OK I have been studying the literature list given me by Joel. I did
study all of this theory stuff back in the 1970s when I first got
into this stand-alone windpower stuff, but it was good to go over it
again, especially with my new enquiry.
I also have three fairly clear personal answers to my question "Is
there an actual loss of amphours in cold weather or is the battery
just not so good at delivering?" The following answers indicate that
the amphours are still there, but the loss of performance results in
a loss of voltage that makes the battery appear to be discharged
prematurely.
At 15:12 -0400 14/1/10, James Surrette wrote:
Regarding temperature effects on capacity, earlier responses are
spot on as the lower capacity is totally as a result of slower
reaction times as a result of lower temperatures.
At 22:58 +1300 16/1/10, Bruce Geddes wrote:
to use your analogy of the bank, in cold weather the money counters
operate more slowly and if the temperature rises they return to
normal speed. The money is still there, it is just the rate at
which it comes out that varies.
At 09:46 -0800 16/1/10, Darryl Thayer wrote:
Now for Discharge: ..... The amp-hrs are there but the watt-hrs are not.
OK so if the battery is not actually discharged but is operating at a
lower voltage due to lower temperature, then it seems to make sense
to use a lower cut-off voltage. And people who fly their systems by
the SOC meter alone will not even be aware of the voltage, and will
not care that it drops below the normal range. So I think I now have
the answer I was looking for there. Low battery threshold voltages
should be temperature compensated (although in the opposite direction
to charging 'bulk' or 'absorption' set-points).
I don't know if anyone is in the mood to take this battery study
further? I'd like to talk about Peukert's Law next. It has some
similarities.
According to Peukert's law (and manufacturers' data bears this out
well) the capacity of a battery depends on the rate of discharge.
Everyone who knows anything about batteries knows that the capacity
is specified at a certain rate whether it be C20 for a 20 hour
discharge or whatever. And the capacity at 100 hours (C100) is about
33% higher than the capacity at the 20 hour rate. If you look at the
way this is measured though, it is based on running the battery down
to a chosen 'discharge limit' voltage. And I have not heard anything
from Wrenches nor seen anything in the literature to suggest that the
battery discharged in 20 hours has actually lost any amphours
compared to the 100 hours one. So it appears to me that if you give
it a rest and then start discharging it again, but now at the 100
hour rate you could still get another 33% extra capacity.
Well now. I don't expect to get away with saying that. But why not?
I can't find any evidence that it is not true.
That's probably enough for now, if anyone has bothered to read this
far. Thanks for any reactions.
--
Hugh Piggott
Scoraig Wind Electric
Scotland
http://www.scoraigwind.co.uk
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