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Running bike off car battery - any risks?

Andrew Vanis

Andrew Vanis

Forum Mentor
Past Site Supporter
Running bike off car battery - any risks?

Outside of the ?keep it original, that?s how they designed it? theme, what considerations are there to using a good-condition car battery that is fully charged as the bike battery while working on the running of bike?

Yes, I understand that I might not be able to fully test the charging system with the car battery in the loop but if I can get the bike to run well, then I'll buy the battery.

I am asking because I?m in the middle of a re-build and want to make sure the bike runs well before buying a dedicated battery.
 
As long as it is a stand alone 12V battery it shouldn't make any difference.
 
no problem at all....

if hooked up to the car, just make sure car is not running

test away.....

.
 
good to hear and glad it is so straight forward.

I didn't expect it to be so easy but since it is...it begs the question, should I be able to test the stator etc. with the car battery in place?
 
Alright. I've read this over and over. "Make sure the car isn't running." What's the difference? 12 volts (nominal) is 12 volts. Why does the bike battery/electric system care if the car is running or not?
 
slayer61 said:
Alright. I've read this over and over. "Make sure the car isn't running." What's the difference? 12 volts (nominal) is 12 volts. Why does the bike battery/electric system care if the car is running or not?
Click to expand...

Because you can't have the two charging systems fighting each other. You could very likely fry one or both.
 
slayer61 said:
Alright. I've read this over and over. "Make sure the car isn't running." What's the difference? 12 volts (nominal) is 12 volts. Why does the bike battery/electric system care if the car is running or not?
Click to expand...

Yes, a nominal 12 volts is a nominal 12 volts, but that is not what a car puts out when it is running.

Most cars put out 14+ volts, and have alternators that are capable of doing that at idle speeds. If you connect that to your bike, the regulator (R/R) on your bike is going to try to regulate it to what it thinks is the proper voltage, which might be exactly 14.0 volts. To regulate voltage, it shorts what it considers to be excess directly to ground. When the car sees this load, it increases its output to try to compensate. Meanwhile your R/R is trying DESPERATELY to regulate the voltage by keeping that short to ground active. The problem here is that the R/R is only rated for about 25 amps, the car's alternator is going to be putting out a LOT more than that.

Guess which system loses? :-k



850GT_Rider said:
Because you can't have the two charging systems fighting each other. You could very likely fry one or both.
Click to expand...
Well, it's not so much about two systems fighting each other, as I have just shown, it's because the weaker one simply can't handle the output of the stronger one.

Interesting point, on this topic: I had a Kawasaki Voyager 1300 for several years. Very interesting to note that it had TWO stators and TWO R/Rs, and the outputs went to the same terminal to feed the battery and the bike. Two separate charging systems, but they worked together, because they were the same type of system.

.
 
Last edited:
Steve said:
Well, it's not so much about two systems fighting each other, as I have just shown, it's because the weaker one simply can't handle the output of the stronger one.
.
Click to expand...

YES, Steve - I understand that and agree with your more detailed explanation. I was trying to give a simple answer.

Plus, there's no guarantee that something's not right on the bike system, and it COULD have a negative effect on the car's electronics. I wouldn't want to bet my car's computer on it......
 
Doubt!!

Doubt!!

You have all managed to sew the seeds of boubt in my mind.For years we have jump started my sons 1200 bandit from the car with the engine running with no adverse effects to either.Maybe we have been lucky.:oops::oops:
 
Maybe you were very quick and got it disconnected before anything fried.

And lucky.
 
warriormonk said:
You have all managed to sew the seeds of boubt in my mind.For years we have jump started my sons 1200 bandit from the car with the engine running with no adverse effects to either.Maybe we have been lucky.:oops::oops:
Click to expand...
That is because the Bandit has an alternator (like a car), not a stator and R/R setup, like a GS.

It is the way the R/R is wired into the system that is the cause of the "problem" when connected to a running car.

.
 
car battery

car battery

Thanks for that Steve.I should have also said I run a gs 850 with a heavy duty car battery, the only electric mods is Dyna S ignition
 
Running it with a (12-volt) car battery is no problem at all, as long as that battery is not also connected to a running car.

.
 
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