Chapter M-2's Technical Corner

Howard Halasz's June 2008 Technical Article

WHAT ABOUT JUMP STARTING A GOLD WING WITH A CAR BATTERY?

I believe that using a car or truck to jump start a motorcycle is perfectly OK as long as the positive goes to positive and the negative goes to negative.

Where we might possibly get into trouble is when we connect to a diesel truck that has a 24-volt system rather than a 12-volt system. An over voltage to a motorcycle battery can cause disastrous results. However, if you connect to a vehicle with a 12 volt system, the current draw depends on the electrical resistance of the motorcycle's electrical system.

A 12 volt starter motor that initially draws 100 amps (locked rotor) and 25 amps (unlocked rotor) will never draw any more than 100 amps (locked rotor) and 25 amps

(unlocked rotor) if the voltage is limited to 12 volts. A car battery capable of supplying 500 amps will never actually pump 500 amps of current into the motorcycle unless the motorcycle has a dead short circuit somewhere. The car battery will supply only the amps that the motorcycle's electrical system draws and no more.

Here's another example: A lighting circuit in your home is protected by a 15-amp fuse or circuit breaker. Therefore, that lighting circuit is capable of supplying no more than 15 amps. You have a lamp with a 60-watt light bulb. A 60-watt light bulb will pull only .5 amps from that 15-amp circuit as long as the voltage is only 120 volts. Remember: the 15 amps are available, but the voltage, or electrical force, along with the resistance of the light bulb, will limit the current draw to .5 amps. You can plug the lamp in to another circuit that might be capable of supplying 25 amps, but the lamp will still only pull .5 amps from the 25-amp outlet.

OK. Now what would happen if we plug the lamp into a 15-amp 240-volt circuit? The lamp will burn out because the 240-volt circuit is pumping 1 amp into a light bulb that was designed to handle only .5 amps! So I hope you can understand that the voltage is what is important here and not the current capability. The same principle is true when jump starting a motorcycle from a car basttery. As long as the voltage is limited to no more than 14.2 volts, we can safely jump-start our motorcycle from a car battery.

If the internal resistance of the motorcycle battery is so low that it draws an excess amount of current from the car battery, we risk having the motorcycle battery explode in our face. But that can also happen even if we try to jump start it from a fully charged motorcycle battery.

Always connect to the motorcycle battery first. Then if it explodes when you connect to the car battery, it won't explode in your face!! Your own health and safety should have priority over the health and safety of your motorcycle. It's usually best to connect the + side first so that you can connect the - side to any good ground away from the car battery in case it sparks and explodes any hydrogen gas from the battery.

Howard Halasz, Technical Coordinator Texas Chapter M-2

Spring, Texas