Charging batteries in series

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Nabiru3
Nabiru3 Registered Users Posts: 13 ✭✭
in Off Grid Solar & Battery Systems #1
Hey everyone, I was connecting my 2 new batteries together when I realized that their voltages are slightly different (one is 12.54 & the other is 12.41) and then I just wondered what will happen if I hooked them up in series and started charging them while both having a slight variation in voltage  :/. some people told me that once you charge your battery bank it will even the difference out, is that true or should I charge them each individually to even their voltages before connecting them together ? 

specs (if it helps)
Battery : two narada AGM 12V.
Capacity : 150Ah each.
desired battery bank : 150Ah 24v.


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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
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    Ideally, I'd charge them separately to get them as close as possible to the same state of charge. They'll likely even out, but initially the higher voltage one may overcharge some, and/or the lower voltage one undercharge a bit. IMHO, better to get them off to as good a start as possible.
    Off-grid.  
    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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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #3
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    Another method is to parrallel both for a few hours the voltage will ballance, then series charge.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
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    Flooded Cell Lead Acids will accept "over charging" (Equalizing charge) of the full cells/batteries and let the less than full batteries/cells fully charge.

    AGM batteries (and other sealed type batteries) allow less current to "leak through" during charging/Equalizing charging. Most AGM batteries do not "like" elevated voltage EQ charging (you can cause the catalyst inside the battery to overheat and/or the batteries to vent).

    By matching the voltage between the series AGM batteries before you connect them (individual charging, parallel connection), this gets the batteries to match their states of charge.

    Other options... AGM EQ charging is just holding the standard charging voltage set point for upwards of 8 hours (a slow "leakage/EQ" charge).

    Another is to get a 12 volt filament bulb (like an automotive brake light), and clip it across the higher voltage battery and bring it down to the same voltage as the other series connected battery.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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