charging 7 amp battries

penrosefred
penrosefred Registered Users Posts: 7 ✭
in Solar Beginners Corner #1
In my camper I have a 135 volt solor charging 2- 875 cranking amps deep cell. Can I charge my fish locator 12 volt 7 amp battry by just hooking up to the camper battries. I hope I explaned this so you know what I mean.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #2
    Re: charging 7 amp battries

    I think you have a 135 Watt solar panel at Vmp=17.5 volts or so...
    • 135 watt / 17.5 volts = 7.7 amps in full sun
    I am not sure of the size of your RV batteries (cranking amps does not tell us)--but assume that the two of them are 12 volts at 100 AH and you have a 12 volt 100 AH trolling motor battery.

    Guess that your RV batteries are fully charged and your trolling battery is near dead:
    • 100 AH / 7.7 amps = 13 hours (plus a few hours to taper to full charge)
    Normally, in summer most people get around 5-6 hour of "full sun" per day...

    So, it will take 2-3 days to recharge the battery from dead (with no other loads).

    In the end, yes, you can connect the battery in parallel to your (I am guessing) your 12 volt camper's battery bank, and it will charge.

    But it will take quite a while and if your camper bank is fairly large--a single 135 watt solar panel is probably too small to fully charge all three batteries consistently. However, for a three day weekend of light fishing, then returning home and charging from your utility power--It would be OK.

    As always, be careful. Lead Acid storage batteries have a tremendous amount of current available (shorts=>fire/damage) and release hydrogen gas during charging (spark=>fire or explosion).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • cfcw
    cfcw Solar Expert Posts: 25 ✭
    #3
    Re: charging 7 amp battries

    I think he is asking if he can take his dead 7 Ah battery that needs charging and just hook it up to his battery bank without damage. i.e what is a practical way to limit the current to a couple of amps?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #4
    Re: charging 7 amp battries

    Opps--I am sorry, I thought he asked about a trolling motor battery (100AH) instead of a small 7 AH battery.

    I don't think there should be an issue... I have jumpered good sized car batteries and only had a little bit of sparking. It would not hurt to have a 10 amp fuse to the small battery to prevent over current / shorts.

    If the small battery is AGM/Sealed and the large bank is flooded cell--just make sure that the small battery is not connected to the large bank when it is being equalized (charged above ~14.5 volts).

    Thank you CFCW and sorry to penrosefred.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: charging 7 amp battries

    i don't know how well that may work as the current will be very large especially if the other batteries are fully charged and there's little to no load on them. this could overcharge easily imho. it would only need about 1/2a to 3/4a over 10hrs or so to charge it.
    jumping car batteries is a bit different than pumping that huge amount of current into a small agm battery.
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  • cfcw
    cfcw Solar Expert Posts: 25 ✭
    #6
    Re: charging 7 amp battries

    So, I am a little ignorant on this subject too. How do we limit current to the smaller battery, with a resistor? My 7 ah battery recommends no greater than a 2.1a charge rate. I wouldn't mind keeping it about about 2C or 1.4 amp
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #7
    Re: charging 7 amp battries

    I've used a automotive light bulb (as a resistor), the ones for brake lights are good, and may allow 2 amps. Also, if you saved any old sealed beam headlights with one side burned out, the high beam side is good for a couple of amps too.

    Your mileage may vary
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  • cfcw
    cfcw Solar Expert Posts: 25 ✭
    #8
    Re: charging 7 amp battries

    Thanks Mike, I'll try that.....
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