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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