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Can a car battery be kept charged with another car battery?

Discussion in 'Electronic Basics' started by billcalley, Jun 20, 2007.

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  1. billcalley

    billcalley Guest

    Hi All,

    I have a car that is not located near any 120VAC power, so I
    cannot use a normal trickle charger to maintain my car's battery at
    full charge (the car sits for long periods). But can I use another
    (fully charged) lead acid car battery, or perhaps even one of those
    portable car jump starter units (like the Black and Decker VEC010BD
    300 AMP JumpStarter), to keep my car's battery fully charged? Every
    week or so I would bring home the "charger battery" for a recharge,
    then hook it back up again to my car's battery through jumper cables.
    Is this possible, or are there some major pitfalls?


  2. AndyS

    AndyS Guest

  3. AndyS

    AndyS Guest

    Andy writes:
    Sure, but there are a couple of tricks.

    1) Use the portable battery to run a small inverter that will boost
    the output
    to a couple of volts higher than the portable battery voltage.
    Then use
    this higher voltage to charge the fixed battery , either thru a
    resistor or
    some time of current limiting regulator.

    2) Use and additional portable battery in series with the first
    battery to provide a higher voltage, and proceed as above.

    3) The simplest way, however, is to just connect the two batteries
    parallel with a 1 ohm resistor in the positive leads. When the
    battery is "freshly charged" it will bring up the charge in the
    battery, slowly. ( The one ohm is to limit the current so
    there's not
    any gassing). The portable battery will need to be charged
    and this is only a trickle arrangement. But, if that's all you
    need, it
    is very simple.

    Perhaps there are other suggestions, such as buying a solar array
    will put a few hundred mils into the fixed battery. That would
    work OK as
    a trickle maintainer, but remember, it will only work for a few
    hours every
    day..... The internet will give you the average daily "full sun"
    numbers for
    various location. In Dallas, for instance, "full sun" only
    averages 5 1/2
    hours per day. So don't use "peak sun" number for the array
    without factoring this in.

    Just a couple suggestions. I'd be interested in reading some of
    the other
    replies...... Lots of creative people on this group.

    Andy W4OAH in Eureka, Texas
  4. Jim Thompson

    Jim Thompson Guest

    The most efficient would be to simply swap the batteries.

    ...Jim Thompson
  5. no_one

    no_one Guest

    or just bring the fresh battery when you actually need to start the car;
    consider disconnecting the in car battery to reduce "nuisance" loads from
    the car.
  6. GregS

    GregS Guest

    I don't think you can charge one with another battery. The charged battery will
    only be 12 volts under load. You need 13.5 volts to charge normally.
    Any time a battery looses a great deal of power, it permanately looses power.
    It will not last. Jump starting is going to be very hard on the battery. You can
    use a battery with increased cells to charge a battery, or use two 12 volters and a
    regulator to deliver a lower charge, longer charge time. There is a FAQ or two
    on batteries which you shaould read. You can keep a battery trickle charged with a
    solar unit, which is what i should have done last winter. i have a very small
    solar unit, thats capable of keeping a battey topped off.

  7. Wimpie

    Wimpie Guest

    Hi Bill,

    I think you can maintain your battery at a reasonable voltage to avoid
    damage, but I don't think that you can give it a full charge or
    maintain a full charge. You can use a head light bulb in series with
    the positive lead to limit the charge current. Make sure that the
    battery that is used for charging is in good condition and fully

    At low temperature the battery to be charged will only accept charge
    when the voltage is higher then the normal room temperature voltage
    (about -30 .. -50mV/degr C. for a 12V battery). So to avoid sulfation,
    you need to charge with higher voltage then your "charger battery" can
    give. So I agree with Andy to use a step up converter when you may
    expect low temperature conditions.

    You may search on "lead acid battery maintenance" for having a table
    with charge voltage versus temperature and/or battery voltage versus
    capacity left.

    If you have regular access to the battery, you may take it home, give
    it a full charge (at room temperature) and put it back. At low
    temperature, the self discharge is not that high for a f r e s h
    lead acid battery. You can leave it for about 6 months after giving it
    a full charge.

    Best regards,

  8. Jim Yanik

    Jim Yanik Guest

    Get one of those SOLAR panels that plug into the cigarette lighter
    socket.They are intended to keep a car battery charged,during storage.
  9. billcalley

    billcalley Guest

    Thanks guys -- your recommendations are great stuff! I'll have to
    look into which way to go next.

    Very Best Regards,

  10. Jasen

    Jasen Guest

    if the car's outdoors get one of those solar charging panels.
    they cost about the same as the battery and there's no maintenance.

    or you could take the battery out,

    or run a 12V cable (with a fuse at the car end) to somewhere where
    you can have a charger....

  11. Jamie

    Jamie Guest

    You need slightly higher voltage than that battery you're attempting
    to charge.
    why not set up some 14..16 volt solar panels to keep the car's battery
    charged ?
    I know some one that does that now with their camper vehicle at a camp
    sight that does not have power all year round. it seems to work very
    nicely. He run's a low power 2 meter repeater from it.
  12. Al

    Al Guest

    Get two 12V motorcycle batteries. Why? smaller, lighter. Hook them in
    series to give you 24V. Get two 12V lamps, say rated at 1 amp, hook them
    in series. Now connect the minus of the 24V pack to the minus of the 12V
    battery. Hook the two series 12V lamps between the plus terminals of the
    24V pack and the 12V battery.

    If your 12V battery is shorted, the worst that can happen is the two 12V
    lamps will light normally. This limites the current to, say, two amps.
    Otherwise the 12V battery will have a 1 amp or so trickle charge from
    the 24V pack. The amount of charge will be indicated by the brightness
    of the two bulbs. The capacity of the two smaller batteries must be
    sufficient to recharge your 12V battery to your satisfaction.

    When you get home, recharge your motorcycle batteries.

  13. z

    z Guest

    Would work best if the "charger battery" is like an enormous truck

    I've tried using the little portable items with a car that isn't
    driven frequently enough to charge the battery, and in the beginning
    when there was enough juice to start the car, for some reason just
    driving it around even for an hour on the highway wouldn't cause the
    car battery to fully charge, and eventually the car battery got low
    enough that the portable thing couldn't put it over the top to start
    the car.

    Now I know what you're thinking, that the car battery had just died,
    because that's what I thought too; but I decided to give it one more
    chance and connected a little 5 amp Sears charger up for two days
    before the current meter looked like zero, and the battery has been
    able to start the car fine even with up to two weeks dead time (as
    long as I dared leave it) even in the winter. ??

    Oh yeah, I had similar bad results with one of those little solar
    panel chargers that sit on the dashboard, at least in the winter.
    Summer, it seemed to do better.
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