Uncle Used Running Car to Jump Bike

[Guest]

So, I'm visiting my uncle and he's a motorcycle enthusiast. I asked him about his bike and he told me it was good except that his battery died. He bought a new battery and the new battery now loses its charge as well. After chatting about it for a minute, I figured out that he had started up the bike by using a running car to jump it, which I know will damage his charging system. So, my question is: What blew? R/R, stator, or something else?
It's a 1100 Honda Shadow. I think it's a '96. I'll double check tomorrow.

[posplayr]

It is called "load dump" and will kill unprotected electronics. R/R is a primary suspect but anything else with semiconductors is possible, the more highly integrated the more likely they get killed (without protection).

https://en.wikipedia.org/wiki/Load_dump

http://webcache.googleusercontent.com/search?q=cache:yRHxlJO6fTwJ:electronics.stackexchange.com/questions/8798/how-do-i-protect-against-an-automotive-load-dump+&cd=14&hl=en&ct=clnk&gl=us

[wymple]

Or his charging system was NG to begin with, when he swapped out the 1st battery.

[Who Dat?]

I agree it's likely the R/R that is not working, whether the running car blew it or it was already gone, as wymple suggested.

Look at it this way: let's assume that the R/R's setpoint is 14.1 volts. It will try to regulate incoming voltage to that point by shunting (shorting) to ground momentarily. When it 'sees' the voltage below that, it will allow current to flow again. It repeats this process about 1000 times per second. If the alternator on the car is regulated to 14.4 volts (typical), the bike's R/R will be living in a dead-short condition. The problem is that the R/R is rated to handle about 25 amps, but the car's alternator is probably capable of pushing over 100 amps. Since it sees the bike's R/R as a heavy load, it's probably pushing close to that 100 amps. Instant toast.

[Gorminrider]

old thread, but regulator aside, even without the car engine running, it seems a bad idea connecting a charged car battery directly to a discharged motorcycle battery.
Depending on the amp capacity of the car battery, there would seem to be a nasty instantaneous current surge involved, probably harmfull to the MC battery. (If you need a boost, maybe better to flag down a Yaris rather than the Dodge Ram?)

Yes,even though the car battery voltage will quickly drop and reduce the current somewhat as they both equalize to same voltage potential,(and then neither will be fully charged but it should start the bike ok) it seems to me it'd be better to insert a low ohm resistor (a high beam headlamp?) in series or...at least, tap the connecting wire repeatedly rather than just hold it on...
I'm pretty sure Charging two differing batteries (even where one is just older, let alone a different amphr rating) from the same charging source at the same time is a bad idea too and degenerates both batteries .

As to hooking up a running vehicle, again, the same sort of Resistor might work? as in, the car regulator sees total voltage including R but the MC regulator sees it without the R and therefore won't "shunt" - the MC battery will never rise to regulation voltage, but it will get enough to start the bike after a short while.

That's a good point above,

let's assume that the R/R's setpoint is 14.1 volts. It will try to regulate incoming voltage to that point by shunting (shorting) to ground momentarily. When it 'sees' the voltage below that, it will allow current to flow again. It repeats this process about 1000 times per second. If the alternator on the car is regulated to 14.4 volts given regulation set points differ.

but not if they are the same or the car's regulation setpoint is lower...?

[posplayr]

Jumper cables are used to jump batteries as a matter of standard practice. The current flow between the two batteries is never going to be that great regardless of the amp hour rating mis-match.

To understand this you need to view the battery in light of what is an idealized model of a battery (i.e. its Thevenin equivalent ) and the basic characteristics of what is known as "internal resistance". I'm not going to go through an entire diatribe here you can find links on the web for that.

https://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem

https://en.wikipedia.org/wiki/Internal_resistance

Basically if a battery is totally discharged, it may have a slightly lower voltage (say 12.5V) than a fully charged battery at 12.8V when sitting. But that is not the big difference; the internal resistance is very different for the two batteries. When State of Charge(SOC) is low, the internal resistance is high and so any attempt to push or pull current from the battery will cause a corresponding large voltage change (V=IR).

So you try and pull a lot of current from a dead battery and the voltage will collapse.
You try to push a lot of current into a dead battery and the voltage will jump. When the voltage jumps ups it goes above the source voltage (the good batteries voltage) and so the current trying to flow is squelched off.

Next time you want to jump a car battery, put the ground on to the batteries and tap the positive side onto the positive terminal. You get a little sparking but not arc welding.

The smaller the amp hour rating of a battery the higher it's internal resistance. So you will see even less current flow by connecting a smaller battery to a fully charge one than a larger dead battery assuming both batteries are at the same SOC.

The discussion of charging cycle times or anything in the motorcycle R/R has nothing to do with the issue of "LOAD DUMP".

The problem to be concerned about mainly is that if you are running an big 90 amp alternator with a high idle car there is quite a bit of energy being produced in that alternator. That energy is in the form of current being pushed into the batteries including the dead one in your motorcycle. Just like when you open teh circuit to an inductor like a coil, the alternator acts the same way (in EE terms it is V=L dI/dt), the voltage output is proportional to how fast you remove the voltage, how big the inductor is L. This is because the energy in the collapsing field has to leave quickly as the voltage drops. It is conservation of energy. If you remove the cables now, you get a big fat spike of current (100+ amps) and a big arch at the battery(100+ volts) but that voltage and current is also applied to your entire electrical system.

Car electronics are designed to be immune to this sort of thing as they have learned the hard way. I'm guessing some of that Automotive level of performance has found it's way into new OEM fuel injection computers on motorcycles, but most of the aftermarket electronics I see has hardly any protection.

The SSPB is not just a bunch of discrete FET's ; there was a lot of though put into the design to meet the basic requirements for +/- 100V spikes associated with these LOADS DUMPS.

[tkent02]

Thanks, I always wondered why it didn't do any damage to hop a big battery up to a small one. Now it makes sense.