>
> >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.
The instalation is a bit more of a problem, but I think the reading is a
more reliable indication of the charge going into the battery. Installation
could be simplified by using an adjustable low voltage meter between the
battery terminal and the alternator (i.e. using the cable as a shunt).
> 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.
It can be useful, and if monitored with the reading from an ammeter you can
tell when the battery is fully charged (volts have gone up but current has
gone down). But you can't determine the state and/or charge of the battery
just from the voltmeter.
> 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?
Or a voltmeter. I haven't seen either on any of the vehicles I've had in
the last 10 years. After various alternator problems on one of my Transits,
I was going to fit an ammeter. I never got round to it though...
> >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.
Yes, but... it could happen when the batter is only a few weeks old. There
may be a 'dry' joint between cells (or some other internal defect) which
could break due to vibration.
> '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.
A very reasonable suggestion, but talking from experiance of batteries on
the various bikes I've had, they last anything from just over 1 year to
about 3 years and they don't give any warning when they're about to go. I
can jump on the bike, hit the start button, ride for an hour but not be able
to start again at the other end (I wish they'd bring back kick start... not
much in a Europa at 5000 ft though).
> 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.
The max. reading will be limited by the regulator on the alternator and I'd
expect the voltage to be low to start with (i.e. after starting the engine)
and go up as the battery charges. But the rate of charge will depend on
lots of things including the weather (temp. of the battery/how much charge
was used to start the engin (more in colder weather)), engine speed,
avionics/lights load. I wouldn't expect to see a higher voltage if a lead
came free than I would if the battery was fully charged. Regularly
monitoring the voltmeter as the battery charges will show a quick jump on
the voltmeter if the battery has a mechanical failure, but the jump may not
be big enough to notice as the battery becomes fully charged.
> Can't remember a battery lead ever falling off - if it was that loose
> it probably wouldn't have started the engine anyway!
It doesn't need to be loose, it may be a work hardened tab on the end of the
wire. Fine to start the engine, but a few minutes of vibration may push it
just a little bit too far. If this happened, there would probably be other
signes though, like additional noise on the audio systems.
> 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.
I don't think it would be unusually high. The regulator will limit it to
the max. voltage that's used to charge the battery, which will be about the
same as you'd see when the batter is fully charged.
> >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.
Sure. Probably best to test in the winter when the battery has less ability
to hold/release it's charge.
> >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.
The voltmeter reading will change on cranking depending on various factors
like engine temp, battery temp. If the cranking voltage is a bit lower than
normal, it could be just because the engine is a bit tighter (due to being
colder) than you were expecting (e.g. -5C over night and +10 when you try to
crank, but it takes a long time for the engine to warm up to the air temp.).
> 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>.
I wouldn't argue with Bob as I'm sure he's been in this game longer than me,
but I do think just a voltmeter can give very misleading readings.
Cheers,
Mark.
________________________________
Mark Jackson - +44 (0)7050 645590
europa-builder@ntlworld.com
http://harley.pcl.ox.ac.uk/~mark/Europa
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