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Current reversal in 2 connected car batteries?

Discussion in 'Electrical Engineering' started by Alex Coleman, Sep 13, 2006.

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  1. Alex Coleman

    Alex Coleman Guest

    I sometimes have to jump start my old car when its battery is too weak
    to start it.

    Some people say I should connect the "donor" battery to my battery and
    then to LEAVE IT FOR A FEW MINUTES so that my weak battery takes in some

    But wouldn't this result in the donor battery forcing a current through
    the weak battery such that it flows through the weak battery in REVERSE?

    If so, then would this result in any damage to the weak battery?
  2. Tim..

    Tim.. Guest


  3. Peter

    Peter Guest

    yes, it will "force" current through the battery. The technical term for
    this is "CHARGING"
  4. Some people would say you should get a new battery...

  5. Dieseldes

    Dieseldes Guest

    The reason I leave the jumpleads for a minute or so after jump starting, is
    to make sure the flat battery actually gets some charge, if it was
    disconnected too soon and the battery was well knackered, the alternator
    may overvolt and may (at the best) just blow bulbs but worst case scenario
    may blow something expensive.....

    Do not even concern yourself with reverse discharge, the internal resistance
    of a battery will be higher than the resistance of a pair of jump leads!!

  6. David Taylor

    David Taylor Guest

    Yes, that would be how your weak battery takes in some charge.
    No, that's how batteries work (rechargable ones at least).

    You put charge in, then take it out.
  7. Roy Lewallen

    Roy Lewallen Guest

    And never, ever, disconnect a battery while the engine is running.
    Google "alternator load dump" to learn why.

    Roy Lewallen
  8. Ian Dalziel

    Ian Dalziel Guest

    Sure - but I've done what he said, take the vehicle battery out of
    circuit till the engine's running, loads of times. You just have to
    have a system you can trust, or trust yourself (unlikely), not to put
    both batteries out of circuit at the same time.
  9. Roy Lewallen

    Roy Lewallen Guest

    Yes, sorry for not being more precise. Disconnecting a second battery
    shouldn't be a problem as long as the main battery remains connected to
    the alternator.

    Roy Lewallen
  10. Roy Lewallen

    Roy Lewallen Guest

    That's an interesting observation, and good question. One thing that
    comes to mind is that it's easier to protect against negative transients
    than positive overvoltage transients -- a reverse connected diode will
    take care of the negative ones without much power dissipation in the
    diode. A zener is a simple possibility for positive transients, but it
    has to be sized carefully so it never comes on under any normal voltage
    condition but still conducts at a voltage below that which would harm
    the circuitry. And it would dissipate a good deal more than the reverse
    connected diode. Of course, there are also other ways of dealing with
    the problem.

    It is true that the automotive power system is a tough environment for
    electronics, and maybe load dump presents no added danger to circuitry.
    But I think I'll keep my battery terminals snug anyway.

    Roy Lewallen
  11. Roy Lewallen

    Roy Lewallen Guest

    No, reverse bias is the normal mode of operation for many
    semiconductors, including the transistor collector-base junction, and
    rectifier diodes which are reverse biased for most of the AC cycle.
    What's potentially damaging to some semiconductors is reverse biasing at
    a voltage high enough to cause breakdown. If done repeatedly to the
    emitter-base junction (which is normally forward biased), it can result
    in reduced transistor hfe. Of course, since substantial current flows
    when breakdown occurs, there's also the possibility of complete
    destruction, e.g., melting of the silicon, due to heating. Zener diodes
    are intentionally operated in this mode, and the designer has to make
    sure they don't get sufficient current to be damaged.

    In contrast, forward overvoltage is difficult to achieve. Because of the
    exponential I-V relationship of a forward biased diode junction, it
    takes a great deal of current to raise the forward voltage much. That's
    why a reverse connected shunt diode makes an effective limiter for
    negative transients. (Negative transients forward bias it.) There is the
    possibility that some junctions in the device to be protected will turn
    on at or near the same voltage as the snubber diode, but they're likely
    to be current limited in some way. If not, a hot carrier diode, with its
    lower forward voltage, might be used.

    The best approach is to use devices like ICs which are specifically
    designed to survive in the automotive environment -- where another very
    significant hazard to electronic components is potentially extreme

    Roy Lewallen
  12. Pretty much, though I've heard the word. Webster says it's
    "English Slang":
  13. Ian Johnston

    Ian Johnston Guest

    : But wouldn't this result in the donor battery forcing a current through
    : the weak battery such that it flows through the weak battery in REVERSE?

    Yes. This is known as "charging".

  14. Common usage here in Australia. Surely it derives from the
    ultimate fate of horses: gone to the knackers yard to be cut up
    for pet food, etc.

  15. DervMan

    DervMan Guest

    Mostly. My American family find the term most amusing.
  16. Bill Darden

    Bill Darden Guest

    Hi Eric,

    This assumes negatively grounded 12-volt vehicles and that the donor
    vehicle's charging system and battery are good. Unless you suspect
    that the dead battery has a shorted cell, a reversed cell or is
    frozen, I believe that it is better to start the donor engine first
    and then connect the donor battery to the dead battery (with the last
    negative connection going to the frame rather than the battery post to
    less the possibility of an explosion). I suggest allowing the donor
    charging system to run at "high idle" for some time to partially
    recharge the dead battery before attempting to restart the dead
    vehicle. The reason is that if the donor battery is not fully
    charged, you could end up with two stranded vehicles. I also
    recommend that the battery that was jumped be fully charged with an
    external battery charger, surface charged removed, and capacity tested
    to check for latent damage, especially wet "Maintenance Free" (Ca/Ca)
    car batteries.

    Please see Section 6 in the Car and Deep Cycle Battery FAQ on for more information on jumping car batteries.

    Kindest regards,

  17. DervMan

    DervMan Guest

    Only an arrogant Brit think that it's "their" language and should deem who
    can or cannot say it, eh?
  18. SteveH

    SteveH Guest

    English and Merkin?
  19. SteveH

    SteveH Guest

    English = Language of the country England.

    It's our language and we'll make the decisions over who can and can't
    call themselves 'English speaking'.

    Americans don't fall into that category.
  20. Sandy Nuts

    Sandy Nuts Guest

    I thought you lived in Wales? :)
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