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

[bluesjr]

Have not read all 6 pages to check if this has been covered, but am going to throw this out anyway.

If you jump from a car, there is some risk to the car's battery. Same thing from one car to another (or one moto to another). The risk comes from one way batteries fail - a shorted cell. If your dead battery reads 10V or less, instead of 12V, (each cell being ~2V), then one of the cells may be shorted. It's an easy thing to check and volt/multi-meters can be had cheap these days.

 

[ST Guy]

So much engineering theory and postulating...

Riders... The donor car is like a charger. It is not loading up the motorcycle RR or stator.

Yes and no. Yes it's like a charger. But it can load up the bike's VRR and blow it. (But not the stator; that's part of the alternator.)

Look at it this way. If you had a dead or low battery on your bike, would you hook up a roughly 1.6 amp charger designed for a motorcycle? Or would you hook up a much higher amp charger? Like 10, 20, 30 amps? The latter is what can effectively happen if you jump from a running auto or truck. Not always, but it can happen. And if it happens, you can often get away with it. But not always. It's a chance you take. Turn off the engine of the car or truck and the risk goes away. Simple.

 

[Burning1]

I cannot see that a newer, electronic (transistorized) regulator would operate completely opposite. That is to say, running full power to the stator and shunting excess power to ground creating a shit-ton of heat in the process.

See the link in one of my previous. The internal schematic for the SV650 rectifier is linked, and dumping to ground is exactly how it operates.

 

[Burning1]

Yes and no. Yes it's like a charger. But it can load up the bike's VRR and blow it. (But not the stator; that's part of the alternator.)

Look at it this way. If you had a dead or low battery on your bike, would you hook up a roughly 1.6 amp charger designed for a motorcycle? Or would you hook up a much higher amp charger? Like 10, 20, 30 amps? The latter is what can effectively happen if you jump from a running auto or truck. Not always, but it can happen. And if it happens, you can often get away with it. But not always. It's a chance you take. Turn off the engine of the car or truck and the risk goes away. Simple.

That's fair, but that sounds like a risk to the battery, not to the VRR. I'm willing to bet that a lot of bike alternators are capable of putting out 20+ amps. The alternator in my GT actually puts out more than 70, and is designed to run not only the full electronics package, but the ESA, heated seats, heated grips, and a full after-market accessory load out... Al lot of which won't be used on the bike during summer months.

 

[Burning1]

The reason you can burn out a stator under heavy load was well-described by Burning1 and was spot on. However what I forgot to consider was that in the case of the r/rs that we're discussing, they effectively create that same load when shunting power to ground. So whether the power is going to charging/running your system or it's being shunted to ground, it seems the stator should be under the same load.

I would expect though that the Regulator / Rectifier wouldn't be under the same load. I think though, that some understanding of the failure modes of diodes would be relevant here, and that's way beyond my pool of knowledge.

What I need to ponder is whether/if a greater load can exist through the draw from the MC system (be it from additional loads or from a deeply discharged battery) than from the shunting of power. My intuition actually says that it *is* possible (which brings us back to the possibility of burning out a stator charging a dead battery), but I can't say that with confidence right now.

I've had my stator burn out 3 times on me, and I had been told (and blindly agreed) that it was because I was running a heavier-than-normal load (aux lights + GPS). I need to sit down and think about whether that's actually what was causing the burn-out.

My suspicion is that the regulator/rectifier heats up and provides some level of resistance to the stator under low loads (since it's shunting power to ground.) Under heavier loads, it's possible that the RR provides less resistance, putting a greater strain on the battery.

That's 100% supposition, however. I'm sure someone with some real practical hands on experience would know better.

 

[ST Guy]

That's fair, but that sounds like a risk to the battery, not to the VRR. I'm willing to bet that a lot of bike alternators are capable of putting out 20+ amps. The alternator in my GT actually puts out more than 70, and is designed to run not only the full electronics package, but the ESA, heated seats, heated grips, and a full after-market accessory load out... Al lot of which won't be used on the bike during summer months.

Well, now we're getting into specifics. And certainly, depending upon the bike in question, it will or won't be a potential problem. With a 70 amp alternator (I am so envious), you'd likely have little to be concerned with jumping from a running car as it's got a VRR that has a rather high capacity. Heck, even the VRR in my ST probably wouldn't have a problem. It's rated at 40 amps but many examples of this particular alternator/VRR put out closer to 55 amps. And the VRR in it is an automotive Nippon Denso unit. But for other bikes where their changing systems don't put out nearly as much and accordingly their VRR's aren't rated to handle as much, jumping from a running auto has the potential to be a problem. That's why I keep harping on having the vehicle doing the jumping off.

 

[Burning1]

80 amps, and BMW in all their wisdom decided that 5-10 amps should be the limit for each of the 3 power sockets... :|

 

[ST Guy]

My suspicion is that the regulator/rectifier heats up and provides some level of resistance to the stator under low loads (since it's shunting power to ground.) Under heavier loads, it's possible that the RR provides less resistance, putting a greater strain on the battery.

Whatever the particular senarios are, it's the heat that get's the VRR and kills it. And so many of them are just potted in a non-conductive epoxy and therefore very thermally insulated. These are the ones that seem to burn out the most often in general use. And for some reason they're the ones mostly favored by the bike manufacturers. Probably cheaper that way. Some have added finning. Some even minimize the potting and expose them to air flow.

With all the bikes that even today still have charging system problems, you'd think that the manufacturers would put a decent charging system on their bikes.

 

[SpikeVFR]

You are waaayyy over my head already, so I can't help you. But interested to hear it too.

I was going to ask this in the thread on the dead battery where it was mentioned, but decided not to pollute that thread.

Can someone with a firm understanding of the electrical/electronics behind this recommendation explain it to me? Note, when gauging the level of explanation I'm an electrical engineer.

I've heard this advice over-and-over and quite frankly it befuddles me, and the only explanations I ever hear are "look at the power requirements of a car, look at the power requirements of a bike, do the math." And similar generalities.

My first and primary point is that car alternators are constant-voltage, not constant current. Meaning they're not going to "force" power into a load. When a load is put on that lowers the voltage, it will increase the amps delivered until it reaches the proper voltage. No load - no amps (for the most part).

My second point is that the internal resistance of a modern motorcycle battery, especially one that is depleted, is so low that most "excess" power will go directly into the battery, not in the systems it's connected to.

My third point is that a car battery can deliver *significantly* more amps than a car alternator can, so saying that it's OK to hook up a car battery as long as the car is off really doesn't make sense to me. If "excess amps" were the problem, simply hooking up the car battery would be the big issue, not having the car running.

My fourth point is that what's often pointed as the weak spot on the bike is the regulator/rectifier. However, as the name implies, the job of that device is to rectify the a/c current from the generator/stator and bleed off excess power from the generator. I suppose that's where the idea that this is the most likely to have problems comes from (bleeding off excess power), but the r/r is "upstream" of the battery and has diodes on it - it's not going to bleed power that's coming from the "battery side" only from the generator side.

Now if the *voltages* were different between a car and a motorcycle - that could be a problem. But every car I know of runs the same voltage levels (aside from hybrids and electrics) that a bike does.

So can someone help me out here? Am I just missing something? Or is this just advice based on loose generalizations/ideas ("car alternator big - motorcycle small - bad").

 

[MotoStark]

So I can I jump my bike with a car battery?

 

[ST Guy]

So I can I jump my bike with a car battery?

You betcha! And to play it safe, make sure the engine of the donor vehicle is not running.

 

[Razel]

So I can I jump my bike with a car battery?

Yes.

Your option on having the vehicle running or not.

 

[SpikeVFR]

Wouldn't the dead cell be more likely to be on the bike battery? I don't think most would use a busted car battery to jump their bike, would anyone? So given a full car battery and a dead bike battery, even with one cell out, how would that create a hazard to the car battery?

Have not read all 6 pages to check if this has been covered, but am going to throw this out anyway.

If you jump from a car, there is some risk to the car's battery. Same thing from one car to another (or one moto to another). The risk comes from one way batteries fail - a shorted cell. If your dead battery reads 10V or less, instead of 12V, (each cell being ~2V), then one of the cells may be shorted. It's an easy thing to check and volt/multi-meters can be had cheap these days.

 

[WhatsYourHandle]

Do not understand a word of what was typed in the past six pages. But from practical experience I have not had any issues starting a ninja 500 with a car not just running but spinning ~4,000 rpm. Also had no issues starting a vulcan 900 from a car that was not running. In both cases the car was an econobox 4 cylinder, no double, water-cooled, 760Li alternators here. Both bikes are just fine many moons later.

 

[Motech]

Yes and no. Yes it's like a charger. But it can load up the bike's VRR and blow it. (But not the stator; that's part of the alternator.)

Look at it this way. If you had a dead or low battery on your bike, would you hook up a roughly 1.6 amp charger designed for a motorcycle? Or would you hook up a much higher amp charger? Like 10, 20, 30 amps? The latter is what can effectively happen if you jump from a running auto or truck. Not always, but it can happen. And if it happens, you can often get away with it. But not always. It's a chance you take. Turn off the engine of the car or truck and the risk goes away. Simple.

I would have no problem slapping my dead moto-battery back to life with a variable output charger set on 20 amps, especially a pulse-modulated charger. Done it many times. I'd not leave it there very long, but that's because I'd want to recharge the battery at a lesser rate, certainly not because I feared mythical damage to the bikes circuitry.

 

[Motech]

80 amps, and BMW in all their wisdom decided that 5-10 amps should be the limit for each of the 3 power sockets... :|

Right, then you plug in heated gear and the smart-ass CAN-BUS brain shuts off your heat for you.

 

[Razel]

Right, then you plug in heated gear and the smart-ass CAN-BUS brain shuts off your heat for you.

Nah, you hook up the fused wire with the battery eyelets at one end and the mating plug for the jacket at the other, run it from the battery to under the seat and out the side ... and you're good for two jackets, no fussin' by either CAN nor BUS.

 

[Jabberwocky]

I ran a motorcycle battery in my camaro for 3 years, then in my rx7 for 2 years and now my subaru for the last 3ish years. Bunch of car racing guys i know do the same. I'm not saying it is wrong or right, just saying.

 

[ST Guy]

I would have no problem slapping my dead moto-battery back to life with a variable output charger set on 20 amps, especially a pulse-modulated charger. Done it many times. I'd not leave it there very long, but that's because I'd want to recharge the battery at a lesser rate, certainly not because I feared mythical damage to the bikes circuitry.

I was trying to make an analogy. Of course, sometimes a dead battery is best shocked back into life. I think even Yuasa has some info on their site on how to do it. And a special charger like the one you seem to have is a good idea. But you wouldn't just slap a generic 20 amp charger on a bike battery and leave it overnight.

The point is that a VRR can get fried a hell of a lot quicker (if circumstances are correct) than a battery could get fried.

 

[LS1Bandit]

The point is that a VRR can get fried a hell of a lot quicker (if circumstances are correct) than a battery could get fried.

You keep saying that, but as of yet you still haven't given anything to support that. Do you actually know that there are any VRRs out there that will, in fact, try to shunt power coming from an external source?