In the boating world they use a special external protection device added
to the alternator terminals to prevent damage if the battery is
disconnected. I think they are zener diodes, 16 volt. These short the
output to ground when the threshold voltage is reached. Only good for
very brief times, spikes in other word.

http://www.emarineinc.com/products/alternators/zapstop.html

This is a real concern in boats since they frequently have battery
switches that allow for batteries to be disconnected and reconnected in
different banks.

I know of several cases of ECU failure when vehicles were driven with
the battery disconnected. One happened at Syncro de Mayo a couple years
ago. Someone driving to the event thought there was a problem with the
battery and disconnected it while the engine was running. Shortly after
the engine died and would not restart. They had it towed to the event
where we determined the ECU was now dead and the alternator regulator as
well. Someone else loaned them a spare ECU and I loaned them a new
regulator/brush pack. They bought a new battery in town IIRC. I know of
a carbed engine vehicle that lost the radio, clock, and various bulbs
after the battery was disconnected on a trip.

In both of the above it was a chicken and egg situation. Both batteries
were disconnected because they seemed to smell funny. The alternator
could have been the original problem.

Mark

John Bange wrote:

>> Is that the problem John, or is it the electronic engine management?
>
>
> Dunno. Could be all that stuff, really. All these new low-voltage
> electronic things are a lot more sensitive to wild swings in voltage,
> I guess. I know a lot of newer regulators are using smaller ICs
> instead of the giant wads of silicon of yesteryear, so they might be
> more touchy as well. I've never been able to get a definitive answer
> on the whys of not disconnecting the battery beyond the citation of
> voltage instability.
>
> --
> John Bange
> '90 Vanagon - "Geldsauger"
>