<!doctype html public "-//W3C//DTD W3 HTML//EN">
<html><head><style type="text/css"><!--
blockquote, dl, ul, ol, li { padding-top: 0 ; padding-bottom: 0 }
--></style><title>Re: [RE-wrenches] discharging Rolls
batteries</title></head><body>
<div>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.</div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div><br></div>
<div>At 15:12 -0400 14/1/10, James Surrette wrote:</div>
<blockquote type="cite" cite><br></blockquote>
<blockquote type="cite" cite><font face="Lucida Grande">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. </font></blockquote>
<div><br></div>
<div><br></div>
<div>At 22:58 +1300 16/1/10, Bruce Geddes wrote:</div>
<blockquote type="cite" cite><font face="Arial" size="-1"> 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.</font></blockquote>
<div><br></div>
<div>At 09:46 -0800 16/1/10, Darryl Thayer wrote:</div>
<blockquote type="cite" cite>Now for Discharge: ..... The
amp-hrs are there but the watt-hrs are not.</blockquote>
<div><br>
<br>
</div>
<div>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).</div>
<div><br></div>
<div>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. </div>
<div><br></div>
<div>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.</div>
<div><br></div>
<div>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.
</div>
<div><br></div>
<div>That's probably enough for now, if anyone has bothered to read
this far. Thanks for any reactions.</div>
<x-sigsep><pre>--
</pre></x-sigsep>
<div>Hugh Piggott<br>
<br>
Scoraig Wind Electric<br>
Scotland<br>
http://www.scoraigwind.co.uk</div>
</body>
</html>