A standard motorcycle battery will give you enough amperage to weld with for at least a few seconds. Accidentally drop a wrench across the terminals and you'll see what I mean. (Do NOT actually do this on purpose; preferably don't do it on accident either, especially not while holding onto the wrench, hurts like a mother------ and you'll have to dig the wrench out from under the workbench where you ended up throwing it.)

In the amount of time you have a bike's battery connected to a car battery for jump starting you're unlikely to seriously damage the battery even though the car battery can pass a lot of amps.

The issue, as cHarley tried to point out and as I took entirely too long to understand, is the way the voltage regulator on our bikes works (a necessity when using a permanent magnet for the alternator instead of a field coil).

When you connect a completely charged car battery to the system, you're getting just under 13 volts with the potential for several hundred amps. With the car's engine running, you're getting ~14.5 volts with the potential for "only" about 150 amps in most cars.

With the bike, the regulator is designed to shunt excess current to ground, which builds up a lot of heat in the regulator (which is why the voltage regulator for a bike is big, heavily finned, and placed in an area with good airflow). If the regulator is designed to shunt as much current as necessary to keep voltage at a steady 14.1 volts, then, with just a battery connected it won't be shunting any more than normal because the battery's voltage will be lower than the regulator's threshold.

However, if the car's engine is running, the car's electrical system is now sitting at 14.5 volts, higher than the bike's design voltage. This isn't enough of a difference to kill circuits immediately, but it does cause the voltage regulator on the bike to attempt to "bleed" those extra 0.4 volts by shunting a bunch of current to ground, including the current coming in from the car's system which, even at "only" 150 amps from the car's alternator, is more than the 30 amps that the regulators in the '07+ Sportsters are designed to handle.

For a couple seconds that may not hurt anything, but there's a real risk of overheating the regulator in this situation.

Now, if the bike used an alternator with a field coil instead of permanent magnets, this would not be an issue because the regulators for those systems work differently (they shut off the field coil when there's excess voltage which basically just turns off the alternator, so instead of "dumping" excess current, they just attempt to stop the production of excess current in the first place). This was why I was initially confused. I was/am familiar with the alternators in cars that use field coils, but hadn't realized how different the permanent magnet alternators were.

 

my thought's exactly.

 

My question is why? To charge the battery, yes. To test the battery, yes. To go for a ride, NO!

 

The original question as asked doesn't state where the car battery is. I have a spare car battery in my garage that I keep charged. I have used it to aid in starting a Sportster during cold weather when the bike battery got weak.

 

In an emergency, I would do it.
But, the jumper car should not be running and once the bike starts remove the cables ASAP.
This should be just like using a booster battery.

But if at home, tinkering, I would charge the battery, disconnected from the bike.

I did not notice any replies above of real world experiences.

Good luck.

 

What do you mean by that?

I have jump started a Sportster with a car battery. Since then there has been no ill effects on the bike starting since as I did it last winter.

​​​​​​​Is that a real world experience?

 

I have also jump started my Sportster from a battery that happened to be in a car . This was an emergency that occurred on a campsite in Scotland when rain got into my electrics and the bike switched itself on in the night . I followed the instructions in the manual . Connect up and wait ten minutes or so then hit the button . The bike started so I grabbed my helmet and went for a memorable ride up and down the road that runs through Glen Coe in the setting sunshine . Marvelous .

 

I thought this thread was about jumping a Sportster with a car..a la Dukes of Hazzard...

 

Seems the argument is still raging after 9 years.

 

Never went away, just woke up !