Not a myth, doing it runs a "Risk" of ruining your charging system, won't happen every time, but it's bad for it every time.

Here's the problem, cars use electromagnets in their charging system to create the field that generates electricity, once the regulator in the car's charging system reaches a certain voltage (say 14.3 or so) it turns off the supply current to the electromagnets which in turn kills the magnetic field thus stopping the alternator from outputing anymore current. They also put out way more wattage than a bikes system.

Since bikes don't use electromagnets in their stator charging system (all the magnets are permanent magnets) theres no way to "turn off" the magnetic field to regulate the bikes current output, instead they have to use a regulator setup that shunts the extra power to ground (say when it hits 14.1 volts or so), the regulators are designed to be able to shunt the bikes max possible output for short periods of time at high rev without burning out, (hence the big cooling fins on the regulator, shunting current straight to a ground generates alot of heat), so far so good.

Here's where the problem comes in, when you hook up a running car to your bike you effectively link the two systems, the car's alternator is going to try to keep its output at a certain lvl by turning the field on and off, the bikes regulator is going to try to keep the bike at it's certain lvl by shunting extra current to ground. Which means if the car is trying to keep it's voltage at a higher lvl than the bikes trying to keep its, could be as little as the car going for 14.3 and the bike going for 14.1 then the cars alternator won't shut off due to the drain of the bike and itself and the bike's regulator will try to shunt the entire car's output, which is far greater than it's designed to deal with, as well as it's own output when it's stator starts to spin, to ground. It's not designed to be able to shunt that much current and the regulator overheats and fries, your effectively running over double the current through the regulator than its rated to handle.

So why is it ok to jump it with the car off you ask? The car batteries voltage when the cars off tends to be under 13 volts. Since the voltage is lower than the bikes regulator is set to maintain it Will NOT shunt the current to ground, won't overheat, and won't pop. The problem only arises when the car's alternator is trying to maintain a voltage higher than the bikes regulator is trying to shunt. Oh, also the battery isn't rated at taking in (charging) amps at the rate that both the running car and the bike are able to generate at the same time, so the battery can boil as well, nifteh huh?

See, it's simple... eh? It's not really a problem of amperge per se. Technically the car battery alone has enough cca's to fry out your bike, its more a matter of the way the regulators on the car and bike work differently and try to deal with all the extra current that's over its voltage setpoint.

Will it happen every time? Nope, will it happen sometimes? Yup. Wanna take that chance?

 

Nice detailed explanation Hangfire ...

well done mate

 

Thanks, took a minute to write that all out. Theres alot of misconceptions about why it's a bad thing to do, or even if it's a bad thing to do. The topic seems to comes up so often it's tempting to just copy that whole explanation to it's own thread and see if I can get an admin to sticky it.