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1. 07:57 AM - Re: lithium facts (Robert L. Nuckolls, III)
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| Subject: | Re: lithium facts |
Subject: Re: AeroElectric-List: lithium facts
At 23:49 2014-11-03, you wrote:
Hi Bob;
Thank you for performing a valuable service in
testing lithium iron batteries.=C2 You have made a
good point that a=C2 lithium iron battery may be
capable of starting the engine, but may fall
short in longevity=C2 while running the instruments.=C2
A conundrum that arises from the inability/
unwillingness of suppliers to offer DATA to
support the customers broader needs.
I've often mentioned the early struggles of
Bolder Technologies to find market niche
for their 1.1 a.h., flashlight-cell sized
jelly-roll products that would crank an
engine but wouldn't run your panel for 10
minutes.
We tested some of those cells at B&C about
15 years ago . . . pretty amazing . . .
when they worked . . . but completely
inappropriate to our mission.
We have a similar situation here. The
energetic hawkers of lithium are quick
to point out engine cranking abilities
in terms of 'equivalencies' but without
hard numbers for EVERY feature of lithium's
performance. So just as we saw with Bolder
products, successful incorporation of
the new technology requires that the
system integrator (that's YOU) understands
the trade offs not only in weight and load
dump but capacity, low temperature performance,
and requirements for exploiting capacity (system
voltage). My early studies have demonstrated
that a 4-cell stack of lithium gets De-rated
in a 14.4v system to approximately 1/2 of
potential capacity.
This means that for cell-paks consisting of
arrays of 26650 cells, the USEFUL capacity
is less than the POTENTIAL capacity. Revisiting
the data published by A123 on their 26650
cell offering we see:
Emacs!
Note that while they speak to a MAX ALLOWABLE charge
voltage of 4.2 (pretty much standard across the spectrum
of lithium cells) they also speak to STANDARD CHARGE
and NOMINAL CAPACITY? with numbers on the same order
as demonstrated by my experiments thus far. A123 data
speaks directly to this 14v system de-rating phenomenon.
One point that has not been made in their favor
is that substitution of a 3 to 4# battery for a
20 to 30# one may solve a weight and balance
problem without needing to=C2 move the battery to
the tail cone and adding long and heavy cables.
Very good. Yes, the lighter weight of a lithium
engine cranking battery may indeed offer such
an opportunity . . . as long as all other trade-offs
do not impact system performance in undesirable ways.
I think it improbable that we're going to see really
profound weight ratios. Just as the Bolder cells would
happily dump 400A when new, it was never demonstrated that
they would do that for say 50 times a year for say 4
years in service. We've not yet seen numbers on fielded
products with having a 1:6 weight savings or even
1:4 . . .
Bob . . .
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