Kevin Taylor wrote:
> ...
> I put a digital AVO on to get an accurate reading and I'm finding the
> battery is reading 12.9 volts when off load and engine off. With the engine
> running its reading 14.2. My interpretation of this is that the alternator
> is permanently charging the battery and therefore overcharging.
Unfortunately, even a digital AVO or "multimeter," but consumer-grade,
can be assumed not suited to the task here. Cumulative errors in the
thing as little as a 2%, ignoring the least-significant-digit problem,
can cause +/-20% error in reading the actual "state of charge" on a
lead-acid battery, and each .1V in charge volts is significant at full
charge.
Indeed, if a used-battery is reading 12.9V after sitting for some
hours, even for a recombinant gas type, or "AGM" or "VRLA" (but not
necessarily gel cell), that means an indicated 14.2 charge volts,
after replenishing starting energy, might be 2-4 tenths more than
actual, which is not over-charging. Though may be upper limit for an
RG battery, as they say never "float" them much beyond 13.5-13.8V.
Due to surface charge, reading standing volts can be irrelevant
without following mfr's recommendation to deal with it, and regulator
voltage doesn't say much more and is particularly dependent upon
battery type.
A measure which ignores all variables is the actual current into any
type battery, and at any temp, using its ampere-hour rating. After
starting power has been replenished, current at float voltage is
typically 1% of its AH rating; "overnight" charge is 10%. Anything
between these two should be fine. At the high end for flooded-cell
batteries, it's overcharging a fully-charged battery, but the way
recreational airplanes are used it will not harm the battery.
Fully-charged RG's may not like 10% x AH.
W/o any panel ammeter that can resolve down to an amp or even less,
there are ways to check it with even a $10 multimeter and not a bad
idea for RG's. Manually bring the batt to full charge. With a
rear-mounted battery, 10 feet of #4 cable will show 2.5 millivolts
across its length, per amp. Scale for shorter/longer cable. Start
engine and read the drop. For firewall mount, exactly one foot of #20
_aircraft_ wire, carefully stripped to not cut any strands, will show
a drop of 10mV per amp. Securely wire it between a battery post and
either cable, jumper-cable across or temporarily attach cable to
start, then disconnect and similarly read the voltage drop at
necessary RPM and time to replenish starting loss. Yank fuses or turn
off everything in the plane not needed, since if the #20 wire is
accidentally disconnected, things can be damaged when powered by
alternator current w/o a battery!
Regards,
Fred F.
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