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

ST Guy said:
The ST's use an automotive type, variable current, field to control alternator output. If the car's voltage regulator set point is above the bike's, the bike's regulator will simply stop the bike's alternator from charging until the voltage drops. No problem.
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That's true of later ST1100s ('96 and newer) and presumably ST1300s. But NOT for early 1100s.
 
I've jumped my bike with the engine running. Bike started fine but then it crashed
 
When you hook jumpers to a dead battery with engine running, ever notice the donor engine idle drop and alternator start whining a little? That's the alternator forcing more current flow to make up the difference. Affect will be the same as a larger charge on the motorcycle battery than recommended, probably between 15-25 amps initially, tapering off pretty quickly as the moto-battery recovers some surface charge.This is effectively similar to using a variable-rate charger on your dead moto-battery set on 20 amp, auto-adjust range. The charge current will be initially high, then taper down to 4-5 amps when battery charges up.

Too much, sure, but it's pretty common to "smack" a discharged battery with higher charging current than recommended for a few minutes to speed up the charging process, even a precious motorbike battery.

It's not recommended, but aside from the already compromised battery, it certainly will not hurt anything on the bike, especially if only for a minute or two.

FWIW though, I would shut the car off to jump-start a bike,unless I did not trust the car battery, in which case I'd leave it running.
 
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ST Guy said:
In this type of system, the voltage regulator acts to limit the output of the alternator which means that it must also act to limit the output voltage of the bike's electrical system if another power source is connected to the bike.

There will be not problem in connecting your car to this type of system if the car's alternator is not operating. The problem occurs if the car's alternator operates and the set point of the car's voltage regulator is above that of the bike (which is likely). In this case, the bike's tiny regulator will attempt to load the car's 100 amp (or so) charging system to limit voltage. The result will be overload and burning out of the bike's regulator.
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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.

Now of course you wouldn't want to leave this all connected up for a long time - there *will* be effects after a while - the internal resistance of the battery will increase as its charge increases, so if the regulator *does* try and shunt power from an external source, it will eventually start trying to shunt that power. And the extra amps available from the car's system is not good long-term for the MC battery (which is a problem in *both* cases - with the car running and the car off, since the car's battery can produce very high amps).

Wormwood said:
Spoke with an electical engineer, he grabbed his #2 pencil and started scribbling, pulled out his calculator and said "just jump the damn thing and see what happens."
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:cool I've actually done exactly that - I've jumped multiple bikes from a running car with no ill effects.

Razel said:
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.

...

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.
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Right. Or at least, that's my train of thought. :)
 
LS1Bandit said:
Unless you have an upgraded alternator, I think it's unlikely to be 100+. I believe most cars are in the range of 40-60amps stock. And that's max output. As I stated earlier, they are not "pushing" 40-60amps all the time - cars have regulators too - the charging system will only put out as many amps as is required to maintain the voltage.
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My car has a 120 amp alternator.

http://www.ecstuning.com/BMW-E34-540i-M60_4.0l/ES259915/

Newer versions of the 540 had a 150 amp liquid cooled alternator.

http://www.fcpgroton.com/product-exec/product_id/52081/nm/BMW+E39+540i+540it+Rebuilt+150+Amp+Alternator+1999-2003+(Bosch)-+$150.00+Core

I agree with you, but have one question: The car's alternator will try to maintain ~14v, and will put out whatever number of amps are needed (up to it's limit) to maintain ~14v. With enough load, the car could potentially put a couple hundred amps through the jumper cables (alternator plus battery). But... how much can the motorcycle battery draw? Is it possible for the motorcycle battery to draw enough to damage itself? Would it be advisable to use a smaller wire guage (10ga, for example, rather than 4ga) to limit the amperage to the motorcycle battery?
 
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Motech said:
When you hook jumpers to a dead battery with engine running, ever notice the donor engine idle drop and alternator start whining a little? That's the alternator forcing more current flow to make up the difference. Affect will be the same as a larger charge on the motorcycle battery than recommended, probably between 15-25 amps initially, tapering off pretty quickly as the moto-battery recovers some surface charge.This is effectively similar to using a variable-rate charger on your dead moto-battery set on 20 amp, auto-adjust range. The charge current will be initially high, then taper down to 4-5 amps when battery charges up.

Too much, sure, but it's pretty common to "smack" a discharged battery with higher charging current than recommended for a few minutes to speed up the charging process, even a precious motorbike battery.
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That is exactly correct. And actually, smacking a discharged battery with a high current does more than just speed the charging process, it actually helps counter some of the sulfation that occurred as a result of running it down. And in fact, I use this exact process to help "revive" batteries. I use batteries extensively in some pretty "mission critical" scenarios (emergency communications and advanced technical SCUBA diving being the primary activities) and thus have a garage full of batteries used in a variety of services.

Based on a lot of research and a fair bit of experimentation, I've found that I can be pretty effective at reviving a battery by running it through a few *deep* discharge cycles, as long as the discharge is *immediately* followed by a very high initial charge cycle (I usually use 40amps - even on MC-sized batteries).

So that high initial draw you get from the car (which, again, you'd get the same thing from just the battery with no car running), as long as it's short-lived, is just fine.

It's not recommended, but aside from the already compromised battery, it certainly will not hurt anything on the bike, especially if only for a minute or two.

FWIW though, I would shut the car off to jump-start a bike,unless I did not trust the car battery, in which case I'd leave it running.
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Yes. In general I would agree that while I don't really see a problem with having the car running, unless you're suspicious of the car battery, there really isn't much of a need to have it running.

Ironically the only times I've had to jump motorcycles, I *have* been suspicious of the car battery, which is why I had them running.

And therein lies the crux of my asking the question. I'm actually fine recommending to people that they leave the car off while jump starting the bike, just to be on the safe side. However I'd like people to understand (as long as I'm actually correct) that it's not likely to be a problem if they do feel the need (for whatever reason) to have the car running while jumping the bike.

And of course, I'm asking the question because I'd like to know whether or not my assertion/understanding is correct.

I guess I'm also asking because it really bugs me when people take a high-and-mighty attitude with people and say things like "well, if you want to blow up your motorcycle, go right ahead" without really having anything substantial backing the statement.

SLOspeed said:
My car has a 120 amp alternator.
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Yeah, after I posted that, I did a little research and found that things have changed a bit since I used to spend a lot of time around car engines. The alternators of today tend to be higher-powered than alternators a while back.

But the fact still remains, regardless of whether you have a 60amp or a 200amp alternator, that's still its maximum and is only going to put out enough current to cover the load.

I agree with you, but have one question: The car's alternator will try to maintain ~14v, and will put out whatever number of amps are needed (up to it's limit) to maintain ~14v. With enough load, the car could potentially put a couple hundred amps through the jumper cables (alternator plus battery). But... how much can the motorcycle battery draw? Is it possible for the motorcycle battery to draw enough to damage itself? Would it be advisable to use a smaller wire guage (10ga, for example, rather than 4ga) to limit the amperage to the motorcycle battery?
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The motorcycle battery will draw according to its internal resistance. As Motech stated earlier, there will be a fairly high draw initially, but as the charge builds up, the resistance will increase, which will cause it to draw less (unless you have a constant-current source, in which case the source will keep raising the voltage to maintain the current). Nothing that you should need to worry about using smaller-gauge wire for though. Just don't leave it hooked up for a long period of time.
 
LS1Bandit said:
I guess I'm also asking because it really bugs me when people take a high-and-mighty attitude with people and say things like "well, if you want to blow up your motorcycle, go right ahead" without really having anything substantial backing the statement.
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Just make sure you don't leave you battery on the pavement or it will discharge into the Earth. :laughing
 
Motech said:
Just make sure you don't leave you battery on the pavement or it will discharge into the Earth. :laughing
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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!
 
The easiest way is stop thinking like an engineer, and just dont have the donor vehicle running when you jump start your bike. :twofinger
Much easier :thumbup
 
stangmx13 said:
if uve gotta start the working car to jump a dead car... then either you are using really shitty jumper cables or your connections are bad/corroded. 1 car, 1 good battery should always be enough. that being said, i have had to start the car b4 cuz my good jumper cables were in the garage and not in the trunk of my car.
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depends on the state of the dead battery, if the battery is truly dead it becomes a capacitor sucking up any energy that would be used to start the car. Note this on old cars in the 80s, but have had the situation with a completely dead battery and can't jump the car, disconnect the dead battery from the car and jump to the battery cables of the dead car and it started.
 
LS1Bandit said:
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.
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My guess is that this could be written off as production tolerances. The target voltage is the same, but the tolerance is different, and the approach to achieving the desired voltage is different.

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.
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The wiring diagram I found for a Suzuki Regulator Rectifier seemed to indicate that the regulator sat on the DC side of the rectifier, and maintained voltage by means of resistance. Thus, the regulator is not protected from the car's alternator by means of the rectifier's diodes.

http://webpages.charter.net/jrandall/vrr/vrr_upgrade.htm

My understanding is that most automotive regulators use a different approach to regulation; they momentarily turn the alternator off when voltage gets too high, and turn it back on as voltage starts to fall. In this way, they don't need to burn any extra energy off as heat.

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.

Now of course you wouldn't want to leave this all connected up for a long time - there *will* be effects after a while - the internal resistance of the battery will increase as its charge increases, so if the regulator *does* try and shunt power from an external source, it will eventually start trying to shunt that power. And the extra amps available from the car's system is not good long-term for the MC battery (which is a problem in *both* cases - with the car running and the car off, since the car's battery can produce very high amps).
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What I'm gathering is that if the cars regulator threshold is high enough above the regulator threshold of the bike, the bike's regulator will attempt to burn off the entire charging capacity of the car's alternator. Because the bike's regulator is keeping the car's alternator output voltage low-enough, the car's regulator may not kick in at all. Because amperage is limited by resistance, and the bike's regulator is designed to burn off energy in order to produce a voltage drop, the regulator will attempt to draw as much amperage out of the car's alternator as it can. By running the car during a jump, your bike's poor little RR may be asked to dissipate heat from a much more potent energy source than it was designed for, and can quickly burn out.
 
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.
 
ST Guy said:
Here's something I've cut/paste from an ST forum I frequent. The guy is very, very sharp:

There are two basic types of alternators used on bikes.

The first is the most common and uses a permanent magnet rotor rather than an electro-magnetic field. This type of alternator simply charges full out at all times, limited only by the RPM of the alternator up to the maximum of which it is capable. This type of alternator generally has the rectifier (changes the alternator's alternating current to direct current) external to the alternator and the rectifier is generally mounted as one unit with the regulator. The regulator acts to limit voltage by either loading the output to ground as would be the case with many accessories or "clips" the output.

This first type of alternator regulator is not used on the ST but is most common for motorcyles and other small devices.

In this type of system, the voltage regulator acts to limit the output of the alternator which means that it must also act to limit the output voltage of the bike's electrical system if another power source is connected to the bike.

There will be not problem in connecting your car to this type of system if the car's alternator is not operating. The problem occurs if the car's alternator operates and the set point of the car's voltage regulator is above that of the bike (which is likely). In this case, the bike's tiny regulator will attempt to load the car's 100 amp (or so) charging system to limit voltage. The result will be overload and burning out of the bike's regulator.

The ST's use an automotive type, variable current, field to control alternator output. If the car's voltage regulator set point is above the bike's, the bike's regulator will simply stop the bike's alternator from charging until the voltage drops. No problem.

Best advice for bikes is to avoid having the car running unless you know for sure. The car's battery will be able to crank the ST until the cows come home so not problem with running the car battery down.
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Thanks !

The only voltage regulator I'm familiar with, is the type that controls the field.

I'd really like to see a schematic for a system that uses the "clipping" or "bypassing" method. It's easy to see how one could easily smoke something designed for 10amps if you ran 50amps through it.
 
I've jumped my car from my bike whilst the bike was running.
 
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.
 
Burning1 said:
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.
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Exactly. That's how I read the schematic.
 
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