12 volt charging 6 volt battery

Can a 12 volt 45 watt solar panel be used to charge a 6 volt battery without harming the battery or the solar panel?
Thanks
Bill

Comments

  • BB.BB. Posts: 25,034Super Moderators admin
    Re: 12 volt charging 6 volt battery

    Simple answer... "It depends"....

    Longer answer--there are several issues.

    One is how much current you have going into the battery... Normally, a "generic" battery should be charged around 5% to 13% of its 20 Hour rated capacity... So, if this is an 80 Amp*Hour battery, a "normal" charging current will be:

    0.05*80=4 amps minimum
    0.13*80=10.4 amps maximum

    Now--this is assuming you have some sort of charge controller (turns off the charging current when the battery is fully charged). If you just left 5%-13% current flowing into the battery, you will "boil" the water away over days-weeks (if you don't refill often, it will boil dry and ruin the battery/exposed plates)--and if it is a sealed battery, you probably would cause it to pop a pressure relief and lose water--permanently damaging the battery.

    If, this panel is not connected through a charge controller, you could probably "trickle charge" it at a 1% rate--while keeping an eye on the water.

    Lastly, I assume you are asking the question because of the 12 volt panel connected to a 6 volt battery--In and of itself, there is no problem with this... A solar panel can be connected to any load; down to a dead short--and there will be no damage.

    With a simple PWM 6 volt charge controller (or a direct connected panel), charging a 6 volt battery will reduce the panel's "wattage" by about 1/2...

    For example (just guessing on the numbers). Assume 45 watt "12 volt panel" (actually, Vmp is ~17 volts for the typical panel). So:

    45 watts / 17 volts = 2.65 amps

    Assuming your 12 volt battery charges at 15 volts maximum (assume your controller and wiring has ~2 volts of drop--wasted energy):

    2.65 amps * 15 volts = 39.8 watts (maximum, full noon sun, cool panel)

    And your 6 volt battery charges at ~7.5 volts maximum:

    2.65 amps * 7.5 volts = 19.9 watts (max, full noon sun, cool panel)

    So--the best thing would be for you to use a solar charge controller for your 6 volt battery (even if it is a "cheap" one). And all else being equal, a "cheap" 6 volt charge controller will "waste" about 1/2 of your solar panel's true power output.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: 12 volt charging 6 volt battery

    Great information and response
    thanks so very much
    Bill Cook
  • BB.BB. Posts: 25,034Super Moderators admin
    Re: 12 volt charging 6 volt battery

    You are very welcome Bill...

    By the way, I have a typo in one line... Should be:
    And your 6 volt battery charges at ~7.5 volts maximum:

    2.65 amps * 7.5 volts = 19.9 watts (max, full noon sun, cool panel)
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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