Bulk Battery Charging

msgiesermsgieser Registered Users Posts: 24 ✭✭
I have an off grid solar system that is working great! I want to add a generator powered (I have an 8kw) battery charger to use only on cloudy days when I can not get enough power from my panels. Based on my 450 amp hour battery bank (4 T105re's) I can use a 50-75 amp charger. I'm looking for one with adjustable voltage, as I want to charge these Trojan's at 14.7 - 14.8 volts thru the "bulk stage only"...get them to 90%, and let the panels do the rest. I want to get them thru the bulk as quickly as possible, as my 8kw generator is a gas hog! Any recommendations on a charger...Iota....Powermax...or ??????. I seem to be having trouble finding one I can set to 14.8. FYI...the batteries are never discharged more than 40%....always are at 60% or better SOC. Thanks!


  • BB.BB. Super Moderators, Administrators Posts: 30,490 admin
    Re: Bulk Battery Charging

    Iota makes some very rugged/simple charge controllers that work well.

    They are not advertized as "adjustable" output, but a couple of people have opened them up and adjusted the ten turn pot (variable resistor) to the exact charging voltage they want.

    I don't know how wide the adjustment range is--But Iota has been very helpful to folks that call with technical questions.

    Regarding the genset... That is a bit on the large size. A 75 amp 12 volt charger will need around:

    75 amps * 14.8 volt charging * 1/0.80 efficiency * 1/0.67 power factor = 2,071 VA minimum rated genest

    (note, there is some disagreement here regarding if my 0.67 PF derating is needed or not--Be warned, I try to be conservative and it can "over design" a system at times).

    Anyway, ideally you would want to run your genset around 50% of its rating or higher for beset fuel efficiency. An 8 kW genset running a smaller battery charger may use 2x (or more) the fuel vs a smaller genset (unless you are running other AC loads at the same time, such as water pumping for irrigation, etc.).

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