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

[LS1Bandit]

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").

 

[Entoptic]

 

[mototireguy]

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.

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.

 

[fast4d]

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

this doesn't sound right.....at idle I guess????

a car battery by itself as more than enough amps to start your motocycle engine.

when you have the car engine running that increases....also increasing the CHANCE of shorting or damaging the bike's electronics

 

[LS1Bandit]

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.

But that's because the *car* is drawing more power than the bike when the engine is running. Those 10amps don't get magically "diverted" to the bike when you hook it up. The car is still drawing those 10amps for its own use.

As I said, these are constant-voltage systems, not constant-current - they're not going to "force" power into a load.

 

[sasquatch]

It is fine to jump start a bike from a car as long as the car's engine is not running.
The bikes starter motor can draw plenty of current from the car's battery through a decent set of jumper cables.

The car's charging system, Alternator et.al., puts out anywhere from 50 to 100+ amps of current when the motor is running at 2-3000 rpm. The motorcycle charging system can't deal with this much power / current.

 

[LS1Bandit]

The car's charging system, Alternator et.al., puts out anywhere from 50 to 100+ amps of current when the motor is running at 2-3000 rpm. The motorcycle charging system can't deal with this much power / current.

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.

 

[kurth83]

This is a very interesting thread if you like EE.
Here is some popcorn fodder for you:

I am only a hobbyist, but I did have a few analog circuit theory classes in college since I was interested in that subject.

Probably the real question is what difference is there between the charging voltage for a car vs one for a bike.

I once had a car that charged at somewhere around 14v, and it put about 4a through the battery when it was full, and more if the battery was low.

I know for a fact that you can fry car electronics by jump starting. My dad had to pay $500 to replace his ECU after jumping someone (this was 10 years ago). I speculated that they dead shorted it while the car was running, but they wouldn't admit to it if they did.

So while basic circuit theory may make it sound safe, there are other factors involved like human error and charging voltage differences between different systems, so I guess I am in the camp of car off too. Safer for the bike and the car.

That said, I once tried to jump a car (with my car) that had a dead battery that acted almost like a short (the cables got hot, yeah, ouch), and my car was fine, so it takes a somewhat unlikely combination of factors to cause damage (poor car electronics, accidental dead short perhaps), but it can happen.

 

[stangmx13]

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.

this thread made me quite a bit curious... and i think i found an answer, up for your confirmation.

if you connected your jumper cables in series w/ the battery, all would be fine. the charging system would "sense" the small extra load that the motorcycle placed on it and would increase the power until the voltage made it back to 13.8 or something.

however, jumper cables are always put on in parallel. so, now the battery and the MC take half of the current that was previously supplied to just the battery. since the battery is a load, the voltage drops and current must increase. if we assume that charging the battery takes more current than the MC can handle... then everything goes to shit for the MC since the battery and MC will receive the same current.

actually, maybe the MC doesnt just get half of the current that was charging the battery... but half of the entire systems current, since everything on a car is grounded to the same place including the battery.

 

[kurth83]

Hey Stan,

You are assuming the internal resistances on the MC are the same as for the car, in parallel the amperage will distribute according to internal resistances, which may not be an even distribution. I think you are right that the car voltage regulator will up the voltage to compensate for the increased load. That is only an issue if the MC charge voltage is lower than that and the MC electronics can't take the example 13.8 charge voltage, since the MC electronics will draw whatever current is appropriate at any given voltage.

 

[stangmx13]

Hey Stan,

You are assuming the internal resistances on the MC are the same as for the car, in parallel they will distribute according to internal resitances, which may not be an even distribution. I think you are right that the car voltage regulator will up the voltage to compensate for the increased load. That is only an issue if the MC charge voltage is lower than that and the MC electronics can't take the example 13.8 charge voltage.

ahh ure right, i forgot about their internal resistance. id think that the chances of the MC have a higher internal resistance are good though. smaller components, smaller wires, small frame to ground too vs. a car with a ton of parallel connections on a huge grounded frame. iono.

can we agree that its not the voltage that kills MC electronics, its the current?? i have personally killed a reg/rec by charging my bike w/ a running car... sure it was my '66 Mustang that has very old electronics, but its not like it had a generator instead of an alternator.

if u actually are responding to me, the username reflects mustang, not stan.

 

[High-Side]

I've read that when connecting the positive cable clamp to the dead battery, you must be sure that you don't touch the clamp to any area of the bike other than the positive battery terminal. Doing so can send an electrical charge through the entire bike which can do serious damage to electrical and computer components.

 

[LS1Bandit]

Probably the real question is what difference is there between the charging voltage for a car vs one for a bike.

As I said, that would be where you might have issues. But except for hybrids and electrics, cars have the same charging voltage as bikes, which makes sense because the batteries are effectively the same (though of different capacities).

I know for a fact that you can fry car electronics by jump starting. My dad had to pay $500 to replace his ECU after jumping someone (this was 10 years ago). I speculated that they dead shorted it while the car was running, but they wouldn't admit to it if they did.

So while basic circuit theory may make it sound safe, there are other factors involved like human error and charging voltage differences between different systems, so I guess I am in the camp of car off too. Safer for the bike and the car.

If it was the human error of shorting, then it wouldn't have mattered if the car was on or not. If you've ever dropped a wrench across the terminals of a car battery and seen it "arc weld" and/or melt the wrench in half, you'd get some appreciation for the amps available from a car battery (even with car off).

this thread made me quite a bit curious... and i think i found an answer, up for your confirmation.

if you connected your jumper cables in series w/ the battery, all would be fine. the charging system would "sense" the small extra load that the motorcycle placed on it and would increase the power until the voltage made it back to 13.8 or something.

Actually, series would be the one time where I would envision there *would* be a problem because that would force the full current draw from the car through the motorcycle.

In parallel, the power will distribute according to resistance, and therefore the draw of the car would be effectively independent (assuming the alternator can keep up) of the load from the bike.

Put another way, since they're in parallel, if your "source" (in this case the car alternator) is able to maintain its constant voltage, the amount of current (amperage) that flows through the "motorcycle branch" is strictly based on the resistance of its load and is effectively independent of the car load.

That is why I'm confuzzled as to why having the car running would have an impact. In my mind, the only difference between the car running and the car not running is the fact that a fully charged car battery will typically put out about 12.6V or so, whereas when the car is running it will typically put out between 13.8 and 14.4V. So it would only be an issue if the bike isn't designed to handle 14.4V. But it is. Hence my "confusion".

To be clear, I'm perfectly willing to concede if I'm wrong. I'm not trying to state that I'm unequivocally right and that there is no problem starting a bike with a running car. What I'm trying to determine is whether this suggestion is strictly "fear based" or whether there is actually some concrete reason why it's a problem.

 

[ST Guy]

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.

 

[tuxumino]

have you ever jumped a car with a car and had to increase the rpms on the car you're jumping from to get the dead car to start?
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.

 

[Wormwood]

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."

 

[stangmx13]

....
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.

....

is this only a problem if you turn the MC key on?? or is the reg/rec "working" even with key off?

have you ever jumped a car with a car and had to increase the rpms on the car you're jumping from to get the dead car to start?
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.

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.

 

[Razel]

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.

Finally, the alternator on the GT I have has a 72amp capacity, which is more than some cars.

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.

 

[Razel]

is this only a problem if you turn the MC key on?? or is the reg/rec "working" even with key off?

Well ... how do we start the motorcycle with the key off? Or, are you only just charging the battery.

 

[stangmx13]

Well ... how do we start the motorcycle with the key off? Or, are you only just charging the battery.

ya, u answered ure own question

two possible scenarios that may or may not lead to MC reg/rec death:
-charge MC battery w/ running car w/ battery cables (MC) still connected
-start MC when 'jumped' with running car



this whole thread just makes me happy that im good at bump starting motorcycles