? for an auto electrician - jumping bike from a running car

[Stig / Major Tom]

For years I have read/heard that one should never jump-start a motorcycle or scooter from a running automobile.
Various reasons have been given.

Would like to hear from a technically trained auto electrician, if we have any here. 
"Is it true that this is not a good thing to do?"

I also hear that it is absolutely unnecessary to have the car engine running - because any car battery is perfectly capable of jump starting a bike or scooter.

Stig

[Kansas kymco]

Opinion, I would say it is because the car alternator  (my truck) puts out 130 amps and would damage the battery.  If you can't start a scooter from just the jumped  battery you have something else wrong.  I beleive the max charge rate is more around 3 amps.

Of course I am not a qualified automobile  electrician, but I did stay at a Holiday Inn last night. I did take training in auto mechanics and  electronics many many decades ago.

[de dee]

the 12 volt battery of car will start the scooot,, car running will put out up to 200 amps will fry the ECU,  as most scoots have # 20 wires and cars have # 12 and 10 wires much thicker wires, canadian batterys are up to 1000 anp hours for our winters,

[CROSSBOLT]

I think "capability" is being confused with "actuality." The output "capability" of an alternator is similar to the output "capability" of a battery expressed in "cold cranking amps" or "amp-hours." The capability of your 12 volt car battery ranges between lighting a single tiny 12 volt side light to starting the car engine with all the lights on. The tiny 12 volt bulb will "actually" draw only a few milliamps of the battery's "capability." Starting the car with the lights on demands several hundred amperes, nearly the maximum "capability" of the battery. It is

E = IR. E = 12 volts. R = what ever the tiny bulb is. I = E/R, the current, I in amps is 12 volts divided by whatever the resistance is. The ECU will not fry unless you supply it with too much voltage.

Jumpstarting while the car is running may, i say MAY affect the car alternator, or more correctly, its voltage regulator. It has been rumored that the diodes in the regulators have been fried while jump starting.

Forbes is a trained, experienced auto tech and will be able to explain this even better. He is actually one of several on here who can chime in with some real insight.

[Kansas kymco]

I think "capability" is being confused with "actuality." The output "capability" of an alternator is similar to the output "capability" of a battery expressed in "cold cranking amps" or "amp-hours." The capability of your 12 volt car battery ranges between lighting a single tiny 12 volt side light to starting the car engine with all the lights on. The tiny 12 volt bulb will "actually" draw only a few milliamps of the battery's "capability." Starting the car with the lights on demands several hundred amperes, nearly the maximum "capability" of the battery. It is

E = IR. E = 12 volts. R = what ever the tiny bulb is. I = E/R, the current, I in amps is 12 volts divided by whatever the resistance is. The ECU will not fry unless you supply it with too much voltage.

Jumpstarting while the car is running may, i say MAY affect the car alternator, or more correctly, its voltage regulator. It has been rumored that the diodes in the regulators have been fried while jump starting.

Forbes is a trained, experienced auto tech and will be able to explain this even better. He is actually one of several on here who can chime in with some real insight.

I will respectfully disagree , the max charge rate for my motorcycle battery was stated at 3 amps. Using a higher powered battery charger can destroy the plates. Motorcycle battery has resistance that resistance turns into heat which warp the plates leading to a internal short.   I could be wrong but that is my understanding.  If this was not the case then why the warning on the battery for a max charge rate of 3 amps? In a filament light bulb as the filament gets hotter the resistance grows limiting the current.

[randyo]

very simple, read instructions that come with a Yuasa battery,  you will find that it tells you not to charge at a rate more than 5amp/hr

I don't know of ANY car alternator that puts out that little, don't do it, jump start from battery only, without car running

[jak51]

The question should be directed to motorcycle mechanics.  I think Stig started this thread because I mentioned in a previous post that I jumpstarted my scoot while my wife's car was running.  If you do a search on whether or not you 1) can jumpstart a bike from a car battery (YES), and 2) should the car be running, the common answer is NO.  However, two, count 'em, two Level 4 BMNW certified motorcycle techs have told me and shown me how to jumpstart a bike battery from a car.  First and foremost, the two vehicles should be at the furthest point from each other while the cables can still connect.  Second, was the sequence to engage the cables, and I've seen this elsewhere.  First positive goes on car, second positive goes on bike.  First negative goes on car, second negative goes on bike.  Car can be started, left idling with no revving.  Start the bike.  It may take one or two cranks to get it started, assuming the battery is drained and not dead.  Once started, turn off the car and quickly remove the cables in reverse order.  Logic behind all this?  If the bike battery is seriously depleted, it may need to draw more than 12 volts from the car.  Once the bike has started, there is no advantage to letting the bike stay connected to the car, so to prevent any possible damage, both techs said to remove the cables immediately after the bike fires up.  If the bike's battery is good, it will build its own charge.

I've rarely had to jumpstart my bikes, and now my scoot, but that's the way a couple of BMW techs I trust taught me, and that's how I do it.

Jeff

[Kansas kymco]

very simple, read instructions that come with a Yuasa battery,  you will find that it tells you not to charge at a rate more than 5amp/hr

I don't know of ANY car alternator that puts out that little, don't do it, jump start from battery only, without car running

Bingo-Because the battery manufacturer said not to.  They design and build the battery and know it's capabilities.  My small scooter battery stated 3 amps the larger ones may be more. 

[Ex-KLR]

The difference between a running (14.5 volts) and non-ruining (12.6 volts) car is approximately 2 volts.
Unless it’s already beyond hope, the unloaded motorcycle battery will be at or near the battery voltage so little current flows.
The increased voltage (14.2 volts) is  significant!
This additional alternator voltage is sufficient to provide a charging current that could damage the smaller battery. 
For this reason, a small (read delicate) battery should not be attached to a running automobile.

[CROSSBOLT]

I will respectfully disagree , the max charge rate for my motorcycle battery was stated at 3 amps. Using a higher powered battery charger can destroy the plates. Motorcycle battery has resistance that resistance turns into heat which warp the plates leading to a internal short.   I could be wrong but that is my understanding.  If this was not the case then why the warning on the battery for a max charge rate of 3 amps? In a filament light bulb as the filament gets hotter the resistance grows limiting the current.

No real arguement there. There is no regulation during a jumpstart. The current will flow from the higher voltage to the lower. The question is for how long. Not long IF the scoot (or whatever) starts quickly. Even using the car system to charge the scoot battery could destroy a battery with a 3 amp charge limit but if only left hooked up for a "long" time. Won't fry the ECU or any other load device. The greatest danger in the scenario you outlined is a deadlined vehicle with a stone dead (or nearly so) battery is a battery explosion.

There is a caveat in a jump start of any kind and definitely the smaller the battery the greater care is required.

[Forbes1964]

Just seeing this since it changed formats. I wondered what had happened when I couldn’t find you all in Tapatalk. As for the question . I’m not 100% certain. But  believe it’s simply out of an abundance of caution and the fact that the demands are low enough that a battery alone should be sufficient without starting the “donor” vehicle.  I’m guessing that there’s the unlikely possibility that the car charging voltage could be too high. Some car voltage regulators allow as much as 14.5 volts of charging voltage in some older vehicles. It’s usually less. But it could happen. I suspect that may be high enough to possibly damage some fragile scooter electronics.

By the way, even when starting another car, the “donor” vehicle is not supposed to be started until AFTER the final connection is made. Many (including myself) often ignore this i
Precaution , and it causes no harm. But in THEORY ignoring  it could .

[Stig / Major Tom]

Just seeing this since it changed formats. I wondered what had happened when I couldn’t find you all in Tapatalk. As for the question . I’m not 100% certain. But  believe it’s simply out of an abundance of caution and the fact that the demands are low enough that a battery alone should be sufficient without starting the “donor” vehicle.  I’m guessing that there’s the unlikely possibility that the car charging voltage could be too high. Some car voltage regulators allow as much as 14.5 volts of charging voltage in some older vehicles. It’s usually less. But it could happen. I suspect that may be high enough to possibly damage some fragile scooter electronics.

By the way, even when starting another car, the “donor” vehicle is not supposed to be started until AFTER the final connection is made. Many (including myself) often ignore this i
Precaution , and it causes no harm. But in THEORY ignoring  it could .

That's all I need.
Good to hear from you again Mr Forbes.
Hope all is well down there in the Sunshine State
Happy Easter!

Tom

[randyo]

Bingo-Because the battery manufacturer said not to.  They design and build the battery and know it's capabilities.  My small scooter battery stated 3 amps the larger ones may be more.

yes, my V-strom & UTV batteries are at least twice as big as most scooter batteries, and both are rated for up to 5 amps charging, but I always set my charger to 1 or 2 amps

[de dee]

I was told by a battery dealer to charge a battery that has been low to long and does not hold a charge at 15 amps,.  and it cleans the plates , and desulfates the batterys. is that true,

[Ex-KLR]

I was told by a battery dealer to charge a battery that has been low to long and does not hold a charge at 15 amps,.  and it cleans the plates , and desulfates the batterys. is that true,

It's worth a try.  The battery is already dead, so you may resurrect it it this works.