>I don't think a volt meter can tell you anything about the battery
Mark - obviously opinions differ on this. Bob Nuckolls of the
AeroElectric Connection might take issue with you if you made the
above statement on the AeroElectric list where he spends a lot of
time. He has said it's nice to have both a voltmeter and an ammeter,
and in the book he seems to regard ammeters as more important.
However, installation is rather more problematic for ammeters than
for voltmeters - witness the number of pages devoted to possible
arrangements.
I believe that a voltmeter which gives good resolution can be useful
on its own, and I'd want one even if I already had an ammeter. I
speak from experience, as I used to have a nice suppressed-zero
voltmeter in my car which I felt gave me better information about the
state of the battery than the ammeter which was also fitted.
Naturally, neither of those was original equipment! When I changed
that car, I moved the voltmeter to the next one, but did not bother
with the ammeter. One of the supporting reasons for that decision was
the extra work involved in getting the appropriate heavy wire in and
out of the panel to connect to the ammeter (it had a local shunt).
When I eventually got rich enough to buy a brand-new car, I didn't
want to cut holes in it, so passed the voltmeter on to someone else,
a decision I've since regretted!
Incidentally, [rhetorical question] when did anyone last see an
ammeter in a car panel? Why aren't they needed anymore?
>1) A cell goes open circuit after starting the engine (it does happen) or
>one of the leads falls off the battery. Your volt meter will show a >13V
>reading, but the battery is getting zero charge (an ammeter would show this
>fault).
A cell going open-circuit is likely towards the end of the battery's
life. 'Lectric Bob commends treating batteries as consumables and
replacing them regularly whether you need to or not. Thus you never
reach the stage of the battery "unexpectdly" letting you down. You
can have a dual-battery setup where the "primary" one gets moved to
the "backup" position each year, and then gets kicked out the second
year. As I've not experienced it, I'm not sure, but I suspect that a
voltmeter would give some indication of this problem in
uncharacteristically high readings, although it might be more subtle
than an ammeter reading.
Can't remember a battery lead ever falling off - if it was that loose
it probably wouldn't have started the engine anyway! Depending on
whether the voltmeter lead stayed attached to the battery or to the
loose cable, you'd see unusal voltage readings too - below charging
voltage if on the battery, and unusually high if on the loose cable.
>2) Battery not holding a charge.
Same as above - that battery should have been pensioned off already.
'Lectric Bob suggests a simple capacity test that you can carry out
at intervals if you want to avoid throwing out a battery that still
has some good life left.
>3) A duff battery may show a reasonable voltage but may have too high an
>internal resistance to let the current out
Regularly keeping an eye on the voltmeter during engine cranking will
tell you something about the battery's internal resistance, and you
can observe trends long before things get to the show-stopping stage.
I guess someone's sure to ask, so Bob's website is at:
<http://www.aeroelectric.com/> and you can find out about subscribing
to the AeroElectric list at
<http://www.matronics.com/aeroelectric-list>.
As I said, my opinons above are based on personal experience - your
mileage may differ.
regards
Rowland
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