Hear hear!
The voltmeter requires the user to be a little bit more knowledgeable to be
able to get the best from it.
It will tell the state of charge of the battery (i.e. in percentage terms
prior to start-up) and a failed or shorted cell will be immediately
apparent.
It tells whether the battery is being charged, overcharged or not charged
at all.
It won't tell accurately the remaining storage capacity of a battery,
which is normally done at annual-time by means of draining down through a
fixed load. In this instance an integrating ammeter can be useful if (and
it is unlikely to be) the degraded storage capacity is actually known. But
integrating ammeters come expensive (>?200).
The voltmeter will however show battery voltage drop against a heavy load
(i.e. cold cranking), which gives a reasonable indication of increasing
internal resistance (and hence battery life/capacity) everytime the
aircraft is used.
The voltmeter doesn't affect the compass either!
Ammeters disappeared on cars along with the dynamo. Probably because the
increased reliability and output capacity of alternators made their use
superfluous.
Duncan McFadyean
On Saturday, March 23, 2002 12:41 PM, Rowland & Wilma Carson
[SMTP:rowil@clara.net] wrote:
>
> >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
>
>
> | PFA 16532 EAA 168386 Young Eagles Flight Leader 017623
> | Europa builder #435 G-ROWI e-mail <rowil@clara.net>
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