Can someone explain to me why...

[psycoreefer]

everyone says you shouldn't jump start a bike with a "running" car. It seems the general consensus among all bike riders is that its ok if the car is off but not if it is running.

I've never heard or found a good reason for this, its just generally stated that you will "fry" the bike's wiring/battery/alternator/electronics/(place gizmo of your choice here).

I can understand why you wouldn't want to leave the bike battery hooked up to the running car as a car alternator at 100+ amps would boil the water out of a dead MC battery but I wouldn't think a few seconds for a jump would hurt, maybe just slightly shorten the life of your battery.

[boyat68]

psycoreefer:

I have to come down on the side of "Old Wives Tales" as a 12V system is still a 12V system. It does not matter if you have a 100 amp alternator or charging system as the battery in your motorcycle acts as a ballast to prevent the charging system from cooking your system. The only caveat that I know of is that be careful not to accidentally hook positive to negative as that may blow the diodes in the charging system, but that is the same caveat as jumping car to car. I have used a car to jump start all types of equipment, mowers in particular as low battery is a PIA to charge up especially if you have set up to mow.

[Crazcnuk]

As above, the battery takes the brunt of it, easily.

Just hook the booster cables directly to the battery posts, if you're worried.

Personally I boost bikes with the car whenever I need to, I just don't start the car. There is no need. The car battery, alone, will crank your bike for about an hour.

[trbig]

I'm no electrical engineer and yes, 12 volts is 12 volts, but the amps are what does the work. The volts simply keep them moving along. I also believe that most car alternators will charge much higher than the 13.5-14 volts that is the battery's (And our bikes charging system) limit.

A little lawnmower battery may have 225 cold cranking amps, and some cars have 1000 or more. A thousand amps moving through a bike's system can't be good, otherwise you'd just throw thousand amp batteries in everything. It would make production SOOOO much easier on battery manufacturers.

On the flip side of that... you are getting all those amps to a bike even if the car is turned off??? Maybe a wive's tail... but I'd rather err on the side of caution on it.


Tod

[Rhumba]

The way I understand it is that you could hook up a 1000 amp car battery to a MC battery and the MC battery will only "draw" as many amps as necessary. Nothing is going to cook. I as well have jumped bikes from car batteries lots o times. The starter motor is what is drawing the power, not the battery. You are hooking it up in parallel (+ to +, - to -) so you would get the same voltage and since the MC battery is dead, no additional amps. The voltage has to match though. Hooking up batteries in serial (+ to - and so on) adds the voltages together, therefore frying the motor.

It's like this, your house serves 115 volts and say 30 amps through a 30 amp breaker. If your toaster only needs 5 amps, then that's all it's gonna use. All your house wiring is 10 or 15 amps and up, most appliances only draw 1-10 amps.

As for the car running or not, IDK.

Hope this helps.

[psycoreefer]

thanks for the input, I was thinking it was a wives tale as well but wondered if someone could give me a solid reason why it was, or wasn't, bad.

otherwise you'd just throw thousand amp batteries in everything

I think the reason they don't do this is size, look how big a car battery is, there is no way they could fit one in a motorcycle. Plus it would make your bike 30lbs heavier.

[Rasputin]

I know with the newer cars you are not supposed to boost while running as it can do bad things to the computers on them. My Ex had to replace two computers on her Dodge pick-up from boosting other cars. Something to do with a spike in the readings the computer gets does it in. I do not know what it could do to the motorcycle system at all. Maybe some more knowledgable member has more info. I do know those computers for that dang truck were $875.00 each. So now whenever I boost anything the vehicle is not running or I disconnect the battery cables from my cars.

[Guy_b_g]

Your battery acts (in a sense) like a buffer, absorbing the output of the charging system from you alternator and putting a load on it..same with the car you're jumping from.

On some bikes (maybe someone here knows if ours is one of them) the ignition unit (computer/brain/blackbox...etc...whatever you want to call it) is fairly sensitive to voltage spikes.

The battery usually loads the system and also dampens the spikes...so usually it's not a problem to jump the bike.....but......be sure your battery doesn't have a dead short in it...especially if you're jumping the bike This can cause the voltage to spike as high as 18 volts on some alternators, and there's no battery to absorb the extra voltage...so on a lot of bikes it's goodbye ignition unit.

I suggest you load test your battery if you have any doubt at all about it's integrity.

And NEVER disconnect a battery lead on a running bike....for the same reasons.

[T99Ford]

Here's my .02 (or now that i've typed it, more like $5):
Beginning in the late 70's and early 80's, in the ever continuing quest for a better running anything, the CDI, black box, brain, etc was introduced into ignition systems in order to replace moving parts (read breaker points!) with solid state transistors that function in the same capacity. Namely, a high voltage is induced in a secondary ignition coil by collapsing a low voltage field generated in a primary ignition coil. This is accomplished by breaking the ground circuit to the primary ignition coil every time the spark is to fire. The switch on the ground side is (in old systems) the breaker point set, and on newer systems, a solid state transistor. The transistor and related solid state components are sensitive to high voltage spikes as has already been stated.
What the engineers of these earlier systems failed to realize (as they were pioneering a new design) was that these voltage spikes can be handled through the proper grounding. The mobile electrical world (meaning bikes, cars, trucks, planes) has since realized that good and numerous grounding points are essential to safe and reliable computer control systems. Just look at the ground points under the hood of a late model car. A voltage spike is looking for a good source to ground (as is any electical current flow). If the easiest path to ground through is the ignition unit, then poof, up in smoke. Grounds are the most overlooked connection in any electrical troublshooting procedure. They are also the wires most often left unattached, dirty, or removed for looks. If you have enough (the xs may or may not, by design--remember, late 70's early 80's) grounding points, you will NOT harm your system by jumping it to anything operating on a 12v negative ground system running or not.
A running car battery is at nearly full charge all the time, if the lights and other circuits are off. The car when running is using current provided by the alternator when running, not the battery. The battery only provides reserve power for surge loads. The battery remains constant. (if you have a large, constant accessory on your car, such as a large spotlight, and it is constantly running the battery down, even with the engine running, your mechanic will tell you to get a bigger alternator, not a bigger battery. If you have a large sound system in your car, that uses a lot of power for a short time, to fire sub woofers for instance, your mechanic will tell you to get a larger or additional battery.
Therefore, jumping from a running car battery is the same as jumping from a fully charged stationary battery. The difference, is that if the car is running, the alternator will sense the additional load caused by the dead bike battery (which is not much), and will cause the alternator of the car to start charging at a regulated output of not more than 14.5 volts. The charging amperage will vary based upon how dead the bike's battery is. An alternator is not full on or full off. It can vary the rate of charge (in amps) based upon load. A bike battery is not a significant enough load to warrant full rated amp charge from your car alternator.
The bottom line is that it does not matter if the car is on or off if your grounds are properly installed and functioning. If they are not chances are you will have a problem either way. Be sure to observe safe jump starting practices always: avoid any sparks near either battery when jump starting! Sorry to run on, but hopefully this will help to dispel any wives tales accumulated about charging systems.

Tyler

[Guy_b_g]

Nice explaination Tyler.

I hope I wasn't misunderstood in that you can't jump from a car..you obviously can.

I was just trying to point out that sometimes a dead battery is caused by a short in the battery itself...as a matter of fact about a month ago I had to replace my car battery (less than 2 years old) because of exactly that problem.

If the battery is shorted the alternator will go to whatever it's max regulated value is, as soon as you disconnect the jumper cables, and there's nothing to buffer that spike.

I had a 97 Centauro, and they have a Ducati alternator, a good number of folks on the COG (Centauro Owners Site) had destroyed their ignition units when running the bike with a bad battery, and that particular alternator/regulator combination was capable of 16-18 volt spikes...we were told by the factory never to bump start, or jump our bikes unless we knew we had a good battery (actually..never to do either period, but we figured out what was happening ourselves).

I don't know what kind of spike our bikes can produce, but in general on modern bikes it's important to have a battery that doesn't have a dead short.

[T99Ford]

Guy--

Nope, not misunderstood. You make a good point. I just wanted to try to clear up some of the other wives tales regarding what actually goes on when you jump something. I think between everyone here we're getting it cleared up!

[Guy_b_g]

My prior post about a shorted battery reminds me of something....there was a modification we posted on the COG site to save our computers.

It was a transil diode which shunted voltage spikes to ground.

I wonder if it would be a worthwhile mod on our bikes...

[T99Ford]

You could put the diodes in your ground cables to prevent backfeed spikes. This is a common feature of most modern automotive relays. However a relay makes and breaks a connection much more often, and the diode is in the power circuit, rather than the grounding circuit, to prevent power from going the wrong way down the power circuit when it is shut off. I think just more regular grounds (ie from frame to rear fender, frame to engine, frame to front headlingt mount, etc) would be just as effective and made with stuff you probably have in your garage already.

[fredintoon]

Does so fritz up the car.

Boosting anything, not just your bike, from a car battery while the car's engine is running, or even just with the ignition switched on connects all the car's modern electronic gizmos to the boosting process.
This can screw up the car.
I too used to think "Old wives tales" about not turning the car on when boosting then I boosted a bike from my Olds Cutlass Ciera while the Ciera's engine was running. Afterwards I noticed the Ciera's tach was reading 2,000rpm with the egine stopped. You can poke a wire in and re-zero the tach, though. Being slow to learn, I did it again with the same result.
So yes Jennifer, you can fritz up your car by boosting while it's running.

[John]

Let me throw this wrench into the works...at my job, over 50% of the trouble calls I make are to jump start a dead/weak battery. The tow truck is an '05 Ford Super Duty F 350 with the Diesel engine. It has two alternators, as in the amblance set up, and it also has two under hood batteries, which is the stock set up. One of the alternators charges an 8D battery that is loated on the back of the truck. For those who are not familiar with an 8D battery, it is about 24 inches long and 12 wide and 12 tall. It is used in heavy duty deisel equipment. Weighs around 75 lbs. Anyhow, the truck has a set of jumper cables that are attached to the 8D battery, and permanently mounted behind the large push plate that is installed where the front bumper would be. The cables are 20 feet long to reach some equipment that may be blocked by other equipment. We never turn the engine off, and we have never had any problems with spiking doing anything adverse to the electrical system of either vehicle. We also have a gas powered tow truck, that has the same set up, and no problems. I jump from my car to bike when ever necessary, but I don't start the engine. Most auto batteries are about 6-700 cold cranking amps, so there is no need to run the car's engine. The only thing that does is increase the voltage available from 12.5 to about 13.5-14.5. I don't know if that will hurt/help anything, but I just don't start the car engine, just because I am extremely frugal, and hate to spend money on something I FU'd. I don't worry about it at work, because I'm spending other peoples (OP's) money there.