Post by RickyCOr better still, I should probably have asked: What is an easy way to
monitor one's alternator output while driving?
I don't know if they still do it, but Maplin used to do a kit - I ended
up building and fitting one to my mum's Belmont, which regularly
suffered from a flat battery because of the short runs and frequent
starts that it used to do. It was basically a voltmeter, but quite a
sensitive one - IIRC, about a dozen LEDs, giving the voltage in
something like 0.25v steps. It wasn't too tricky to build, provided you
know which way round to hold a soldering iron.
As others have said, an ammeter has to be wired in series - generally
between the battery positive and 'everything else.' An ammeter will
only tell you how rapidly your system is charging or discharging. An
accurate voltmeter will give you a better indication of the charge state
of the system.
And even a voltmeter shouldn't tell you much if the engine is running.
It'll show up a fault (overcharging or no charging) quite quickly, or
let you know that the alternator can't keep up with your 10Kw stereo
from hell, but it won't give you any indication of a full or flat
battery on an otherwise working system. The alternator's job is to do
as much work as is necessary to pull up the car's electrical system to a
certain voltage (which will, in turn, charge the battery.) If the
battery charge is low, the alternator will produce more power (but be
harder for the engine to turn and so use more fuel.) If the battery
doesn't need charging, the alternator will more or less free-wheel.
For that reason, I wired the Maplin voltmeter so that it was always on.
It used bugger all current, and gave you a good idea of the state of
charge when the car had been standing without the engine running for
some time, which is the only time you're likely to get a meaningful
reading.
--
David Thornber