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Seeking Dual Car Battery Installation Advice

Discussion in 'Electronic Basics' started by W. Watson, Jun 8, 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?
  2. W. Watson

    W. Watson Guest

    My first problem is to drill a 1 7/8" through the floor. It's fairly soft
    metal. At most 1/4" thick.
  3. Ross Herbert

    Ross Herbert Guest

    There are a couple of companies which are called Valmar. Your switch
    comes from Valmar Marine

    Unfortunately, the above link appears to be obsolete or is non

    In my opinion a simple battery selector switch is not ideal nor the
    preferred method of connecting two automotive batteries in a vehicle,
    especially when they are of different types and capacities as in your
    situation. You should use a Smart Battery Isolator for your dual
    battery setup.

    You might investigate using one of the battery isolators from Newmar

    There are numerous sources of similar items if you wish to investigate
    for yourself.
  4. W. Watson

    W. Watson Guest

    I couldn't reach the URL you found either. The carton shoes Pompano Beach,
    FL.There server may be down.

    Here's an interesting one <>.

    Because of just too many needs on my time, I didn't get to this until
    Tuesday when I ordered the battery box Tuesday. I live in a small community,
    and it took until 1 pm today to get the box. I plan to leave for a 3 week
    trip tomorrow, and difficulty with cutting a hole in the rear area of the
    van has put further work off until tomorrow. I think I'm just going to
    return the battery, box, etc. until I have more time to think this through.
    This looks a bit chancy at the moment.

    I had hoped to use it with my CPAP medical unit on a 3 week car trip into
    some fairly remote areas. There's no threat to me if I don't use it. Last
    year I resorted on a similar trip to getting some AC camp sites a few times,
    but had hoped to get this squared away weeks ago.
  5. Jasen

    Jasen Guest

    for that you could use a hole saw, you may be able to get by with a jugsaw
    with steel cutting blade or a burr in a die grinder, if you remove the
    fuel tank, oxy and plasma become options too.

    OTOH I know some guys with a drill press you could drive under.....

  6. Chris

    Chris Guest

    Hi, Wayne. I'm wondering if you might not just be overthinking this a
    bit. The selection of a marine battery seems to be a good one. I'd
    guess, though, that the CPAP only requires about 2 amps continuous to
    run. Using a switch rated for 160 amps/210 amps starting seems to be
    a little much (the Valmar website is down -- can't give you an
    immediate response on the switch positions).

    How about just switching between the van electric and the marine
    battery locally (at or near your sleeping space) with a standard
    switch? You can minimize the extra wiring, and not have to drill
    holes that way. To be safe, I'd use a 10 amp double throw switch with
    center-off (available at any hardware store).

    Good luck
  7. Chuck

    Chuck Guest

    If you're going to be drawing only 2
    amps for 8 hours each night, your van
    battery should be able to handle that
    provided you do an hour or so of driving
    the next day. If the van battery is in
    good condition, there should be no problem.

    I agree that it't not good to mix
    battery types. But there are other
    options. One is to use a larger (greater
    capacity) battery in your van. The
    larger battery could be a deep cycle type.

    If your concern is that you don't want
    to risk discharging the van's battery to
    the point that it won't start the
    engine, then you could construct a
    simple sensor and relay that would
    automatically disconnect the CPAP when
    voltage dropped to some predetermined
    level. No need to make any mods to the
    van or its wiring. Just a small box
    between the cigarette lighter plug and
    the CPAP unit.

    Or you could use something like this:
    Vector 300 Amp Jump Starter Model 010S -
    Jump Starter

    Simple, versatile, cheaper than a second

  8. neon


    Oct 21, 2006
    how much power is the req?. for your machine a simple diode will isolate the second battery if connected in parallel with car battery. the difference will be allways car battery minus .7 to.9v drop at the secondary battery now both can be charged at the same time but if the extra equipment is left on both batteries will die at the same rate unless you disconnet car battery by a switch.cathode to sec battery with a switch in series.
  9. W. Watson

    W. Watson Guest

    Hi, I left for my 3 week vacation on the 9th, my last post. I was unable to
    really start installing it, so figured I'd do it on my trip. Fortunately, I
    found a much better solution on my way before I tinkered with anything. It's
    called a power pack.

    When I happened into the Tower campground in Yellowstone, I asked the
    campground host if he had any electric outlets I could borrow. One thing led
    to another, and he offered me the use of his Xantera 600W PP. It worked fine
    in that it powered my CPAP unit for 8 hours. One can recharge it by plugging
    it into the car's 12v outlet.


    Although I don't need it, it has a radio. alarm, and a light. Allows for 3
    AC devices. I used it for 3 nights. I think Xantrex makes an ever higher
    wattage one. Amazon has it for a lower price, much lower depending upon how
    shipping figures into it. The campground host told me there might be a
    bigger (wattage) model.

    For Amazon,

    I should be ordering it in a few days. It seems like a very good solution to
    me, and I can use it around the house as necessary.
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