Explain jump starting from a started car is bad for your bike

[Ray]

I've owned over a dozen bikes from a 1978 Yamaha 250 Enduro to a 2001 Kawasaki ZRX 1200, and I've jump started all of them from a running car with no appreciable negative side effects. Granted I could probably could have jumped them without the car's engine running, but it never occurred to me not to.

I hate to actually bring in any real world experiences to bust up the electrical engineering wank fest, but there ya' go.

 

[Burning1]

Is it possible that if your battery is dead from some other electrical problem (bad RR, short) that jumping from a running car could make it much worse? Might be how this idea got started.

 

[Motech]

Oh that's one of my favorites - but it's concrete, not pavement. Pavement is OK.

But don't you dare derail my thread. If we somehow end up talking about synthetic oil and MC clutches I'm a gunna whup yur ass!

It's asphalt Jackhole.

 

[sasquatch]

My interpretation is based on some research, based on the info that ST Guy posted earlier.

I agree, the answer 'a car alternator puts out more amperage than a bike alternator' never quite worked for me. It's like saying that my fridge burned out, because I moved from a coal powered community to a nuclear powered community.

I think of it as drinking from a garden hose vs. drinking from a fire hose.

 

[Burning1]

I think of it as drinking from a garden hose vs. drinking from a fire hose.

That analogy is only true if you have no resistance in the line.

In science and engineering, the intuitive answer is often incorrect.

 

[ST Guy]

I don't think jumping from a running car will necessarily do damge to the bikes charging system, but I see the potential for to much amperage toasting componets on the bike if the car is revved.
safer to just not turn on the car.

Even with the engine idling, it will depend on the car in question whether it will be putting out enough to kick the bike's VRR into draining all those extra amps.

Also, remember that there's absolutely no reason to have the jump vehicle running (unless it's another bike) as any car or light truck battery is going to have more than enough capacity to turn over a bike motor from here until the cows come home. So, the solution is simple. Just don't start the jumping vehicle's engine. Period.

 

[ST Guy]

Srsly ...

Interesting read, that's for sure.

14v is normally the maximum that the charging system puts out on both cars and motorcycles.

Amps are going to be drawn where there is a load connected. Since a dead M/C battery being charged and cranking the starter is going to be the biggest load, that's the maximum draw you are going to pull from the car. Oddly, the M/C charging system can also deal with this kind of load.

So, if you make the connections properly, and start the helper vehicle after all the jumper connections are made, you're gonna be just fine.

Wrong. See my earlier post. Depending upon the bike and the car, you could easily fry the VRR. Why take the chance?? The jumping vehicle doesn't need to be started and run at all because the load that a motorcycle starter puts on it is so much smaller than a car starter. Leave the jumping vehicle off, hook things up, start the bike, ride away.

 

[ST Guy]

That's true of later ST1100s ('96 and newer) and presumably ST1300s. But NOT for early 1100s.

True. But the systems are still very similar in that the earlier alternator still uses an electrically generated magnetic field in the alternator. No magnets involved.

 

[auntiebling]

i started a similar thread years ago in the garage about this and came to much the same conclusion as i'm getting from this thread, though this thread has much more info than mine did:

-operator error, or some other electrical system flaw, is probably what causes most of the "i fried my bike trying to jump start it" reports
-hooking everything up, then starting the car, will likely cause no harm and might get you home (add me to the list of people that have done it successfully)
-i'll always start with the car OFF, and that usually works
-i'll always recommend to those that ask to have the car OFF

 

[ST Guy]

So this is the closest I've seen to an explanation that I can appreciate. But the few points I still don't understand are:

1) I don't understand why he says it's likely that the car's target voltage would be higher than the bike's. First of all, given that the batteries use the same technology, the ideal charging voltage would be the same for both, so they should be designed to provide the same voltage. Further, anecdotally every car system I've actually tested has put out the same (or actually *less*) voltage than I've seen from the bike systems I've checked.

2) When I looked at the design of the regulator, at least on my suzuki (which is of the type he's describing), it looked like it would only shunt power from the alternator, not from a source external to the bike. But I guess it's possible that either I incorrectly interpreted the schematic or that the reg on suzuki's is different than on other, similar, systems.

3) Even if the regulator would try and shunt extra power from an external system, *and* the car's target voltage were higher, the very low resistance in the dead battery would cause it to sink power "first" and should still have a minimal impact on the regulator.

1. Well, yea, many bikes might actually have a higher threshold than a car. But unless you really know what those numbers are for the particular bike and particular car in question, it's safest not to have the car running. It's not necessary anyway.

2. As for the shunting, there are so many differences between various bike charging systems that again, it's safest not to have the car running because again, it's just not necessary.

3. As for the battery being a sort of buffer or temporary capacitor, yea I'm sure that happens. And it may be the reason you and others have gotten away with having the jumping vehicle running.

Bottom line is that it is possible to fry your bike's charging system if conditions aren't right. Are you going to know if conditions are "right" when the time comes? Likely not. So, again, the safe thing is to simply not have the jump vehicle running. And it's not needed anyway.

Cheers!

 

[Lovedoc]

I have cheap cables and the bike won't crank without starting the car

 

[elskipador]

i've jumped hundreds of bike with running cars & never once had any problem. personally, i think its bs stemming from the days when cars were mostly 12v systems & most bikes were 6v systems. my integra puts out between 30~33 ac volts @ idle, most sport bikes put out 15~25 ac volts @ idle. they're close enough not to worry about it. basically you need to know how much each vehicles ac charging system puts out @ idle, if they're close it won't be a problem. my experience is that pretty much all 4 cylinder cars of 2.0 liter or smaller are totally safe to just bikes with while running.

 

[Razel]

Wrong.

Nope.

See my earlier post. Depending upon the bike and the car, you could easily fry the VRR. Why take the chance?? The jumping vehicle doesn't need to be started and run at all because the load that a motorcycle starter puts on it is so much smaller than a car starter.

The load of the starter is irrelevant. You're not going to force more current into a load that doesn't demand it. If your starting current is 10 amps, and you have a 20 amp source or a 1000 amp source, it's not going to draw anymore than 10 amps from either one.

Leave the jumping vehicle off, hook things up, start the bike, ride away.

First off, yes, the helper vehicle may not need to be started. But, if it is, its own battery is now going to be drained from starting up, and the engine is at idle. So, it's not going to be putting out anything close to frying the VRR. Second, the dead/weak motorcycle battery is going to be sucking up most of the incoming current, so the VRR isn't going to be sinking that much more than it normally would.

 

[Wormwood]

Megadataincomingfloodservercrash

your gonna break teh barf.

 

[SD Hornet]

Actually... My reading of the diagram was wrong. In the SV diagram posted above, the regulator is a voltage sensor. When it senses that voltage is too high, it appears to short the output from each winding of the alternator to ground.

So, it does appear to be protected by the rectifiers diodes. At worst, it would dump the entire output of the bike's alternator to ground.

There is the answer for the OP.

Why would you have two independant regulatory circuits trying to control the same voltage bus without some sort of communication between them? Sounds like a good way to damage the charging circuit of the SV650. Either the stator coils could be damaged or the shunt regulating scr's will.

 

[Carlo]

Yes car and motorcycle batteries both use 12v systems but you're overlooking the amperage difference.

Most bike charging systems when the engine is running are generating 14.4volts at 2amps.

Most car charging systems when the engine is running are generating 14.4volts at 10amps.

I've started a bike from my truck with the engine running.

What you're failing to take into account is that the system drawing the current only draws what it needs (unless something is shorting out), so even though the car charging system can put out 10 amps, the motorcycle's starter motor will only draw as much current as it needs.
The car can't "pump" extra current into the motorcycle's system.

 

[ST Guy]

i've jumped hundreds of bike with running cars & never once had any problem. personally, i think its bs stemming from the days when cars were mostly 12v systems & most bikes were 6v systems. my integra puts out between 30~33 ac volts @ idle, most sport bikes put out 15~25 ac volts @ idle. they're close enough not to worry about it. basically you need to know how much each vehicles ac charging system puts out @ idle, if they're close it won't be a problem. my experience is that pretty much all 4 cylinder cars of 2.0 liter or smaller are totally safe to just bikes with while running.

'Has nothing to do with the alternator AC output.

Here's the potential problem in a nutshell. If the bike's VRR "dump point" (when it starts to send current to ground) is lower than that of the car's VRR, then there's the possibility that the bike's VRR will then be dumping all the current from the higher capacity car alternator and the attempt to dump much more current than it was designed for means it can overheat and suffer a melt down. Will it always happen? No. Is it possible? Hell yes. To be perfectly safe, you'd need to know a lot more information about both the bike and the car than is practical to know. So, the simple, safe thing to do is have the car's engine off.

 

[Burning1]

'Has nothing to do with the alternator AC output.

Here's the potential problem in a nutshell. If the bike's VRR "dump point" (when it starts to send current to ground) is lower than that of the car's VRR, then there's the possibility that the bike's VRR will then be dumping all the current from the higher capacity car alternator and the attempt to dump much more current than it was designed for means it can overheat and suffer a melt down. Will it always happen? No. Is it possible? Hell yes. To be perfectly safe, you'd need to know a lot more information about both the bike and the car than is practical to know. So, the simple, safe thing to do is have the car's engine off.

From what I can tell, that is not possible due to the way the regulator is designed. The RR dumps the AC output of the alternator to ground. The dumping circuit is protected from the DC side of things by means of diodes.

At worst, the RR will be dumping the entire output of the bike's alternator to ground. With that said, it will typically be the entire output at idle, rather than the peak output.

 

[Carlo]

Even with the engine idling, it will depend on the car in question whether it will be putting out enough to kick the bike's VRR into draining all those extra amps.

Also, remember that there's absolutely no reason to have the jump vehicle running (unless it's another bike) as any car or light truck battery is going to have more than enough capacity to turn over a bike motor from here until the cows come home. So, the solution is simple. Just don't start the jumping vehicle's engine. Period.

How is this supposed to work? You've got the jumper cables connected to the battery. Is current going to flow into the bike's rectifier/regulator, against the bias of the circuitry in the R/R? I think you'd have to have a pretty extreme DC voltage difference in order to have current flowing anywhere but through the bike's starter motor.
I'm willing to accept a logical explanation of how this works. I have a basic understanding of how diode juntions, transistors, etc. work, so you probably won't be talking over my head if you can explain it.

I've had to jumpstart my Aprilia a couple of times, and both times, there wasn't enough current until we started the engines on the jump vehicles (a Ford truck, and my Toyota truck)

 

[ST Guy]

Nope. The load of the starter is irrelevant. You're not going to force more current into a load that doesn't demand it. If your starting current is 10 amps, and you have a 20 amp source or a 1000 amp source, it's not going to draw anymore than 10 amps from either one.
First off, yes, the helper vehicle may not need to be started. But, if it is, its own battery is now going to be drained from starting up, and the engine is at idle. So, it's not going to be putting out anything close to frying the VRR. Second, the dead/weak motorcycle battery is going to be sucking up most of the incoming current, so the VRR isn't going to be sinking that much more than it normally would.

I only mentioned the lower draw of the starter in the context of it's draw on a car battery where the car's engine is not running. In other words, unless you've got a real problem with the bike, a quick jump from the car battery to start the bike is hardly going to phase the car's battery. Hence, it's no problem leaving the car's engine off.

And yes, depending upon the car and the bike, the bike's VRR could still end up getting fried. It's specific to the bike/car combination. If the set/dump point of the car's VRR is higher than the bike's VRR, then it's possible. Of course, this may not happen immediately. Depending upon the current the bike's VRR is trying to dump to ground, it may take awhile. Or it could happen before you had a chance to disconnect the jumper cables.

In my book, it's just not worth the risk. Leave the engine on the car off.