> I did not quite understand how to identify a pending battery failure
> when we discussed ignition systems a couple of months back. I have
> the following questions:
>
> 1. Is low voltage a sure sign of impending failure (provided the rest
> of the charging system is healthy)?
Voltage alone is not a very good indicator of anything. The two
conditions that voltage alone will call out are (1) if the
alternator
is running and the bus voltage where it belongs (13.8 to 14.6 volts)
then the charging system is carrying all sytem loads with something
left over to charge the battery. IF the battery is CAPABLE of
storing a useful amount of energy for later use, it will do so
under these conditions. (2) if the alternator is NOT working and
the bus voltage is greater than 10.5 volts, then there is SOME
energy still left in the battery. Again, the question goes to
CAPACITY which I'll address below.
> 2. How about specific gravity? Do the testers that float the little
> balls work?
Hydrometers have been around since the first acid was dropped
between two lead plates to run the first electric cars. While they
will yield some useful information about battery condition, they're
totally useless in recombinant gas, sealed lead-acid batteries.
Personally, given the outstanding performance of the RG/SLA
batteries, I wouldn't even consider putting a wet battery into
and airplane. I just replaced the wet battery in my GMC van with
an RG battery.
> 3. Just what are the 'load testers' used at the parts / tire stores
> and how do they work? Where can I buy one of these?
Load testing speaks mostly to the ability of a battery to
deliver power fast enough to crank an engine . . . while loosely
associated with battery condition, it's not a good measure
of battery capacity. You "load test" your battery every time
you successfully crank the engine. None-the-less, I'll suggest
that 1/2 of all single-engine airplanes flying today have
FAILED batteries aboard . . . if the alternator craps, there
is insufficient energy stored in the battery to be a practical
second source of power. None the less, the battery probably
got the engine going . . .
I'm doing some work on the AGATE program for Raytheon (Beech).
I'm going to propose an automatic battery capacity tester
be included on board the airplane so that the owner can
make periodic tests of battery capacity and replace them
based on good data as opposed to waiting until they no longer
crank the engine.
> My concern is having what I think are 2 healthy batteries, only to
> find that they have only about 20% storage. This will not last long
> with an EFI fuel pump sucking on it.
This is why we developed the two battery, yearly swapout
technique for electrically dependent airplanes. Install
two, RG batteries of identical size in your airplane. Use
out ops and the "aux" battery to power panel essentials.
Swap out the main battery into the aux slot each annual and
put a new battery into the main slot. Now, you always have
one battery less than one-year old and no batteries more than
two years old. The CAPACITY of your on-board energy storage
system is proven and can be relied upon to get your wheels
back on the ground with comfort. The goal is to make sure
that batteries have 75-80% of original capacity before you
go flying.
> At this point I am in favor of 2 alternators unless I can be sure of
> identifying a battery that is prepairing to die. But 2 batteries and
> 2 alternators on what is a basically day VFR plane??? Sounds like to
> much weight and complexity to me. Maybe 2 alternators and 1 battery
> is a better idea. The idea of eliminating failure modes has several
> permutations here, but I was not educated enough in our last visit on
> this subject to make an informed decision.
Two alternators is an excellent idea . . . especially IF you can
deal with an all-electric panel. B&C has light weight alternators
in the 8 and 20 amp class than fit on the vacuum pump pad of
the engine. Wiring for these systems is shown in diagrams
downloadable from http://www.aeroelectric.com/errata/Z8_0299.pdf
I would still have two batteries so that each alternator
runs it's own stand-alone electrical system . . . see
drawings.
However, you can comfortably run one alternator and two
batteries as well if you're willing to remove "a perfectly
good battery" from the aux slot every year . . . the $50-70
expense of this exercise is trivial compared to the total
cost of airplane ownership and it affords a level of reliability
that is unequaled in ANY single engine, type certificated
airplane flying today.
>
> Maybe we can get enough info to start a folder like was done for fuel
> systems. This is very important since many of us are planning to run
> EFI, which has to have juice.
It's not a big deal. The design criteria lend themselves
very well to analysis of the energy needed to meet your
mission requirements. You just need to think and act a
little bit "outside the box" . . . your experiences with
TC'd airplanes are best filled in the "how not to do it"
category going instead with a considered, calculated and
MAINTAINED approach to fabrication and operation of your
airplane.
Bob . . .
////
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