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Join Date: Nov 2007
Location: Chandler, Arizona
With the jumper cables disconnected and the bike running, use a multimeter to check the voltage across the battery terminals at idle. Then rev it up to 3000 RPM. You should get around 12V at idle, and over 14V at 3000 RPM. If not, you definitely have charging system problems. A car battery also has around 12V with the engine off, but a car has a much stronger charging system than any motorcycle. So it is likely the cars charging system that is making the difference.
BTW, it has long been said that it is not a good idea not to use a running car to jumpstart a motorcycle. I never though much about it, but I don't see why not. The cars charging system put's out the same voltage as the bikes, it is just capable of a lot more amperage. And the motorcycle will only draw the amps it needs.
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike