this might explain the Bikes "that have had no problem" directly jumping from a running car...GIVEN their battery IS in good shape.
as in (with my changed highlighting):
Originally Posted by eddiev
My take is this: If you have a healthy battery that is discharged there is no problem jumping from a car battery. The issue is that a car battery has the potential to push out a lot of current. That current is determined by the load, your battery in this case. If on the other hand your battery is defective (a shorted out cell etc.), the load, essentially a short circuit or close to it, now can cause a tremendous current draw from that monster car battery. Even more with the running engines alternator. I think this is were we hear cases of batteries blowing up
All that said, I would not use a motorcycle battery as a car battery.
and I would never connect a car battery directly to a motorcycle battery.
I am a cautious fellow, or try to be. I would first employ, is to place a brake-light bulb in "series" with a car battery (running vehicle or not), to my DISCONNECTED motorcycle battery first, to bring it back from a discharged level. This lightbulb acts as a series resistor and limits current to the smaller motorcycle battery....1/10 of a battery's amphr capacity is considered to be a safe charge rate. more than that would be ok for a short period.
THEN, disconnect that roadside charging mechanism, reconnect the motorcycle battery to motorcycle and try to start it..... if no go, try with the lightbulb still attached.still No go? possibly up the ante to a headlight in series... nogo? Quit.
added...but I would not worry to strap a well-conditioned car battery onto my bike to get home with...or to troubleshoot my system. A well-conditioned car battery is one that has no dead cells and is fully charged ...It will reach the set-point of the motorcycle's regulator when the bike is just using it's headlight, turnsignals etc and not overburden the bike's charging system
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