Re: [RE-wrenches] discharging Rolls batteriesA 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.
----- Original Message -----
From: Hugh
To: RE-wrenches
Sent: Friday, January 15, 2010 12:02 AM
Subject: Re: [RE-wrenches] discharging Rolls batteries
Hi Jamie,
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.
But earlier you put it this way:
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.
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?
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?
(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?)
I hope you can follow my descriptions.
--
Hugh Piggott
Scoraig Wind Electric
Scotland
http://www.scoraigwind.co.uk
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