monitor the temperature of the battery bank--It is possible to overheat the battery bank with higher rates of charge (and 13% rate of charge is usually the "maximum cost effective rate of charge" for solar systems--Larger systems will work, but system will sit at "float voltage" for much of that second 1/2 of the day).

    Many fork lift batteries may be sized to 6-8 Hour discharge rate (a fixed current that takes a battery from 100% to 0% state of charge over 6/8 hours)--And a few of the manufacturers seem to have difficulties properly mapping into the 20 Hour capacity specification.

    A 210 AH @ 20 hour capacity battery may have 175 AH @ 5 hour rate:

    So using the proper rate of charge against the correct capacity rule of thumb is important:
    • 0.13 * 210 AH = 27.3 Amps charging
    • 0.15 * 175 AH = 26.25 Amps charging
    Rolls uses a different % rate of charge and C/x capacity number for their rule of thumb charging calculations (last I saw). Always review the Mfg's data sheet and instructions. Many deep cycle battery manufactures recommend 10% minimum rate of charge... And you should have at least 5% minimum rate of charge to bring battery back to full charge over a reasonable amount of time, and to perform equalization (2.5% to 5% rate of charge).

    Our rules of thumbs are short cuts to get to roughly correct designs that will work for most everyone. And the rules are not "exact"--Usually anything within 10% (watts, amps, etc.) is close enough for solar work.

    Note that it is possible to overheat a battery bank with 2.5% to 5% rate of charge too--When the battery bank is full and you are pumping charge current into the battery bank for "equalization charging". During equalization, most of the energy goes into making hydrogen and oxygen gasses, and heating the battery bank.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    Depending on charge voltage setting and battery temp comp, at 236adc you may be setting max current a bit higher than 12kw. Also, I would be reluctant to run a generator at 100% load for extended periods as the alternator will run pretty hot etc.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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