Battery Absorb Time



  • petertearaipetertearai Solar Expert Posts: 471 ✭✭✭✭
    Re: Battery Absorb Time

    My understanding is that gel battery's are sensitive to over voltage, Absorb is the controller holding the voltage at a constant . As the battery charges the last bit the amps go down. So being gell should not be an issue.
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited March 2017 #33
    Old topic, but Rolls provides a formula for absorb time with lead acid.   8.4 hours at a 5% (of capacity at C/20) charge rate, 4.2 hours at 10%, 3.2 hours at 13%.    Or end absorb at 2%.

    I am available for custom hardware/firmware development

  • BB.BB. Super Moderators, Administrators Posts: 32,747 admin
    For GEL batteries (at least in North America--Not sure, but (some?) European GEL batteries may be less affected by gassing????), it is the voltage level that is a problem. Generally, charging around 14.0 to 14.2 volts maximum is recommended to limit gassing.

    GEL batteries have very low self discharge, can supply high surge currents, and are reasonably reliable+cheap (lead acid battery variation). However, because they take longer to charge (typically C/20 or 5% rate of charge maximum), they are difficult to solar installations where you have a limited number of hours of sun per day (6 hours or so to fill your battery).

    If you get to much charging (high charging voltage, longer times at high charging voltage), the hydrogen+oxygen gas from charging forms pockets in the GEL--And I understand, those pockets are permanent and reduce the contact area between the plates and the gelled electrolyte.

    GEL generally make very good UPS batteries--But just are not good for off grid solar/wind installations.

    Do you have the charging information for your batteries?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    Rolls suggests an initial GEL bulk charge rate of "0.35 X C20", which is quite far from 5%.

    Also interesting is "VRLA battries should be brought to a full charge on each cycle or, at minimum, once every 6-7 days."  I didn't see a similar warning for lead acid (although I've always assumed a full absorb cycle every 2 weeks).

    I am available for custom hardware/firmware development

  • EstragonEstragon Registered Users Posts: 4,496 ✭✭✭✭✭
    I put a VRLA (agm) battery on my seldom used diesel generator specifically so I could leave it without a charge source for months at a time. I guess it's a good thing it isn't a Rolls!
    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
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    Rolls says 6 months storage for their AGM batteries (page 26).

    I am available for custom hardware/firmware development

  • Marc KurthMarc Kurth Solar Expert Posts: 964 ✭✭✭✭
    I would add that storage time is highly temperature dependent.
    I always have more questions than answers. That's the nature of life.
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