Michael Bremer wrote:
: It is a Piper Cherokee 180 (1968). We are talking about an alternator
: (pretty sure it is a Chrysler). The ammeter shows total load as opposed to
: charge/discharge. I also have a volt meter which shows steady at all RPM
: and loads.

I've got more or less the same plane. The stock ammeter doesn't show total
load, but rather total current supplied by the alternator. If the voltage is high
enough so the battery isn't providing any current, it is the "total load." How
accurate is the voltmeter? A 0.5V difference makes a *big* difference in how much
current the battery is taking.

: When I turn on the light the meter rises. It appears to be a stock meter
: that doesn't have a lot of numbers, basically "0" on the left, "30" in the
: center and "60" on the right. If I extrapolate/interpolate/guesstimate the
: readings, it is about 45-50 amps with everything turned on at idle RPM. At
: takeoff power, the needle swings as fat to the righ as it will travel. As
: the power is reduced, the needle settles back to the same 40-50 area.

Remember that alternators can't put out their full rated current at low RPMS.
It's quite likely that there's a slight discharge of the battery at idle RPM and full
load. Between that an a healthy but low battery, I could envision seeing LOTS of
current at high RPM with everything on. Remember that the 60A (or 80% of it anyway)
is a rating for continuous load... not continuous load + low-battery charging.

The 45A reading at idle seems a bit high. My mechanic has been ferreting out
a similar problem with his Arrow. We're suspect that the ammeter is reading too high
on his, but haven't been able to measure it directly. Any corrosion on the current
shunt in the ammeter would cause it to read too high.

I would say put a digital voltmeter on the battery/bus and see how much the
voltage changes between idle and full RPM. If it's less than 12.0V at 600-1200 RPM
under full load or over 14.5V at full RPM under full load (or not full load for that
matter), there's a charging problem. Bus voltage will tell all (unless there's a bad
wire somewhere), but the current is hard to judge where is coming from/going to.

-Cory


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* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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