ear.navy.mil>
> I just experienced a battery failure with my new 18 Amp Powersonic.
>Yesterday, I stopped by the airport and flipped on the "master" to find
>almost total darkness on everything. The battery had a grand total of 5 VDC
>indicating voltage (no, I did not leave my master on). I got out the
>battery charger and set it to 12VDC @ 15 AMPS for about 30 minutes or so
>then switched it to 12VDC @ 2 AMPS for several hours. At that time, I
>showed 12.2 VDC static on the EIS. An hour later, I showed 12 VDC static.
>I've been told that I toasted the battery when
>I first put the charger at 15 AMPS. Also, I should "NEVER" select more than
>2 AMPS at the charger. The battery already had a problem so, charging
>wasn't the main cause (that was the first time I've ever put a charger on
>the battery. My Key West charging system is putting out 14.2 VDC normally.
I'm not sure this is a valid deduction. By-in-large, vent-regulated,
sealed lead-acid, recombinant-gas batteries are quite tolerant of
high rate recharge.
Consider that "charging" reverses a chemical reaction by pushing electrons
into as system that was discharged by pulling electrons out. Starting
and engine can take hundreds of amps of current flow (albeit for seconds)
that causes a whole lot of molecules to change state as they give up
their available electrons. The battery is not a perfect device . . . it
has some internal resistance that adds to external resistances in the
system (like wire, contactors, bolted joints, etc) to reduce the amount
of energy available to crank the engine.
The VERY low internal resistance of RG batteries is what makes them
so powerful for their size. This same internal resistance has an
effect on recharging too . . . some of the energy that might otherwise
be used to trip some molecules to their charged state will warm the
battery up internally . . . resistors do one and one thing only with
energy that moves through them . . . warm up.
The admonition "never select more that 2 amps at the charger" argues
with how things work in the airplane (or any other vehicle). As soon
as the engine starts, the bus voltage goes to whatever the regulator is
set for . . . current flowing into the battery's chemistry is whatever
it is going to be at that bus voltage. If the battery is only partially
discharged due to normal cranking, the current goes up pretty high because
the battery's internal resistance is still quite low and it will readily
accept charging. If the battery is flat from a total discharge, connecting
it to a constant voltage like a running alternator may not produce much
inital charge . . . the battery's internal resistance is much higher
when it's completely discharge.
The internal resistance can do only one thing with the energy you're
trying to stuff back into the battery . . . get warm. This
is a transient condition . . As the battery begins to charge,
the resistance drops which improves on the energy conversion
efficiency. Internal heating during initial phases of charging
a totally flat battery can be detrimental to the cells in some
cases: Very small batteries (camcorder, cellphone, etc) may have
some initial charging limits due to their inherent higher internal
resistances and low mass. Batteries used to crank engines are pretty
hard to
warm up and much more tolerant of increased losses during rapid
recharge.
Consider the case where an engine has been hard to start and you've
used up most of the battery's capacity trying to get it going. When
you finally do get it started, the bus jumps to 14.2 (or wherever the
regulator is set) and recharging of the battery will begin . . . limited
only by internal resistance of the battery and the output rating of
your alternator . . . sometimes this current is well over 20 amps
even for a small battery.
The concern about battery chargers is how well their voltage is
regulated while a totally discharged battery is getting its chemistry
awake and ready to go to work. Unless the charger mentioned soared
really up there . . . like over 15-16 volts during initial phases
of recharge, I think it unlikely that the recharge scenario described
toasted the battery . . . the way to check is to put a voltmeter
on the battery and watch it for the first few minutes of rechaged.
Depending on voltage regulation built into the charger it MAY
go pretty high initially (like over 15 volts) but it should come
down in ten minutes or so as the battery beings stirring back to
life.
I think most RG batteries roll belly-up because they loose some of
the water from a cell. Remember, these batteries have very little
moisture in them . . . the fiberglas separators are only 80-90%
saturated when the battery is new. If the factory was having a bad-
battery-day and didn't get the cells wet enough, the battery may
be pretty fragile. Also, the cell might be compromised and liquid
may have been expelled causing pre-mature failure. The battery
in this story wasn't run down by leaving a load on it . . . it was
DOA for a routine start up. This suggests the battery had already
departed to where all good batteries go when they die . . . this
makes charger behavior irrelevant to the story.
It would be interesting to do a diagnosis on how this battery
failed. If you're not going to turn it back in for recycling
and/or warranty claim, I'd like to have it to disect.
As an interesting aside to this discussion, check out Bolder
TMF batteries and some of the things coming over the hill
at: http://www.boldertmf.com/news/news_details.asp?ID=25
These are VERY tiny cells with VERY low internal resistances.
If there was ever a seal lead-acid battery that might favor
being pampered, this has to be a good candidate. Yet they
are finding their way into some pretty heavy-duty applications.
They are not going to be beneficiaries of smart charging
systems. They'll get stuck on a constant voltage bus and the
recharge current will be what it will be . . .
I have seen a few battery sites caution about rapid recharge
of RG batteries . . . Hawker (Genesis, Oddysey, Cyclon battery
guys) doesn't caution the system designer to pamper these batteries
with itty-bitty chargers. In fact, there's a statement in
the engineering manual specifically allowing the battery to
be recharged at whatever rate it will accept on a constant
voltage bus. I think the cautions come from dealers and
distributors that don't understand their products (like
calling them "gel cells") . . . and may be looking for
ways to mitigate their warranty hassles . . . there's nothing
WRONG with soft recharging but it's not necessary either.
When in doubt . . . go to the engieering data for the brand
of battery you're working with and see what the manufacturer
says. Dealers are among the most unreliable sources of good
info on batteries.
Bob . . .
--------------------------------------------
( Knowing about a thing is different than )
( understanding it. One can know a lot )
( and still understand nothing. )
( C.F. Kettering )
--------------------------------------------
http://www.aeroelectric.com
|