Modern car alternators seem to be able to give out a huge amount of current at engine idle speed. I'm sure my friend got his to give out pretty much the full 80 amps without revving the engine. He was powering a small disco on a campsite :-)


On Sat, 22 Jun 2019 11:00:34 +0100, Brian Gaff wrote:

Yes indeed, the nominal output of an alternator can be as high as 15 volts,
but even a fully charged car battery is only 13.8v as far as I know and
these days, I'm sure the direction of current flow and voltages are
monitored very well by the computers. In the old days it was a bit of a
black art just relying on the ability of the alternator or dynamo in the old
bangers.
Normally the output will change due to engine speed, but in alternators
there is a voltage regulator inbuilt to keep the thing pretty nominal and
of course the thing that then suffers is the charging rate, ie its going to
be be slower when its not running very fast. I think if a battery dips below
about 11v outside of starter transients, you have to charge it or get a new
one. This very accurate sensing these days can often mask a battery on its
last legs though, as people tend to ignore warnings if the car still works,
then they leave it a couple of days and it won't start!

Brian