Charge two batteries in parallel?

I am setting up a small system to be used when camping, for charging a pair of 12v marine (size 27 I believe) deep cycle batteries I'd like to connect in parallel and. I have two 43w Kyocera panels and a Xantrex C35 charge controller.

Is it possible to operate as described and achieve a full charge in both batteries?



  • BB.BB. Super Moderators, Administrators Posts: 29,980 admin
    Re: Charge two batteries in parallel?

    As long as the wiring gives near identical voltage/resistance to both batteries, and the two batteries are pretty much the same age/lot/prior use--then they should parallel charge/discharge just fine.

    Regarding wiring a pair of parallel batteries, generally, you either connect both batteries to a common point with equal size/gauge wires--or you, for example, connect the bank + to the first battery plus, and the bank - to the second battery minus--and run two identical parallel cables + to + and - to -... That pretty much insures that both batteries see the same voltage under all normal load conditions.

    Also, a marine battery is typically a hybrid between a starting battery and a true deep cycle storage battery (with advantages and disadvantages of both kinds)... Next time, if this is a storage battery application, you should try to aim at a true storage battery.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Charge two batteries in parallel?

    it might charge, but then that depends on many factors because you're borderline between being able to really slow charge and just maintain. if it does charge it may take weeks of full sun to do it with nothing else draining the batteries. many batteries lose a percentage every month and if your batteries' percentage is high then you may not see the full charge realized. my assumptions are that the pv current is about 5a and the battery capacity would be about 220ah and not discharged beyond 50%. my opinion is that it will charge them, but it may take upwards of a few weeks between losses and cloudy days(maybe never reaching full charge in winter). if it is just a lack of getting a charge into the batteries then by all means use the presented setup and get something into the batteries. on the other hand if the battery is to be periodically used you will need more pvs to charge the batteries without a doubt.
  • BB.BB. Super Moderators, Administrators Posts: 29,980 admin
    Re: Charge two batteries in parallel?

    Niel is, of course, correct...

    The 43 watts of panel is, more or less, just a very big trickle charger...

    When all is said and done, you will get, very roughly, about(assume ~60% over all system efficiency, STC to PTC panel derating, Controller loses, battery losses, possible inverter losses, etc...--actual efficiency would require more information--just a SWAG for starting estimate):

    43 watts * number of sun hours * 0.6 system efficiency = "useful power"

    So, assuming 5 hours of sun:

    43 w*5H/d*0.6=129 watt hours per day...

    If you have, for example, a couple 12 watt lights, a 10 watt boom box:

    129WH/day / (12+12+10) = 3.8 hours per day of use (sunny summer day)...

    This may be enough for your needs, or not... Your choice.

    If you are camping in shade/winter/etc. conditions--obviously less sun.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Charge two batteries in parallel?

    One big problem I have with some of the chain stores here, is that, to make a sale, they will lead customers to believe that all they need do is buy their 15 or 30 watt panel and all power problems are over. Just give them your money and the next time the power goes off, you can run your fridge, freezer lights, you name it, until the grid comes back on. So people not knowing any difference, buy into it, and of course it doesn't work - - so they conclude that "solar is no good".
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