Need input for Glass mat Batt

I have 2 glass mat batteries that are used only 2 times a years to start generators...
Will a float voltage of 12.98 volts maintain these batteries at the proper charge? They are sunextenders 12 volt


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Need input for Glass mat Batt

    AGM's have a very low self-discharge rate. Maintaining them with a constant float charge isn't necessary. If they start the generators a couple of times a year they're probably okay. It might be advisable to check the resting Voltage once a month and recharge if necessary.
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Need input for Glass mat Batt

    From their website:
    Initial charge or recharge: 2.37 to 2.40 volts per cell at 25 C (77 F). Float charge: 2.20 to 2.22 volts per cell at 25 C (77 F). Equalize charge: 2.40 volts per cell at 25 C (77 F). Temperature compensation = ±3.75 mV. per cell per degree C [Reference to 25 C (77 F)]. This is for battery temperature (not ambient temperature) and is useful for battery temperatures from 0 C (32 F) to 40 C (104 F). Contact Concorde Battery Corporation for temperatures that exceed this range.

    For a 12 volt battery, that would be 13.20 to 13.32 volts float voltage. Also, remember that this is at ~77F... If your batteries are stored in a cold area (batteries last longer if kept cool)--say 77F-18F=59F, then:
    • 6 cells * -10C * -0.00375 volts + ~13.25 volts float = ~13.475 volts
    What I have done on the cheap side--Get a 1-2 amp trickle charger and connect it to a 24 hour lamp timer. I normally set the timer to charge 1 hour per day... That is usually enough to keep the battery fully charged and not overcharge (probably, up to a 100 AH or a bit larger battery--If a bigger battery, either a larger charger or 2 hours per day, etc.).

    I have done with with a few different cars and it works out well. Before, the "float" chargers plugged in 24x7 to a "maintenance free battery" would require fulling every month or three.

    Or, monitor the resting voltage of the batteries... As long as they are over 75% state of charge, they are "OK".

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Need input for Glass mat Batt

    an unregulated charge source could continue to raise the voltage on the batteries so some sort of regulation should be used as agm batteries are sensitive to overcharging. you cannot put a unregulated 1 or 2 amp charge source on say a 100ah agm long term.
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