On 22/6/19 8:00 pm, 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

A 12 Volt lead acid battery will show 13.2 volts straight off the
charger, about 2.2 volts per cell. That will drop to about 12.7 volts
after a day or so, a tad over 2.1 volts per cell.

On the other hand, a vehicle's nominal *system voltage* is 14 Volts.
That's because the *alternator typically operates in the 13.8-14.2 range.

Due to increasing loads on vehicle electrical equipment, manufacturers
were pushing to a nominal 42 Volt electrical system on cars. They were
to be equipped with a 36 Volt battery. It may not happen now.
https://en.wikipedia.org/wiki/42-volt_electrical_system


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

Most batteries fail gracefully as they progressively lose *capacity*.
The first sign of impending doom is a lack of gusto on the first turn of
the engine on the first cold start of the day. Batteries don't like cold
weather, their chemistry just isn't happy. The rest of the day they will
start pretty much as normal. Most people however aren't as attuned to
this so fail to observe that subtle change. It won't get better however
and it will eventually be noticeable by all and sundry as it will, one
slightly colder morning, fail to start the car at all.

--

Xeno


Nothing astonishes Noddy so much as common sense and plain dealing.
(with apologies to Ralph Waldo Emerson)