Myth busted: Jump-starting your bike from a car battery

[Zerosum]

I was always told by my Dad that if you tried to jump-start a motorcycle off a car's battery, the higher amperage of the car's electrical system would fry all of the bike's electronics.  So, for all these years, jumping off a car battery was positively verbotten in my book.

Then, yesterday afternoon, I find that my Hawk's battery was dead.  I left the stupid rear parking light on.  So, I go back into the office and mention my plight to an electrical engineer.

"Is your bike a 12 volt system?"

"Yeah"

"Then you can jump it off a car.  Your starter motor doesn't car about the incoming amerage, it'll only draw what it's designed to draw."

So, I gave it a try, half expecting my Hawk's CDI box to explode.  Sure enough, it started right up!  

Huh.  All these years I've believed in my Dad's myth.  I think he must have gotten that in his head back when bikes with 6-volt systems were common.  Now that WOULD blow something up, according to my coworker.

Maybe you all knew this already, but it was news to me!  

[UFO]

V = IR

[1KPerDay]

If I have to jump from a car battery I do it with the car not running. The battery alone will have enough juice to jump the bike. Someone told me once there's less chance of trouble this way; dunno if it's true but I have perpetuated that belief.

[DavidLSI]

If I have to jump from a car battery I do it with the car not running. The battery alone will have enough juice to jump the bike. Someone told me once there's less chance of trouble this way; dunno if it's true but I have perpetuated that belief.

+1

[bomber]

your Dad is laughing.

;-}

[Papa Lazarou]

I did fry the ECU/ CDI on my Tbird doing just this.....

[1KPerDay]

I did fry the ECU/ CDI on my Tbird doing just this.....

Car running or not?

Was it some funky British 17 volt car battery?  

[Papa Lazarou]

car running

[frayne48]

I have jumped bikes from cars with no problems.

[Papa Lazarou]

I have jumped bikes from cars with no problems.

For me-old bikes yes, those with computer thingies-no.

[Leanintree]

Never had any issues, but only with the car not running. Used to have to do that every spring before I learned about the wonders of a Battery Tender.

Oh, and given a choice, not off of a pickup, particularly a 3/4-1ton that might be packing a spare battery in series. Always jump a bike off of the smallest battery available.

Used to weird me right out... going from rr-rr-rr-...clickclickclick... to  RR-RR-RR-RR-RR-RR-RR-RR-RR-RR-RR-RR VROOOM!!! BANG!!!  <----flooded backfire  

Leanintree

[scottzilla]

If I have to jump from a car battery I do it with the car not running. The battery alone will have enough juice to jump the bike. Someone told me once there's less chance of trouble this way; dunno if it's true but I have perpetuated that belief.

Yup, that's what I do too.

[Zerosum]

If I have to jump from a car battery I do it with the car not running. The battery alone will have enough juice to jump the bike. Someone told me once there's less chance of trouble this way; dunno if it's true but I have perpetuated that belief.

Huh.  I'll keep that in mind.  The car was running when I jumped it.  Everything on my Hawk still works.  

[tigertom1965]

Forgive my ignorance but.......Your bike has a parking light????

I thought the lights went out when the key was off?

I know I could possibly leave the hazards on but a parking light?

[Medic09]

Forgive my ignorance but.......Your bike has a parking light????

I thought the lights went out when the key was off?

I know I could possibly leave the hazards on but a parking light?

In addition to the 'lock' position on the ignition lock, there is often (usually? always?) a 'park' position.  The 'park' position on older bikes left the taillight lit; on my '01 Sprint it leaves a little white light up front and the taillight lit.  Every bike I've owned (AFAIR) has had some arrangement like this.  Yours doesn't?  

[tvpierce]

12 volts is 12 volts.  Extra amps won't hurt the bike.

[tigertom1965]

I haven't tried it to see if the lights stay on. I will try that tonight when I get home.

[1KPerDay]

It's a holdover from the European bikes... in some/most countries they do (or did) require that bikes parked on the street have their parking lights on.

Or so my dad tells me.

I for one wish my hazards would work with the ignition off. But that's another story.

[Zerosum]

Forgive my ignorance but.......Your bike has a parking light????

I thought the lights went out when the key was off?

I know I could possibly leave the hazards on but a parking light?

Yeah.  I hate it.  It's the last notch on the ignition switch before "off."  You can take the key out and the rear lamp stays on.  I don't ride my Hawk that much, and I sometimes forget about it.  On a bright sunny day, you don't even notice.  Some time after '88, Honda must have figured out that this was a battery-killer.

[Mydlyfkryzis]

There can be a problem. On bikes that use a Zener Diode regulator, the basically work by shorting the excess voltage to ground. If your zener diode is, say, dumping at 14.6 volts, and the car, running, is putting out 14.8 volts, the diode will try to regulate the voltage by diverting current to ground. since cars have a pretty large charging capacity, the voltage regulator in the car will increase current output, trying to maintain 14.8 volts.  Most motorcycles have a 30 amp or less charging system. The car keeps adding current until the fuse in the motorcycle circuit blows or the zener diode burns up.

If you jump your bike from a non-running car, the battery is never over 14 volts, so it is safe.

If you have a none zener diode system, then it doesn't matter.

If the car regulator is putting uot less then the Zener diode short to ground voltage, then you won't hurt anything.

So depending on the bike, the actual rating of the Zener Diode, and the output of the car, you may or may not fry your charging system.

You will not fry the other electronics as they use the smae voltage as Car electronics.

For those who say 12 Volts is 12 Volts, I remind you that even a 12V battery isn't 12V. And charging systems run between 14 and 15 volts on a properly operating system. If the car is putting out 16 volts, or 15.8 or something else, you may fry the bike regulator or electronis.

For safety, just make sure the car motor is not running and you will never ruin any electronics.

If the engine on the car is running, then, good luck, you are taking a chance.