This article was originally posted by Kenneth Murray to the VFR List, and he has graciously allowed me to post it here as a reference for others. It is an excellent post that may be useful to other riders.
Anyway, Kenneth was saying ... "I've jump started motorcycles from car batteries several times and not suffered any ill effects that I know about. Worse than that, I've jumped a dead bike from a *running* car successfully.
Jumping a deeply discharged lead-acid battery will cause it to start producing hydrogen gas. If the concentration of hydrogen in the air space above the battery plates reaches 4% (not a lot), it is potentially explosive. Keep ignition sources away from the discharged battery!
In particular, be careful with the jumper cables, they can produce the deadly spark. Here are the instructions that came with my jumper cables.
(i) Connect +ve of dead battery to +ve of live battery.
(ii) Connect -ve of live battery to chassis of dead vehicle.
(iii) Crank it.
(iv) Disconnect cables in the reverse order.
Note that (ii) puts the last connection away from the dead battery. Please, please take this warning seriously. An exploding battery will scatter acid everywhere, maybe in your face and eyes.
I've heard the horror stories and I've heard advice and opinions both ways. So in an effort to find out the straight dope, I called Yuasa, Interstate, and Hawker Energy Products (used to be Gates).
Gates (800 546 9611) didn't return my call.
Yuasa (800 523 3553) said "we do not recommend jumping a motorcycle battery from an automotive battery." I asked why and the technical rep said something about the car battery being "too strong." I pressed him for a slightly more precise explanation, but I didn't get anywhere.
Interstate (800 541 8419) had a real engineer answer the 'phone. He said as long as both systems (car and bike) are 12 volts, there shouldn't be any problem. I told him about Yuasa's objections and he didn't understand them. In simple terms, the size of the car battery is a capacity. The bike will only draw on that capacity what it wants or needs. That last statement probably needs to be qualified with "if the motorbikes electricals are in good shape and all that's wrong is that the battery is discharged".
I also asked about jump starting a bike from a running car and he said he could see that might be a problem since the voltage at a running car's battery terminals rises and this might be too much for the bike's electricals to handle.
I know next to nothing about lead-acid batteries and that whole electrochemical nightmare, but my background is in electronics engineering so I'm going to indulge in a little speculation.
- I think it is unlikely you'll harm a motorbike by jump starting it from a car battery (car engine OFF). This is the answer everyone wants to hear on Sunday morning when the dang thing won't start.
- The only way you'll damage the bike is if there's a fault in its charging system or wiring. I can imagine if there's a short circuit somewhere on the bike, the car battery with its greater capacity could fry the wiring harness, but the bike's fuses *should* protect you.
- Jumping the bike from a running car is a much bigger risk. Off the top off my head, I could see that the extra voltage from the car could blow the diodes that normally prevent the bike's battery running the alternator as a motor.
The above is only my opinion folks. I kid myself it's an informed opinion, but confronted with lawyers I'll deny everything ;).
If it'll make anybody feel any better, around here (Tucson AZ), car batteries die like flies in the summer heat. When they die, they're DEAD. A jump start from another car won't even turn on the idiot lights. What you do is you call AAA and they send "Bob" in his tow truck. "Bob" has the BIGGEST BATTERIES YOU EVER SAW and that's what's used to restart your dead vehicle. What I mean by this parable is that battery size, physical or amp*hours shouldn't matter. It's the voltage that counts. In the case of the heat-expired cars, the battery isn't simply discharged, often one of the cells has an internal short circuit. This short draws so many amps from the tow truck that even 4 gauge leads (and dirty battery terminals) drop a significant voltage. To compensate, "Bob" revs up the truck. At some point enough current is delivered to blow the short and voila, the car starts. The car battery however is toast. It will hold a charge, but not very well.
I'd love to hear the real bottom line on this subject. Anybody out there?"