Capacity left in batteries

squarebobsquarebob Registered Users Posts: 25 ✭✭✭
I have 4, 6V golf cart batteries that I took from my motor home. They were in service for 3 to 4 years and were well taken care of. Hardly used, usually on float and watered as needed. They were hooked up in a 12 volt configuration. I now have them set up as my experimental batteries for my backup home system. They are set up in series for a 24 volt bank at, I assume, 220 Ah capacity. Being that they are "old", I want to determine their loss of capacity, if any. I fully charged them and took an SG reading of approx. 1.270. I put an approx. load of 250w for 6 hours on the bank. I withdrew 57Ah in the 6 hours ( trimetric ). The SG at the end was approx. 1.225 and the resting Volts ( after 5 minutes ) was 24.9. Can some one smarter than me tell from this data if the bank has a diminished capacity? Is there a different / better way to determine the capacity of the bank. I am all ears!!!

Thanks

Bob

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,026 admin
    Re: Capacity left in batteries

    You only used 57/220 AH = 0.259 = ~26% of your bank's capacity... In theory, you bank will be "worn out" when it no longer will support 80% of its rated capacity.

    Although, if you never discharge lower than 50%, you could also use that as your "recycle" point.

    Note that heavily sulfated batteries do suffer a loss of "surge current" capability... So that will be important to you too (typically a deep cycle flooded cell battery should be able to support ~C/2.5 or C*0.40 of rated capacity for surge current; and C/8 for continuous current):
    • 220 AH * 0.40 = 88 Amps of surge current (without crashing battery voltage)
    • 220 AH * 1/8 * 100% = 27.5 Amps for 8 hours (for 100% discharge)
    • 220 AH * 1/8 * 80% = 22 AH for 8 hours (80% discharge)
    Note, in reality this is a bit more more complex.. The AH rating of the battery is different at different current levels... It may be 220 AH at 20 Hour Rate, and 200 AH at 10 Hour rate, and 245 AH at 100 Hour rate (the higher the current, the less apparent capacity).

    According to one battery FAQ, roughly 12.42 volts (24.82 volts for a 24 volt bank at 77F and after 3 to 24 hours of "resting"--No charging or discharging current) is 80% state of charge (20% discharged)--So they do sound OK, but you need to rest the battery more.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Capacity left in batteries

    bill,
    "and 20 AH at 10 Hour rate"

    i assume you meant that to be 200 and not 20.

    squarebob,
    the way it is figured is for a 20hr load rate down to 10.5v. this means for a 220ah battery that it would be a load of 11a for 20hrs to be 100% dod. you can check with the manufacturer to see if they give anything further so that you won't have to try and deplete the batteries to see how short of time it is before it's at 10.5v.

    edit to add: i inadvertently put the voltages for a 12v battery instead of a 24v one. for 24v just multiply the voltages i listed x2.
  • BB.BB. Super Moderators, Administrators Posts: 32,026 admin
    Re: Capacity left in batteries

    Thank you Niel,

    Fixed typo...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • squarebobsquarebob Registered Users Posts: 25 ✭✭✭
    Re: Capacity left in batteries

    After a 12 hour resting period, the voltage is 25.1

    Bob
  • RCinFLARCinFLA Solar Expert Posts: 1,399 ✭✭✭✭
    Re: Capacity left in batteries

    Your batteries look in good shape. Attached is chart of SOC vs. SG and rested voltage. Both your open rested voltage and SG agree that you discharged batteries to about 75% SOC.
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