On 6/14/2013 11:20 PM, David Nelson wrote:
So how do you calculate the theoretical charge storage difference between
cells when the ESR isn't really the same as what a resistor does? Diffusion
differences alone will cause an effective resistance to differ and yet not
change the charge storage efficiency. If there is a theoretical value of
the SOC difference between two cells which started out at the same SOC
after they have been cycled a given number of Ah then we could check with
actual use.
The theoretical charge storage (coulombmetric efficiency) is 100%. In
theory, every single electron that goes in gets stored, and you get
every single one of them back on discharge.
In practice, some of them *do* get lost to unrecoverable side reactions.
Measuring the actual coulombmetric efficiency is hard because it is so
close to 100%, and there is essentially no voltage change with state of
charge. It's common to measure lithium cell voltages, find them "the
same", and conclude that they must be at "the same" state of charge.
It takes very carefully controlled conditions and very precise
measurements to find out what the real state of charge is, and to
measure its change as a consequence of different testing conditions.
All this doesn't matter in the short term. But it *does* matter in the
long run, because the errors accumulate over time! Imagine that you have
an ICE car that leaks oil. The leak is so slow that you're not down a
quart before the next oil change. So it costs you nothing, and you don't
care, and don't fix it. But it makes a big mess of your garage to have
almost a quart of oil leaking onto the floor every few months!
I think that one of the things that is possibly confusing things is that
there is no theoretical self discharge in a LiFePO4 cell where there is in
a lead acid cell.
Even with lead-acid, I think the theoretical self-discharge rate is so
low that it is difficult to measure. High quality batteries with pure
lead plates can sit for many years without self-discharging. I think the
more common high self-discharge rates are a consequence of impurities,
either deliberately placed there (like antimony or calcium in the plates
to harden the lead), or accidentally (by people using tap water instead
of distilled water).
--
The most dangerous enemy of a better solution is an existing one that
is just good enough. -- Eric S. Raymond
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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