nuckolls.bob(at)aeroelect
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 Posted: Thu Nov 03, 2011 9:03 am Post subject: RG Batteries - I learned something last night |
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Last week I related the outcome of some tests performed
on a 10+ year old battery in my stable of portable power
sources. A 32 a.h. battery that once delivered 800+
cranking amps was down to about 300, yet the useful
capacity of the battery was not much different than when
the battery was new.
My long adopted model of hundreds of parallel-series
wired mini-cells suggested that 1/2 the cranking power
translates to 1/2 capacity too. The test data argued
with that model. Further, a number of folks here on the
List reported similar experiences.
I wrote to Skip Koss at Concorde who in turn passed it to
one of his capable battery designers. Here's his reply:
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Skip & Bob,
There are a variety of degradation modes of VRLA batteries, and the
relationship between C1 capacity and High Rate (HR) performance depends on
the degradation mode.
Most of the time the C1 capacity and HR performance degrade equally,
as Bob states below. This situation can occur if the degradation mode
is due to electrolyte dry-out. Sometimes the C1 capacity degrades,
but not the HR performance. This can occur if the active material
degrades, but not the grids.
Sometimes the HR performance degrades, but not the C1 capacity. This
can occur if the positive grids becomes heavily corroded thereby
increasing the internal impedance, but the active material is still
in good condition.
This last mode is probably what Bob experienced. We have seen this same
result on some batteries returned from the field. Given the differing
degradation modes, it should be apparent why a C1 capacity test must be
used to establish airworthiness instead of a HR Load test. And if a battery
passes the C1 capacity test but has degraded HR performance, it will be
detected during engine start (prior to flight!). Of course, the best
test method is to measure both the C1 capacity and HR performance, but
battery shops normally do not have the capability to properly measure HR
performance.
Hope this helps!
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Indeed it does. In further conversation with Skip, the
parallel-series analogy was quite valid for flooded
batteries where entire cell-sites would fall out of
a grid and fall to the bottom of the battery.
Tightly compacted plates and insulators in the RG
battery prevents cell contents from physically moving
and gives rise to a new dual-path down the road to
battery wear-out.
Of course, we load test he battery every time we
crank the engine but be aware that depending on a variety
of factors, a poor cranking battery might still demonstrate
useful performance in battery only operations.
This new data will be included in 'Connection's
13th edition. Any day you go to bed knowing something
you did not know that morning is a GOOD day.
Bob . . .
[quote][b]
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