>OK, A&P's and EE's, jump in here and correct my thinking. This could be the
>case if you were talking about a car, but I'm not 100% sure about alt's in
>aircraft. Automobile alt's have diodes in them, that when they go bad will
>allow a battery to completely discharge back through the alternator.
When the diodes in an alternator go "bad" they either open
(do nothing) or short (lots of smoke) . . . actually, you
have to fail a minimum of two of the six to eight diodes
in an alternator to effect the reverse feed of energy from
battery back into alternator and it will not be any whimpy
current flow . . . we're talking HUNDREDS of amps.
This is why your b-lead on the alternator has a circuit breaker
or fuse in it . . .
>. . . . . When
>this happens, depending on how long the battery as sat (or discharged), you
>may never get it to come back to life with just a battery charger.
First, alternator diode failure is a VERY rare event. This is one
of the reasons why I've recommended firewall mounted fuses in
the alternator b-lead for homebuilts . . . if the fuse is properly
sized to eliminate nuisance trips, then most likely it will
NEVER trip for the lifetime of the airplane.
> . . . To keep
>things simple, think of batteries as having a "memory". When they lose it,
>through complete discharge, they don't know which side is positive and which
>is negative (and can't be recharged until pos and neg are established). One
>way to overcome this (sometimes) is by simply hooking another battery to the
>dead one, to reestablish neg and pos sides. Once this is done, the battery
>"may" be able to be fully charged with a charger. Alternators can also be
>checked to see if the diodes are bad. Again, whether or not this applies to
>aircraft type or not...I don't know.
The MEMORY effect alluded to was first improperly applied to liquid
Ni-Cads used mostly in BIG airplanes. I could cite about a half
dozen articles that appeared in various electronics journals over
the past 15 years debunking the memory theory but suffice it to say
here that "memory" doesn't happen in other batteries . . . and
especially in lead acid ones.
Lead acid batteries have a shelf life . . . meaning that once a battery
has acquired a certain age, it's capacity has degraded to a non-useful
level. The RATE at which a battery degrades to useless is a function
of state of charge and where in the life cycle the battery presently
resides. For example, a 3 year old battery that's down to
40% of capacity already may loose half of that by sitting in
a totally dischaged for a week and become NON-recoverable. While
a brand new battery can take that 20% whack and still appear to have
"recovered" . . . .
It's true that a totally discharged battery can be charged up reversed
with some apparent capacity of a reverse polarity but it doesn't
take another battery to properly "polarize" a totally discharged
battery . . . just hook your charger to it in the normal manner
and say a few kind words over it, . . and hope you get back some
utility for having done so.
If you have bad diodes in an alternator that are at risk of
discharging the battery, you're going to know it in a hurry.
If you have open diodes, you may NOT know it. I bought a used
car a few years back with a crippled alternator. The car seemed
to have a pretty noisy bus and I filtered the +14V lead going
into my ham rig to control the noise. It wasn't until I got
the a/c fixed and had the blower running on HI along with
headlights at night that I discovered the alternator's output
was insufficient to keep the battery charged. I must have driven
the car six months or more with a funky alternator.
If you have a 60 amp alternator and fly only day/vfr, you might
fly for years with a half dead alternator and not know it.
Bob . . .
////
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