Al Gietzen wrote:

       big     SNIP

 If you wonder about drawdown time of a good battery; go turn on the
headlights and interior lights of your car and see how long it takes for the
voltage to drop to about 8 1/2 volts.  If you shed non-essential loads when
you have a failure, and assuming EFI/IG, I'm guessing that you would be able
to fly about twice that long.

Anything wrong with my logic?

Al Gietzen

Al,

Bill Freeman wrote:

I think the weakest point of this argument is the unknown actual capacity
of the backup battery.  As the battery ages, it will lose capacity at an unknown
(probably unknowable) rate.  If a cell starts to go bad, the battery voltage
will
be apparently fine, but at a significant load the internal losses across that
bad cell can draw down the battery very quickly. Even a very small internal
resistance can be quite serious.  For example, if the battery develops even
0.25 ohms internal resistance with a 15 amp load the voltage drop is 3.75
volts,
dramatically reducing the system voltage.  This resistance will not be a
substantial problem when floating on charge, and the battery will have a good
open circuit voltage.

If one is going to depend on a backup battery, I would suggest a periodic
(monthly? quarterly?) load test of the backup battery.  Apply a resistance load
to pull perhaps 1.5 - 2X the expected amperage of a min load emergency
power situation and plot the voltage at several points in time, say
after 30 seconds, 2 minutes and 15 minutes.  Doing this with a new
battery gives the baseline, and repeating periodically would give a
reasonable picture of actual battery performance under load.  This
test would pick up a subtly deteriorating backup battery which might
otherwise lull you into a false confidence.  This would require a large,
cooled resistance (originally trimmable) and a good voltmeter.

Bill

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