dr bob

I'm on board with the idea of using the other-car alternator to charge the 928 battery, but the cables need to be connected --before-- the engine in the other car is started. Go ahead and start the other car, run it for a bit to give the 928 battery some juice. Then shut off the donor car, start the 928.

I respectfully disagree.

This is one of the parts of the discussion that bothers me. If there was zero resistance between the two batteries you'd be OK. But the batteries are separated by jumper cables (typically by cheap 10ga jumper cables), poor clamp connections, the connection (small) between the jump-start post and the starter, and only then the battery cable to the battery. I'd be less bothered if you attached the cables to the battery directly.

See my suggestion on jumper cable size and connection position. You need to have the donor car running so it can pass starting current through 10ga jumper cables and still have some voltage left at the other end under the starter load. Spend well on premium cables and clamps. Or do as I did, and make a set of cheap jumper cables yourself from some 8ga, knowing that they are good for a booster charge on the battery but not enough to start the car.


The challenge in jump-starting comes when two charging systems, two batteries, and wildly varying loads, are connected together with tiny wires and expected to somehow come to a perfect electrical balance between the cranking current and the initiation of charging. All it takes is a little bit of load juggling between the two alternators, one sixth of one alternator revolution as it tries to adjust charging current demands based on sensed voltage, and decides for that instant as current draw changes that it needs full excitation. It corrects itself on the next cycle, but the damage to the brain is done. Probably would only happen once in a while. Worth the risk? Not to me. It's too darn easy to do it correctly and avoid that risk completely.

YMMV, of course. Proceed at your own risk.

Mrmerlin

Bob, I agree with your prognosis, I would add a few other thoughts, one keep the battery charged this is first defence to fend off other problems that will arise with low voltage batteries.
Dont try to jump start another car with your 928 unless your jumpers are connected to the 928 battery and the engine is off.
This prevents the old diode board on the your expensive alternator, from killing a diode, you will know in a few days when your battery all of a sudden wornt start the car and its charging at 13.1 or so. SB 13.6 or better.
Dont try to jump your 928 with the jumpers to the hot post, as Bob stated the other elex connections in the car can come into play and since most of the wire connections are 20 years old it wont be like a new car, IE old ground strap connections, CE panel connections, and hot post connections. Its best to connect your starting battery to the 928 battery with the other cars engine off, as this can also spike the brains, we have had to replace a few batteries and then to discover that the brains were also dead, only to find out that the car had been jumpstarted a few days previous, since the battery was dead to begin with.
So if your battery is low charge it with a trickle , and if your not going to be driving the car for over 2 or 3 weeks disconnect the battery and still charge it every few weeks, Keep em flying