Jump Starting a Vintage Motorcycle?

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 good example of when NOT to jump start your motorcycle with a car battery is if the battery is showing its age and trying to die with dignity. If this is the case, the dying battery could have a low enough internal resistance to cause an over current – 150 amps or more of a current draw. Let the battery die with dignity and go buy a new one. Batteries are cheap compared to the cost of repairing electrical and electronic damage on a Gold Wing. Our motorcycle electrical systems were not designed to handle much more than 30 amps for the 4-cylinder Wings and 55 amps for the 6-cylinder Wings.

Jump-starting is a bad idea if there are underlying compression, ignition or carburetion problems that are preventing a normal quick start. Incessant grinding of the starter in the face of a “no start” situation is a highly advanced form of futility. It’s a much better use of your energy to turn your attention to diagnosing the no-start problem rather than throwing yet another battery at the no start. Eventually, excess grinding will fry your starter. Then you will have another problem to solve!

Here’s still another example: We’ve been riding on a rather hot summer day, and we stop to take a little 5-minute break. We get back to our ‘Wing that had a brand new battery put in just before the ride. We hit the starter button, and the starter growls and drags, but it won’t crank the engine fast enough to start it. This is another bad time to jump start the Wing. If the battery is new and fully charged, but the starter struggles, our best bet is to push start the Wing. If you are fortunate enough to have a 1975, 1976, or 1977 Gold Wing, you can use the kick starter. But if you jump start with either a car or a motorcycle battery, you could damage a repairable starter to where it is no longer repairable. When you finally make it home, remove the starter and get it repaired or replaced.

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 no more than 14.2 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 100 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 we 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 battery. 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 provided that our battery is accidentally discharged and our starter motor current draw is between 15 and 25 amps when in operation.

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 battery in case it sparks and explodes any hydrogen gas from the battery.