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Seeking Dual Car Battery Installation Advice

Discussion in 'Electronic Repair' started by W. Watson, Jun 9, 2007.

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  1. W. Watson

    W. Watson Guest

    I'm about to configure my 97 Plymouth van so that it has two batteries. The
    second battery will be kept in the rear of the van in a battery box with a
    vent through the floor. I'm using a Mega-Tron RV-Marine Deep Cycle 12v
    battery, which will provide power to a medical device at night. I bought a
    Valmar (Model 31122) battery selector switch. The switch has four settings:
    Off, 1, All and 2. I suspect that 1 means batter 1, and 2 battery 2.

    I have a pretty fair idea how to install it ignition and ground wise. I know
    the difference between +/-, for example. I'm going to need some wire to go
    from the rear to front under the chassis. Does #10 seem right? I'll try to
    keep it tight and away from the fuel tank. It seems best to put the switch
    inside the engine compartment. I think I'll be able to drill a 1" or so hole
    through the floor in the rear of the van. Maybe I'll call the Plymouth and
    ask for advice on that. Any suggestions or comments on all this?

    I plan to put a 30A fuse inline.
  2. Your best source for wire, connectors, fuse holders, etc. would be to
    maybe buy one of those amplifier installation kits.
    Get the heaviest duty one you can get.
  3. Yukio YANO

    Yukio YANO Guest

    a 30 Amp fuse seems to be a little on the small size as does the 10
    gage wiring.

    Check with a RV Shop on wiring size !!! or else use the same gage as the
    battery cables already on the car.

    the 30 Amp fuse might be suitable for your load but not for the charging
    side of the circuit.

    I am a little suspicious of a Dual Battery setup without Isolation Diodes.

    Usually you would expect to be able to Start the engine using the
    auxillary Battery in an emergency, hence the need for heavier cables
    than 10 gage.

    Yukio YANO
  4. Arfa Daily

    Arfa Daily Guest

    I'm no expert on this, but I do know that it's not as simple as just having
    two batteries that can be switched between each other. As Yukio says,
    there's normally a dual charge control box, which does indeed contain at
    least isolation diodes. My next door neighbour went through all this when he
    converted a van to an RV for touring round Europe. I think it would be wise
    to take professional advice from a reputable auto electrical shop on this
    one, or at least post the question on one of the auto repair groups that
    might have a genuine expert in this field.


  5. Meat Plow

    Meat Plow Guest

    The batteries also have a different charge rate.
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  6. PeterD

    PeterD Guest

    Good idea? Humm...
    For what? Charging? Probalby not large enough. I'd suggest 4 AWG (both
    positive and negative, don't use the chassies for ground).
    Define 'tight'...
    If you are not going to use the switch, why bother installing it? You
    going to open the hood each time you use the battery? I doubt it. Also
    I suspect the switch, and your understanding of how it works, and what
    it does, is flawed. You should have a battery management system, not a
    simple switch.

    BTW, Valmar's web site is dead so I can't look anything up there.
    OK, but use proper installation practices in running the cables.
    Bwah-ha-ha-ha... They won't know, or care. Don't waste your time.
    Yes, you are going about it without considering everything. BTW, if
    this 'medical' equipment is life support/life critical, I'd recommend
    you get professional help.
    For what? That dinky 10 AWG wire? It will blow the first time the
    battery needs a complete charge (and the 10 AWG wire would also fail).
  7. neon


    Oct 21, 2006
    i think i allready told you what to do. one battery for extra equipment tie the anode of a diode 30amp or more to car battery put a switch in series to disconnect car battery from secondary battery close it when driving to charge the secondary battery the battery if totaly discarged can charge at a big rate so the wires must be 12 gauge or less.
  8. Jim Yanik

    Jim Yanik Guest

    Or check with an RV group.
  9. James Sweet

    James Sweet Guest

    Years ago my family had a sailboat with dual batteries and my grandparents
    had an RV with a similar setup. In the boat, there was a switch which was
    wired to each battery with very heavy cables, it could select 1, 2, or both
    in parallel. Only the selected battery would be charged or drawn from. The
    RV had a similar switch, but charging was accomplished through an isolator
    which was nothing but a couple diodes potted with a large heatsink. Both
    were charged simultaneously but the switch selected which one loads draw

    30A should be adequate for charging the battery, but it has to be heavy
    enough to handle both the charge current and the load current. Either way
    talk to RV or boat guys for solid advice on this one, it's a very common
  10. GregS

    GregS Guest

    Separating the batteries poses problems. If the battery is cranking the the starter,
    you really can't use a fuse. Thats the way its been done for a long time.
    Ordinarily I recommed using batteries in parallel. Draining down a battery
    reduces life, and with two batteries, there is less sag, improving overall
    performance. With the widely separated batteries, using a diode steering
    unit is going to be mandatory. In any case, install a battery close fuse, unless it
    cranking the starter, and that lead should be as short as possible. There are
    race cars and other vehicles using a long starting lead, but its
    not common.

  11. mike

    mike Guest

    Use a diode isolator. Avaialable at any rv store or auto parts store or
    target or wal-mart or K-mart or most any place else.
    Switches are a bad idea 'cause eventually, you'll forget and the car
    won't start. You also don't want to switch a dead battery across a
    charged one. Wears the swtich out big-time...or blows fuses.
    Depending on how fast you want to charge the battery, #10 may limit the
    current below what you want. Do the math on wire resistance.
    Don't forget that sharp metal cuts insulation. Grommet the holes.

    Be aware that the hydrogen released during overcharge won't go out the
    floor vent. It'll accumulate in the top of the box. Rethink your vent

    Are we having fun yet?
  12. Guest

    With respect to you, your life and all the other answers here, keep a
    couple of things in mind:

    Lead-Acid storage batteries contain HUGE amounts of energy, surrounded
    by nasty acid and generating explosive gases. Do not trust half-assed
    solutions or take half-assed advice (including mine) without several
    pounds of finely ground salt.

    Then: Do the set-up right. Use a purpose-built automatic isolation
    system. This is one:


    There are more than a few others.

    Presumably, these systems are designed to do exactly what you want the
    way you want it. And, presumably they will not melt down you or the

    As to venting: Obtain one of the newer batteries with a collective
    vent to a hose that you may direct as you wish. My Volvo has the
    battery under the floor in the rear, my '87 VW camper has it under the
    right front seat, the other '99 VW camper has its second battery
    underneath the rear floor (right under the head of the bed). All of
    them are connected to such a venting system.

    By the way, the battery cables on the Volvo and both VWs are massive,
    I would guess the metric equivalent of #4 stranded, at least as thick
    as my index finger. The second battery on the '99 camper is a marine-
    type and only to run the housekeeping systems while the engine is off.
    It charges off the engine as well as a built-in inverter if there is
    line-power available. Even though it does not serve as a back-up
    starting battery, the cables to it are massive as described.

    Peter Wieck
    Wyncote, PA
  13. jakdedert

    jakdedert Guest

    And the venerable MGB has a pair of 6v batteries (in series) under the
    back package shelf...properly vented.

    I think it's useful to go back to the OP which states that the second
    battery in his application is NEVER to be used for starting the vehicle.
    That's all well and good, but several posters apparently missed it.

    Consequently, a '0' or '00' cable in his case is probably overkill. The
    cable would be sized based on the output capacity of the charging
    system on his vehicle vs. the expected voltage drop in the length of the
    conductor to the second battery. A look under the hood of most vehicles
    confirms that that the wire run from the alternator to the battery is
    rarely sized above 12ga or so.

    That's not to say that a complete rethink of the OP's original setup is
    not in order. IMO, either of the above setups would be far superior to
    what he originally had in mind. OTOH, a properly engineered system
    could be designed to do exactly what he envisioned: provide a clean,
    capacious source of 12 volts DC to run *outboard* equipment...said
    source to be replenished by the vehicle charging system.

    The OP apparently does not have the requisite knowledge or capability to
    design such a system, however. Unless that expertise can be purchased
    locally, I'd suggest he go with something like the above. What he wants
    is theoretically possible, but too difficult to design safely from a

  14. Guest

    When I read "medical equipment", the term "overkill" leaps immediately
    to mind. Followed by "reliablity" and "caution".

    Peter Wieck
    Wyncote, PA
  15. jakdedert

    jakdedert Guest

    Granted, thus my recommendation for local examination of the situation
    and competent local help. This is not a case where a good determination
    can be made remotely.

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