Posted by Michael Pardee on February 25, 2007, 2:34 am
One caveat - modern alternators put out an awful lot of current into a
discharged battery. Back in the '60s alternators could run all day at full
output (often 35 amps) without overheating, but no longer.
It should be safe enough to drive for 15 minutes while charging a depleted
battery, but I'd hesitate to go longer than that. The battery would also be
happier with a low current charger.
Posted by Jack on February 25, 2007, 5:40 pm
Just wanted to make sure I understood your comment.
Is it your contention that a car that has been jump started should only be
driven for 15 minutes following the jump start.
Posted by Michael Pardee on February 25, 2007, 9:50 pm
Yes - it can be driven farther in a pinch, but the alternator gets awfully
hot and gradually cooks itself. This has been kicked around in
alt.autos.honda for a few years. What we noticed is that alternators failed
either because the brushes wore out or because the car was driven a lot
after being jump started. It's really obvious when dying batteries were
nursed along - the two complaints appeared together frequently. There are
those who feel the alternators killed the batteries, but the causality is
hard to pin down if that is happening.
Personally, back in the '80s I stopped charging dead batteries by driving.
Living in Phoenix, I noticed none of the batteries lasted a month after I
did that. Too much heat, I presume.
Posted by Claudio on February 26, 2007, 10:22 am
Last year, just some month after receiving a new Prius, I left it for
some days parked without actioning the automatic door closing system.
The light on the trunk was switched on and the 12 V battery went out.
I had problems in restarting the car, but then I realized it was a
stupid think: the secondary battery is needed only to start the
computer, so also a normal torch with 12 V DC is sufficient. The
motion battery is very well protected and it is improbably fully
In the engine bay of the car, in the fuzes box, there is a +12V DC
with a red cover: you can use this and the vehicle ground, applying
12V DC to restart the computer (less than 1 Amp. is required).
For this reason you cannot use the Prius to start another car, but, of
course, you can use another car to Jump Start the Prius.