Commander Kinsey wrote
Rod Speed wrote
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

They are a bit higher than that just after being charged.

Depends what voltage you were charging them with.

We're discussing cars.

13.8V is the recommended voltage for a continuous charge.

That's wrong too.

14.4V for fast, and 15V for very fast.

And so is that.

But the last two need to be stopped before boil over.

I used to have a home made solar array, with thirty old car batteries in
parallel. Bad idea. One of the batteries died - it lost a cell. The
other batteries immediately then charged it very quickly as it was 10V and
they were 12V, and evaporated the other cells one by one.

Trivially easy to avoid that.

There was eventually an explosion

That was the hydrogen you stupidly didn't get rid of.

and the battery was blasted across the garage in several pieces, with a
bloody strong smell of rotten eggs.

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

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"Rod Speed" wrote in message
...
Commander Kinsey wrote

How does a lead acid battery charger (or car alternator) know when to
switch to trickle charge?

From the current the battery takes.

I can understand it noticing a drop in charging current if the battery
is on its own, but what if a random changing load is connected, as
there
is in a running car?

You just look at the current going to the battery. The variably
loads like with lights isnt supplied by the battery when the
engine is running, its supplied by the alternator.