Why is jump starting bad?

[theklanchXL1200N]

Hopefully I'll never have to jump my bike. I would try like heck to catch it in gear first.

I recently had to jump my diesel box truck. I left the lights on for about 2 1/2 hours and found myself without enough juice to crank the motor. I talked AAA into coming out (since anything with dual rear wheels isn't covered by an AAA auto membership) and the tow truck guy and I tried in vain to get the truck started. The truck is a Mitusbishi Fuso cab-over. It has two batteries. We tried every combination of the four posts we could think of. After about an hour the tow guys boss was telling him to leave me, but it seems like Chad the tow driver is like me, he likes a challenge he wasn't gonna leave me stranded. I said to him, "Hey, why don't we try the positive jumper to the starter selenoid and the neg to the frame?" Sure enough it fired right up. No cables on the battery.

That is my suggestion to anybody that finds themselves in that situation with a bike, find some metal for the ground (swingarm, shock stud, axle nut) and the terminal on the starter selenoid. It works on my quad too which has a bad stator and needs jumped after an hour of riding.

I would still try catching it in gear (although it is easier said than done on a HD) but be careful when doing this.......

 

[NEWHD74FAN]

I won't give you anymore reasons why not but not having the car running is a good choice. A good friend of mine had to replace the computer on his bike because he jumped it with the car running. I can't tell you why only that it happened. Fortunately the dealer replaced it under warranty because they're the ones that said it would be ok to jump it with a car.

Your friends' computer was fried because of voltage transients when trying to start a dead battery and motorcycle while running with a fully charged on board battery with surface charge. Power supply design is my bag...:s

An automotive charging system would supply greater than 17V, if not for the voltage regulator to shunt away the excess voltage, but if momentarily attached while running to a fully discharged battery and motorcycle starter trying to start, plus inductive "flyback" energy stored in your alternator windings can induce spikes easily exceeding the voltage regulator ability to shunt the excess voltage away at the same time the load goes almost to 10V during the attempted starting, the voltage swings could go from no load (hi voltage/lo current) to full load (lo voltage/hi current) in milli-seconds...plenty of time to provide excess voltage spikes to fry motorcycle computer electronics downstream which utilize a secondary "point of load" step down DC-DC converter/regulator from the 12-14V battery/alternator to the 3-5VDC the for your computer micro-circuits run at...no big surprise there, sorry for the expensive lesson. :small3d008:

 

[dangerdan]

Jump starting is bad for the following reason.

Your car battery is capable of producing 200- 300 amps.

The condition of your HD battery, will determine how many amps are transferred from the car battery to your HD battery. That surge only takes a few milli seconds to occur. You know that spark you see when you connect those jumper cables. The larger that spark, the greater the amperage.

This is where the stator in your ride takes over. That surge in amps causes the stator to make a very large magnetic field and produce voltage that may exceed 100 V in just a few milli seconds. The voltage regulator is to slow to compensate for this peak and your electronics get fried.

 

[glider]

Your car battery is capable of producing 200- 300 amps.

I think if you check out that figure, you will find it's double that in most car batteries.:s
Most are capable of producing 650 CCA.

 

[NEWHD74FAN]

Krikket, contrary to some beliefs, bigger is not always better!
Since you're an old auto mech, turned electrician, you'll understand this little tale; In the heavy equipment busness, when the batteries went dead on a dozer, loader, etc. It was common practice (in the old days) to fire up a welder and use the DC voltage as a source to jump start the machine.
Worked fine on the old stuff with not much more than a starting system and lights.
However, when the age of computer electronics came about, the 1st person who tried that old trick just cost his employer several thousand dollars in fried electronics.
That welder had the same DC voltage, however, the amperage output was way more than the systems on the equipment could stand.
Same same car/truck to bike.
And, the safest way to jump anything is not battery to battery, but from the good battery directly to the starter of the vehicle you are trying to start, and the negative to a suitable ground.
If you were ever around a battery that exploded, you would understand why.

Some of those welders provide huge amounts of pulsing DC with plenty of overshoot and transients, why do you think it is so easy to weld with a DC Welder, if it was clean DC the rod would just weld itself solid to the material until the area vaporizized rather than drop sufficiently in voltage to allow the welder to "walk the stick" in a pool of molten metal! Transients and sensitive electronics don't mix... :small3d038:

 

[mijanro]

The consequences of screwing up are high. Cross the leads, or ground the positive and you usually fry electronic components. Some newer vehicles protect against that with fuses better if you have a spare. Connecting the jumpers to the positive starter terminal and the negative to a stud on the transmission works peachy on my 06 e-glide and is much more accessable than the battery terminals. Don't wanna tell how I know.

 

[dbmg]

Quite simply the battery cables are at least twice in size. So the difference in power is a concern. Second the size of clamps on auto jumper cables make for a difficult connection on motorcycle battery and the close proximity of shorting bikes electronics.

 

[Dan.1977P]

I'm concerned about jumping a car or a bike and frying the ECM as I feel the chances of that happening are much higher than blowing the battery up even if you pay attention.
This day and age with battery boxes being so cheap, every cage and garage should have one. Takes away the running car issue.

 

[sportster 2001]

for some cars there are owners manuals which warn against jump starting. luckily harley is has neither the problem of "over complication" or of being junk.

V=I*R

if you connect batteries than your volage regulator should do the rest. assuming you follow connection instructions - not getting ground return going the wrong way i've connect things backwards quickly on a car: still ran. i don't advise it.

your "real" question is if voltage sensitive parts can be damaged if voltage is same. NO.

your other question is: can current sensitive parts be damaged if volage is the same. NO - not if the voltage is the same and the current ability your pulling from is larger.

if you run an 15 amp electric motor on a 10 amp circuit you will get problems and eventually burn out the motor (the motor part is current sensitive and current driven). but 10 amp on a 15 amp your happy as can be.

what you can't do is run at higher voltage since (1) bad for voltage sensitive parts and (2) draws more curret. (obvioulsy much higher R would do that do, see your elec. mech quiz)

V = I * R

it's possible some part would be damage if not "exactly 12v" and also needed "at least 1 amp" (thus R is critical)

BUT NOT AUTOMOTIVE ELECTRONIC (the more sensitive computer circuits aside)

12 V auto chips are much more resilient than say, a 1.5v radio circuit

any circuit cheap or good can protect itself and parts from poor power supply or intermitten failure with good design, fuses, and such - if they bother. if they bother.

 

[NickD]

You can push start a Harley with a magneto, but not so much the newer models. In earlier bikes a mechanically turning magneto provided the power to fire the spark plugs. This is why, with a kick starter, those magneto bikes ran fine with no battery installed at all.

On modern bikes the power to engage the starter motor, and power the coil (which in turn provides the power to the spark plugs) comes directly from the battery. The alternator does nothing but replenish the battery. That's why with a very weak or broken battery you can push and try bump starting all day and nothing is going to happen.

As for jump starting, in a pinch you can use a car battery with the car not running. If the car is running, and you have computers and micro-processors on-board, as soon as the running car's alternator kicks in (which is instantly) the amount of amperage (higher than a Harley alternator) is probably going to fry things on the bike.