Jump Start

[Phu Cat]

Coyote, I see Garry's points, all correct regarding Ohm's law, current flow, voltage, etc..  I also see your point regarding the car's slightly higher regulator voltage being a problem. But since both regulators are charging 12 volt systems, are car regulators voltage systems generally higher???  I wouldn't think they would be.

I respect your opinion as I've seen you answer some tough questions here, so fill me in on what I'm not understanding about the difference on car regulators.  (Over the last 25 years I've had two HD regulators go bad a week or two after jump starting from a running car.  I originally proposed the idea years ago that regulators might get trashed from a running car jump is because the sign waves from the alternator might be close enough to being in synch they'd arch thru the regulator's insulation..  I'm full of ideas about how trashing the regulator might happen, but short on fact.

PC

[Garry in AZ]

Hi PC,

The AC waveforms from the bikes (or cars) charging system are converted to DC current by a diode bridge, (otherwise known as a rectifier) before the voltage is regulated. For this reason, it's not possible for the individual waveforms from two different charging systems to ever become phase aligned and increase in voltage, they are removed long before the charging systems ever get connected.
This is not a perfect process, and some energy is lost as heat, which is why regulators have cooling fins. Also, in all cases a small amount of 3 phase AC ripple remains on the DC voltage, but it's generally insignificant.  One exception to this is when one or more of the diodes in the rectifier bridge is shorted, and then as Hippo said, you can get a bunch of AC ripple which has the effect of lowering the available DC voltage and if the vehicle has a radio, there will be a lot of whine in the sound.

Cars, trucks and bikes that use a 12 volt battery all have voltage regulator set point maximums within a few of tenths of a volt (14.2 - 14.5) to prevent overcharging the battery. Remember, the regulators output voltage has to be higher than the battery voltage to cause current to flow. New cars actually have the field coil current controlled by the computer, which automatically develops a historical reference of your driving habits, RPM ranges, battery temp, average electrical loads... and only allows enough current development to cover those needs. This is all done in the interest of fuel economy, believe it or not.

Although a modern cars charging system is much more sophisticated than what our Harleys use, the general principles, and voltages are very similar. I hope this really condensed version of what happens helps.

Garry

[Coyote]

Coyote, I see Garry's points, all correct regarding Ohm's law, current flow, voltage, etc..  I also see your point regarding the car's slightly higher regulator voltage being a problem. But since both regulators are charging 12 volt systems, are car regulators voltage systems generally higher???  I wouldn't think they would be.

I respect your opinion as I've seen you answer some tough questions here, so fill me in on what I'm not understanding about the difference on car regulators.  (Over the last 25 years I've had two HD regulators go bad a week or two after jump starting from a running car.  I originally proposed the idea years ago that regulators might get trashed from a running car jump is because the sign waves from the alternator might be close enough to being in synch they'd arch thru the regulator's insulation..  I'm full of ideas about how trashing the regulator might happen, but short on fact.

PC

The charging systems are DC (well after rectification) so the sync thing is not an issue. Charging systems for vehicle batteries are basically charging to a voltage. Depending on how the charging regulator is designed it might be damaged if it's designed to source OR sink to a set point voltage. Since the system on a bike is fairly low capacity (in amps that is) it probably could not fight against a car system. Also, since every regulator set point voltage is slightly dependent on the tolerance of parts used to build it, it's a given that one regulator might be targeting 14.3 volts, and another 14.4 volts. Not much difference but if you connect two different regulators that are 0.1 volts apart, they can exchange a lot of current trying to correct. If the car regulator was on the low side, you would probably not have an issue as the bike could never win. But if the car regulator was on the high side, it would do it's best to pull up the voltage on both the car and the bike. The bike regulator might be damaged trying to do the same. Most likely the damage would occur if you were using good quality jumper cables with a minimum of resistive losses. All of this depends on how the regulator is designed. It could survive this if designed correctly but most of the electronics on our Harley's are pretty marginal to start with. The automotive industry seems to do pretty well though. Probably why you don't see these failures on cars.

[FLTRI]

In other words, if you are stuck in the middle of BFE and there is an other 12volt vehicle available to jump from do it without the other vehicle running.
Otherwise it is not reccommended, especially if there is a proper charger available.

[Phu Cat]

Great explanations Garry and Coyote.  I understand wheatstone bridges, but can't get that goofy idea of the ac ripple out of my mind.  Thanks for the info.

PC

[ANNIEFATS]

Good thread. My question isn't worth a new thread, so I'll respectfully throw it in here:

How 'bout a bump start on an EFI? I grew up dumping the clutch on just about every bike I've owned, but never tried it with an EFI. Does the start button open any other sensors?

Looks like your question, was well deserved of a new thread.  Great question.
Even better explanations guys.
Thanks.
Todd