The reason I leave the jumpleads for a minute or so after jump starting, is to make sure the flat battery actually gets some charge, if it was disconnected too soon and the battery was well knackered, the alternator may overvolt and may (at the best) just blow bulbs but worst case scenario may blow something expensive.....
Do not even concern yourself with reverse discharge, the internal resistance of a battery will be higher than the resistance of a pair of jump leads!!
Oops, meant to add that in some cases, like when your battery is very dead, you *have* to do this, because so much of the donor battery current is going to charging the dead battery, that it can't supply the current required to start the engine.
I suppose you could disconnect the dead battery, hook up the cables to the donor, start the engine, then reconnect the dead battery (with the donor connected or disconnected.) Since running the engine takes much less current than starting the engine, the current siphoned off the alternator to charge the dead battery shouldn't be enough to stall the engine (should it?) However, it's usually simpler to just leave the dead battery hooked up and let the donor charge it for a while before attempting to start the engine.
Question for other folks: what would be the downside of disconnecting the dead battery to use a donor to start the engine, then reconnecting the dead battery once the engine is started? Any chance for damage to the engine or electrical system?
Sure - but I've done what he said, take the vehicle battery out of circuit till the engine's running, loads of times. You just have to have a system you can trust, or trust yourself (unlikely), not to put both batteries out of circuit at the same time.
Fair enough. However, if you carefully read what I suggested, there is a battery connected to both engines while each engine is running, throughout the operation:
1) Disconnect dead battery from its car, for example, by disconnecting the ground strap from its connection to the car body (to avoid near-battery arcing on reconnection)
2) Connect jumper cable from the + terminal of the live battery to the + terminal of the dead battery, which is still connected to its car wiring
3) Connect jumper cable from the - terimanl of the live battery to a good ground on the body of the car with the dead battery
4) Start car with live battery
5) Start car with dead battery
6) Reconnect dead-battery-car wiring to the dead battery, being careful to not disconnect the jumper cables
7) Disconnect jumper cables.
During steps 5 - 6, it is imperative to make sure the jumper cables connecting the dead-battery-car engine to the live battery do not break their connection. OK, in what I originally posted, I allowed for the possibility of reversing steps 5 & 6. Given your comments about never having an electrically isolated alternator, it seems that 5 & 6 have to be done in this order.
For example, is it a problem to have both alternators running with both connected to one battery? Is the sudden current flow due to re-connection of the dead battery to the circuit with the two running alternators a problem?
I understand what you say about getting a new battery, and I agree. I'm just curious.
Finally got around to the recommended Google, and I came up with the following:
"Field Decay Transients These are high energy, high voltage NEGATIVE transients. These typically occur if an ignition switch is turned off while current is flowing in an inductive load such as an electric motor or alternator field coil. Therefore, it can occur several times per day."
This seems to say that each time you shut off the car (and thus turn off the alternator field coil), you have transients of -60 to -120 V lasting up to
200 ms. These are similar in magnitude and duration to the load dump of 60 to 120 V lasting up to 400 ms. If the former doesn't hurt the car when you do it repeatedly, tens of thousands of times over the life of the car, why would the latter happening once? Is there some sort of isolation on the field coil that is not present on the stator coil? I'm not being argumentative, I just want to understand.
Thanks, that is what I was looking for. I guess the weakest (least trustworthy) link, aside from my own ability to remember to do things in the right order, is the jumper cabling, which is only held on with giant aligator clips and can slip off fairly easily. It's one thing if that happens and you've got a battery hard-wired to each alternator, it's a whole other thing if one battery has been disconnected. That's enough reason for me not to try it, I was just curious if there was potential for other damage aside from caveats about batteryless alternators.
That's an interesting observation, and good question. One thing that comes to mind is that it's easier to protect against negative transients than positive overvoltage transients -- a reverse connected diode will take care of the negative ones without much power dissipation in the diode. A zener is a simple possibility for positive transients, but it has to be sized carefully so it never comes on under any normal voltage condition but still conducts at a voltage below that which would harm the circuitry. And it would dissipate a good deal more than the reverse connected diode. Of course, there are also other ways of dealing with the problem.
It is true that the automotive power system is a tough environment for electronics, and maybe load dump presents no added danger to circuitry. But I think I'll keep my battery terminals snug anyway.
Really? My memory of electronics is not great, but I thought that reverse biasing a semicon junction at voltages like that would cause permanent damage, much worse than forward overvoltage. Really must refresh my knowledge of electronic circuitry....
Agreed. Field-dump transient damage is a well-known phenomenon, albeit not really "exciting" as one poster described it--more like some part of the electrical system just won't work the next time you attempt to start the car. So I'm not suggesting it doesn't happen or that anyone should ignore the possibility, I'm just trying to understand why it happens then and not in relation to the various workings of the alternator field coil.
Perhaps the answer is that, when the engine is turned off, the electrical curcuit is completely broken, whereas when the battery is disconnected, the alternator is still connected across the rest of the electrical system. When the circuit is broken, the entire transient appears not across any circuit components, because they represent finite resistances, but rather across the open circuit of the key switch, since it represents an essentially infinite resistance (unless the transient reaches the breakdown voltage of the air.)
There's also the issue of the field decay transients resulting from reversing of polarity of the alternator field coils twice per revolution of the alternator (or some other number of times per revolution, depending on how many field coils there are). At 2000 engine rpm (assuming 1:1 gearing to the alternator rotor), that field flip is happening about every 15 msec. This is on the same order as the field decay transient rise time, and much faster than the time constant for the field decay transient decay, so perhaps the transient essentially never gets established under these conditions before its polarity is reversed--or maybe a very large capacitor across the field coil is used to smooth it all out to essentally zero.
In a very loose sense, I guess the battery is meant to act like a transient-damping capacitor for the stator coil of the alternator. Perhaps it's just not practical to provide such an actual capacitor to damp transients across the stator coil, since such large currents are involved, so you just trust the battery to work and stay connected, and concede that you're not going to be very happy at all in the rare occurrence that the battery accidentally becomes disconnected with the car running.