finer points of rigging 12v batts in parallel?

Discussion in 'OnBoard Electronics & Controls' started by Squidly-Diddly, Dec 18, 2017.

  1. Squidly-Diddly
    Joined: Sep 2007
    Posts: 1,823
    Likes: 143, Points: 63, Legacy Rep: 304
    Location: SF bay

    Squidly-Diddly Senior Member

    Parallel Batteries - Incorrect Wiring by Compass Marine How To

    image says something about 'bank imbalance' if batts are rigged a certain way from strongest to weakest

    Is 'imbalance' good or bad? Sounds bad, but maybe its good for one type of use and bad for another?

    How much diff does it make?

    I've got two possible Parallel Batt projects in mind.

    1)Adding an extra Battery or 2 to normal car, so I can power a laptop for extended periods via 12v to 18v purpose built laptop adapter, and not have to worry TOO much about a dead battery, and recharge as I'm driving around. Yeah, I hear "don't use car's alternator for charging" but this wont be all the time. Any reason why 15amp rated wires wouldn't do the job of connecting the extra batts to primary?

    Laptop adapter will be run off factory cig lighter.

    2)Using two standard car batts to run small electric trolling motor on small runabout, and power running lights.

    Yeah, I know auto batts aren't meant for this sort of duty, but due to "bad luck" I'm the proud owner of 3 extra fairly new car batteries I've got no other use for.
  2. Ike
    Joined: Apr 2006
    Posts: 2,574
    Likes: 403, Points: 83, Legacy Rep: 1669
    Location: Washington

    Ike Senior Member

    #1. First, paralleling your car battery not a good idea, especially if the car is still under warranty. Do that and warranty goes bye bye.

    Anyway. Bank imbalance: that is putting batteries that are not the same in parallel. By the same I mean identical amp hrs, and CCA ratings. If you put a smaller rated battery in parallel with a bigger rated one, the bigger will charge the smaller and then you will have a pair of batteries that cannot reach full charge. not a good thing. Paralleling starting batteries is risky because you are increasing the number of amps released when you start the car. That may just burn out your starter or the starter relay.

    If you want to use a battery to run your laptop, don't run it out of your cig. lighter socket. I've burned up a couple of cig./lighter sockets that way. Use a small inverter off a separate battery and just plug your laptop into the inverter. Use a smart charger to charge the battery at home or where you can plug into normal AC.

    Take a look at the wires coming off your car's battery. There is a reason they are so big (between 1 and 4 ga) When you turn on the ignition hundreds of amps flow through those wires. 15amp wire would fry in an instant.

    #2. No reason why you can't parallel two batteries to run a trolling motor. Use heavy gage wire, 1 or even 0 gage, But it will deplete starting batteries far faster than deep cycle. But if you can live with that it should work fine.
  3. jorgepease
    Joined: Feb 2012
    Posts: 1,620
    Likes: 51, Points: 48, Legacy Rep: 75
    Location: Florida

    jorgepease Senior Member

  4. Barry
    Joined: Mar 2002
    Posts: 1,600
    Likes: 347, Points: 83, Legacy Rep: 158

    Barry Senior Member

    Adding another battery will not void your warranty
    The Magnuson-Moss Warranty Act prohibits this in the US
    But if the dealership can prove that an aftermarket accessory or modification caused a specific problem, then only that part that was damaged from the accessory can be denied warranty
    Say you install a 6 inch lift kit in your pickup and the u-joints keep failing due to the increased angle of the drive shaft. The u joints would not be covered. If the engine self destructed, the engine would be covered. The dealer has to prove that the accessory that you installed caused the problem.

    Putting another battery in parallel will not increase the amount of amps that the starter battery will use when starting your vehicle. The starter motor will draw only the amps at 12 volts that it requires to deal with the load. No difference with having a 350 CCA battery or a 1000 CCA battery, the resistance/load of the starter determines the draw. (assume fully charged batteries)
    In fact having two batteries in parallel will mean that you have twice the CCA available and that the amp draw will be more constant than a single battery.
    When you turn the starter on, you might see 250 amps, (somewhere above 200 plus), if the car does not start right away, the voltage drops in the battery as the voltage drops the amps goes up as the starter is designed to work around x amount of watts. A watt is a volt times amp, so the voltage drops the amps increase. If you have lots of amps available, ie CCA, then the amp draw
    will be more constant as you have a larger energy source to draw from

    I think that every Ford diesel that I have owned has had two batteries in parallel, I suspect the same for Chev and the other OEM,
    Higher compression ratios, bigger displacement, two batteries required

  5. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    1) is not a good idea. During starting both batteries supply 1/2 of the 200 Amps your starter needs. 15amp rated wires will heat up very quickly.
    If you absolutely need 2 batteries, make sure the wires can handle the current involved.
    You can use the cig. lighter socket if it's a good one. My Fiat van has 2 sockets rated at 35amps, which is an exception.

    2)Use a 'star' configuration with fuses for each battery. Read the 'exploded battery thread' of this forum for an explanation.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.