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<DIV><FONT size=2 face=Arial>A lead-acid battery is an electro-chemical
processor (just like you and other living things). When you and your battery are
cold or hot, performance changes because the chemical process is affected by
temperature. Cold equals sluggish chemical reaction, reduces the
capacity to perform work, and affects battery performance
linearly. Battery chemistry is well understood. When I get
some time, I'll google for temperature-based formulas and charts
unless someone else posts the links first.</FONT></DIV>
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B>
<A title=hugh@scoraigwind.co.uk href="mailto:hugh@scoraigwind.co.uk">Hugh</A>
</DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=re-wrenches@lists.re-wrenches.org
href="mailto:re-wrenches@lists.re-wrenches.org">RE-wrenches</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Friday, January 15, 2010 12:02
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [RE-wrenches] discharging
Rolls batteries</DIV>
<DIV><BR></DIV>
<DIV>Hi Jamie,</DIV>
<DIV><BR></DIV>
<BLOCKQUOTE cite="" type="cite"><BR></BLOCKQUOTE>
<BLOCKQUOTE cite="" type="cite"><FONT face="Lucida Grande">Remember, as
batteries cool actual capacity is reduced, so if 200AH is 50% @ 25C it is
significantly more than 50% @ 5C. Thus, you are discharging more
deeply.</FONT></BLOCKQUOTE>
<DIV><BR></DIV>
<DIV>But earlier you put it this way:</DIV>
<DIV><BR></DIV>
<BLOCKQUOTE cite="" type="cite">
<BLOCKQUOTE cite="" type="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></BLOCKQUOTE>
<DIV><BR></DIV>
<DIV>There is an issue here that I need to understand better. You state
that a battery has lower capacity in low temperatures. Suppose you take
a fully charged, 400 Ah battery and cool it down to -5 degrees C where
according to our numbers it will only have 80% of its nominal capacity.
You then remove 160 Ah (say 10 amps for 16 hours). It will then be 50%
discharged. Now warm it up again to 20 degrees or whatever. My
question is: will you only have 200 amphours left in it now? And if so,
what happened to the other 40 amphours? Does low temperature operation
actually lose amphours, or is it just more sluggish? What is the
chemical explanation for the lost amphours?</DIV>
<DIV><BR></DIV>
<DIV>I understand batteries as a chemical process of converting amphours into
chemical changes. I assume that a given amount of electrical charge
converts a given amount of lead into lead sulphate (and likewise) back
again. I understand that cooling will make this process less efficient
and thereby result in a rise in charging voltage and a drop in discharging
voltage. But does a low temperature actually mean that a given amount of
lead being converted to sulphate actually give you less amphours
electrically?</DIV>
<DIV><BR></DIV>
<DIV>(I have similar questions in relation to Peukert's equation where high
discharge rates impact on the amphour capacity. The capacity apparently
'recovers' when the discharge rate is reduced. To what extent is the
capacity actually lost by using high discharge rates and to what extent is it
just a voltage effect that impacts on the terminal voltage, rather than the
actual chemical state of the battery?)</DIV>
<DIV><BR></DIV>
<DIV>I hope you can follow my descriptions.</DIV><X-SIGSEP><PRE>--
</PRE></X-SIGSEP>
<DIV>Hugh Piggott<BR><BR>Scoraig Wind
Electric<BR>Scotland<BR>http://www.scoraigwind.co.uk</DIV>
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