amperage available vs used question

[R_W_B]

I've heard that when jump starting a motorcycle from a car, or jumping any vehicle that could have a lower amperage designed system than the "one doing the jumping", one should always have the "jumping vehicle" NOT running. Otherwise it could damage the diodes in the lower amp system, as well as possibly damage the charging regulator of the weaker system.

I realize the some car alternators can put out way higher amperages than most motorcycles, but I'm a bit confused on how the amp consumption of the lesser vehicle would draw said "available" amps from the larger system.

I.e. it's all still 12 volts, it appears to my novice intelligence that the motorcycle starter would still only pull the needed amps offered by 12 volts, regardless of how high the amperage capacity of a the "jumping" alternator (and or high amp battery). (?)

Further how is 12volts (14 to 15 running) of (anything) going to force itself back down the throat of the lesser regulator line (?)

Obviously I am not seeing something.

 

[crutschow]

No, you are seeing it correctly. It's whoever stated what you "heard" that doesn't know what they are talking about. Having the larger vehicle running when you do a jump start will not damage anything in the smaller vehicle (as long as they are the same voltage). As you correctly noted the amp capability of the vehicle is not factor, only the voltage.

 

[R_W_B]

Oh ok, that's enligtening. I actually had it visualized correct for once. I read a post on a motorcycle forum that said he had diode damage from doing such. I'm no expert but I just couldn't see how the amps (available) could force their way up a normal running system.

Now if he had short or bad connection somewhere I could understand then it would draw the "available" amps then, but that's kinda academic since there's a problem there either way.

 

[KeepItSimpleStupid]

Jumping any vehicle is not without risks. Having the "jumpee" vehicle not running is a lower risk situation.

First make sure both vehicles are 12 V. Yep, 24 V and 6V positive and negative ground vehicles did exist.

This ASSUMES two 12 V negative ground vehicles.

The correct way to jump a vehicle is to connect to the good battery terminals first and keep the cables separated. Then connect to the positive terminal of the bad battery and finally to the frame of the engine.

The frame connection does two things for you: 1) It lowers the resistance of the connection because the negative battery cable is out of the picture when starting and 2) Any connection or disconnection sparks get created away from the vents of the vehicle battery.

Remove the ground clamp first so that sparks created are away from the battery and then the positive.

 

[Mr RB]

I've heard that when jump starting a motorcycle from a car, or jumping any vehicle that could have a lower amperage designed system than the "one doing the jumping", one should always have the "jumping vehicle" NOT running. Otherwise it could damage the diodes in the lower amp system, as well as possibly damage the charging regulator of the weaker system.
...

I would question that! The alternators have a fairly high output impedance and make a very finite max output power. They will handle quite a lot of abuse, in all types vehicles including very high short term loads.

In older AC regulators on car alternators and most motorcycle regulators, the regulator actually shorts any excess energy to ground once the battery >X volts! So the default method of regulating charging is for the regulator "diodes" to actually short the entire alternator to ground which the vehicle is running, even at highway speeds! Hence the magnetics on the alternator is designed to have high source impedance and a finite max power and max current into a short.

I've never heard not to run the car doing the jumping, maybe that is based on some modern issues of car electronics to do with the electronic ECM ignition or electronic fuel injection etc. I really doubt it is an issue with the "diodes" in the car's alternator!

 

[dr pepper]

The first half of your op is correct, the second half on very old engines with dc generators if you had the bikes battery and the car battery paralled for a long period the gen would overload, and quite often with ye olde mechanical regulators overload protection wasnt fantastic and could result is something burnt out.

That is not the case for machines made after the 70's, I suspect what you heard is an old wives tale from way back when.

 

[Mike odom]

The only thing I would question or suggest is to make sure the AC on the charging vehicle is not running. If the compressor drops out, the impending "LOAD DUMP" may overwhelm the whimpy motorcycle regulation system, since it is not built to handle such large voltage jumps, as motorcycles generally do not have AC's... this is also why there is an entire class of electronics for automotive use, to take the abuse of the 'noisy' automotive power generation systems. This was a problem we saw with Aircraft electronics... our PE felt the power supplies we used to bench test the flight data recorders was too clean, and systems that passed on the bench, through burn in and post burn in testing, would be returned with burned out parts.

 

[Mike odom]

on very old engines with dc generators .

ah yes, ye olde DC generators... "make sure the engine runs (at highway speeds) for 30 minutes or the battery won't be charged enough and the car won't start (again)... with the newer AC (three phase, no less), 5 minutes at idle yields a full charge! I worked with an engineer in New England that when the diodes in his alternator burned out (remember the old Chevette?), instead of paying the whopping $100 they wanted for a new one, he just mounted 6 diodes to a heatsink and mounted it to the top of his alternator. The only problem we had with it, and the only reason I knew it existed, was when we splashed through a puddle and it stalled out. We had to wait for it to dry before we could continue on...

 

[Mr RB]

Yeah a lot of the old car alternators just had diode packs bolted to the back of the alternator, and a separate regulator (often a mechanical vibrator type!) mounted to the car body in the engine bay. That was common up to the late 70's. You could swap diode packs (or regulators) between models in a pinch if you could solder some wires.

 

[dr pepper]

In the uk lucas systems that had the 3 coil electromechanical regulators were a dc generator, no diode packs, you had to service the slip rings every so often.

 

[Mike odom]

oh yeah, come to think of it, when I replaced the alternator on my '91 camry two summers ago, the voltage regulator is built in to the alternator! The way they regulate voltage is to change the bias in the stator windings. This is also why you can't push start a car with a dead battery, as there needs to be some type of voltage present to generate a voltage in the field windings.

Back in '77 I was driving from VA to MO and my battery light came on. I stopped at a service station in west VA to have my alternator checked (68 Chevelle). They said it was good, and we were baffled about what could be causing it, until another mechanic came over and said, "you have one more chance" and checked my regulator, which was mounted to the sidewall. It was bad, so I drove to a parts store and picked up a new one, replaced it in the parking lot, and was on my way. I still remember the old red brick buildings and the steep steep mountains right behind them.

Also, in '79, when I was stationed in CT, a friend bought a lobster boat that had sunk (halfway) in the harbor as the kid that owned it (his parents were in Europe) had let the battery run dead and the bildge pump quit working. The engine was a chrysler marine, and the electrical system was completely shot, including water in the voltage regulator box. I didn't know anything about MOPAR, so I bought all delco parts and re-wired the thing like a chevy. All we had to do was flush the engine, pour oil in each cylinder, and the thing started up and ran like new... whoo hoo... free lobster (for the summer, at least).

ahhh, the good ol' days (pre electronic ignition, electronic fuel injection, smog controls, and controller based timing advance) when you could work on your own cars!

 

[dr pepper]

I think you mean rotor fieldc rather than stator field, the stator in modern alty's is just the generation winding.
Theres a boat in the marina where mine is, its a canal boat, noone is impressed by the chrysler engine it has, but I spose its 40 odd years old now.
Mines getting on to 60, the old bmc still runs really well, they only need a service and a new cam chain every 50 years or so.

 

[crutschow]

After following this discussion, I realize there may be a possibility of damage to the motorcycle charging circuit if the motorcycle has a shunt type regulator which simply dumps excess current to ground above a certain voltage. A car alternator current could overload this type of regulator and burn it out. It thus would be best to leave the car engine off when jumping to a motorcycle, since that would eliminate the possibility of any excess current through the motorcycle regulator. The car battery will have more than enough energy to start the motorcycle without having to run the engine.

 

[Mr RB]

Sorry to argue Crutschow but I'm not sure that's neccessary as a general rule.

The motorcycle shunt regulators of that type (zener shunts that go across the battery) are extremely old and rare, ie 1950's British bikes, maybe some very small cheap Jap dirt bikes ie 1970's although they tend to be 6v anyway.

Modern road bikes etc have "shunt" style regulators but they use SCRs in their own regulator diode pack, so they only short their own alternators to reduce voltage, they don't shunt anything to do with their own battery or 12v system.