[RE-wrenches] discharging Rolls batteries

[Hugh]

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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