Why is jump starting bad?

Discussion in 'General Harley Davidson Topic' started by krikket, Oct 14, 2008.

  1. krikket

    krikket Active Member

    327
    26
    0
    I've been trying to figure out why jump starting a motorcylce from a car is such a destructive thing? Here's why I question it:

    1- The battery voltage on a car battery (fully charged) is around 12.6 volts... the same as a motorcycle.

    2- The charging system in a car keeps the voltage around 13.5 dc(+/- 1 volt)... the same as a motorcycle. Any ac voltage (usually 1 volt or more) indicates a bad diode in the charging system

    3- The current draw when starting is very large, but will only draw what is required...Ohm's law

    4- Electricity follows the path of least resistance, which would be your starter at the time of starting.

    With all that being said, I still feel like I'm missing something because I've read it sooo many times in other threads, blogs and heard it from many people. Can anyone clarify?
     
  2. NEWHD74FAN

    NEWHD74FAN Experienced Member Retired Moderators

    8,176
    98
    16
    Your points are well made, and jump starting a Harley from an auto is by itself not bad, just hazardous because finding a good connection for the cables is the real problem and the fact that so much current is concentrated in the center of the bike and the fuel tank is so nearby, the vibration is so high as to unseat the cable clamps, etc...so just be careful. :s

    Also, the need to jump a bike is resolving a symptom and not the cause, which should be investigated rather than masked! :newsmile105:
     
  3. krikket

    krikket Active Member

    327
    26
    0
    Obviously, if the battery's dead you should find out why... no arguement there!! I'm just questioning why people say it's a no-no.
     
  4. NEWHD74FAN

    NEWHD74FAN Experienced Member Retired Moderators

    8,176
    98
    16
    Basically the jumping of a Harley is okay, but can be hard on the battery (it is a gel type made for high short term delivery and always with a surface charge) as Hobbit states. It is just hazardous, your HD manual even shows you how to do it, so it is not BAD par se...or as advised by others, as is mine, they are just personal opinions.

    I use a jumper cable that has a middle disconnect plug and polarity LEDS on it so I can connect both sides securely first with out spark/shorting hazard (I even showed my gal how to do it just in case), then plug the center one together away from the vehicles. This type cable is especially handy because on our Honda CRV, finding a good ground in the bowels of the engine compartment is really tough to find (modern vehicles have so much plastic shrouding hardware it is tough to even see the engine).

    Also remember your Harley probably has a lot of chrome, and small jumpers to ground the rear swingarm and frame, along with a high delivery battery, cables and starter tightly confined, gas tank nearby and plenty of vibration when you are cranking and starting with hi current or heating at the clamps which can discolor chrome or arc if your connection is not good.
     
  5. glider

    glider Veteran Member

    25,464
    108
    44
    Aside from the points brought out above, consider the amperage of the car battery verses the bikes battery and the fact that you would be hooking up a car battery with a full charge and high amps to a pretty much dead bike battery. Just consider the difference in size. :s
     
  6. hntwrobin

    hntwrobin Active Member

    236
    26
    0
    :sIt's ok to jump it if you have to but make sure if you jump from a car that the car is not running, that's where the problems start.
     
  7. krikket

    krikket Active Member

    327
    26
    0
    I think I get it now. Here's an more technical explanation. When you charge a battery, the liquid inside (a mix of sulfuric acid and de-ionized water) gets hot and can boil from the reaction with the lead plates. In car batteries, you can check the water levels (on some) but a big tell-tale sign that a battery has been over heated is by the sides of the battery...they bulge from the pressure. A little motorcycle battery PHYSICALLY cannot handle the strain which can crack or sulfate plates (coat the plate in a layer of sulfur, reducing the available lead for chemical reaction). It should be slowly brought back up to charge to keep battery heating under control via a trickle charge.

    You'' have to forgive me. I'm an auto mechanic turned electrician. Every time I heard someone say "jumping a bike off a car is bad" it didn't make any sense... electrically. If I ever had to jump a bike, I would definitely do it with the car in the not running.
     
  8. wildspirit97

    wildspirit97 Senior Member

    1,088
    46
    0


    I won't give you anymore reasons why not but not having the car running is a good choice. A good friend of mine had to replace the computer on his bike because he jumped it with the car running. I can't tell you why only that it happened. Fortunately the dealer replaced it under warranty because they're the ones that said it would be ok to jump it with a car.
     
  9. gs34

    gs34 Junior Member

    639
    28
    5
    Krikket, contrary to some beliefs, bigger is not always better!
    Since you're an old auto mech, turned electrician, you'll understand this little tale; In the heavy equipment busness, when the batteries went dead on a dozer, loader, etc. It was common practice (in the old days) to fire up a welder and use the DC voltage as a source to jump start the machine.
    Worked fine on the old stuff with not much more than a starting system and lights.
    However, when the age of computer electronics came about, the 1st person who tried that old trick just cost his employer several thousand dollars in fried electronics.
    That welder had the same DC voltage, however, the amperage output was way more than the systems on the equipment could stand.
    Same same car/truck to bike.
    And, the safest way to jump anything is not battery to battery, but from the good battery directly to the starter of the vehicle you are trying to start, and the negative to a suitable ground.
    If you were ever around a battery that exploded, you would understand why.
     
  10. Romain

    Romain Active Member

    228
    26
    0
    This might be irrelevant to the thread but, if the bike battery is flat, is it possible to push start, as was done with racing bikes instead of jump starting?