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On 01/19/2014 04:56 AM, Fred Klein wrote:
> Up til now, I've planned to measure the voltage at the plus side
> of the battery...Is that the best place to measure?...or are there
> multiple places where voltage should be measured?...and how is
> that accomplished?...with a rotary switch?
Voltage should be the same over the entire system. Unless there is too
much resistance or loose contacts. I would put the voltmeter over the
panel bus, since that voltage is the most interesting during flight. I
wouldn't care too much if the battery contact is loose and the battery
voltage drops, as long as the alternator keeps up the panel voltage,
this failure will only cause problems with the next engine start, and
since you are then on the ground you can do all the trouble shooting.
There is not much else you need to know.
> Similarly, I've planned to place my ammeter in series on the
> output side of the alternator...will that do?...
Depends again on what you want to know. I have the ammeter in the
battery wire. This gives me the following information:
1) During flight the amp reading should be close to zero. This
indicates that the battery is fully charged and not taking current,
and also that apparently the alternator is feeding the electrical
system because no current is taken from the battery. (Directly after
starting I see current going to the battery to compensate the charge
taken out for starting, but it should run down close to zero after a
few minutes. If the battery continues to take current all the time,
this indicates that the battery is not so healthy anymore).
2) On the ground, with the engine off, it is easy to check the health
of the system, because every electrical load will reveal itself on the
ammeter since the battery is now feeding the system. I regularly
perform these tests: I know how much current sub systems are supposed
to consume, I toggle all switches individually and see how much load
is taken, if the value has changed since last time this tells you
something before real problems arise. If, let's say, the fuel pump
normally uses 2 amps but suddenly it consumes 4, it means something is
starting to go wrong with it (internal friction, partly clogged?) and
it can be expected to fail in the near future. ALso, this test,
combined with checking the voltmeter, gives an idication about the
health of the battery. If the voltage sags too much under load this
usually means that the battery is going to need replacement.
> places where amperage should be measured?...and if so,
> where?...and how is that generally accomplished?...with a rotary
> switch?
I try to avoid switches. Switches, conbined with their associated
wiring, connectors etc, are failure points. If you start to measure
weird values, it can be that something is wrong with the alternator,
but it can also (more likely?) mean that there is something wrong with
the switch, contacts or wires.
Just keep it simple: measure voltage in the panel, this is where all
sensitive avionics is and thus most important, and also it makes
connecting the voltmeter much easier and failure proof because the
voltage to measure is directly there and no long wires are required.
To measure amps, you need to have a resistor somewhere in the wire. I
prefer to have this directly at the battery because this resistor is
doing the least harm at that place, and if it fails, has the least
impact on your flight. And it gives you the most useful information at
that location. All other information can be derived from what you are
measuring: if the battery provides current it means that the
alternator isn't working properly; you don't have to measure the amps
coming out of the alternator to find out that that it is failing.
Frans
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