[RE-wrenches] discharging Rolls batteries

[Hugh]

hi

We know that batteries deliver less amphours at low temperature and 
at high currents.  Volts drop quicker.  That's my starting point.  My 
question that I still do not hear an answer to is this:

If the battery is a bank account and its harder to get the money out 
in cold weather and when you want to get your hands on a lot at 
once...  Does this actually mean that some of the money gets lost? 
What happens to it?  Is it perhaps available later when the bank 
warms up or the demand gets less hectic?  Is there really less money 
in there or does it just seem like less due to the conditions?

I notice that Ah capacity is actually defined as how much Amphours 
you can get out before the battery reaches a certain terminal 
voltage.  I am wondering whether it is the ability to maintain 
voltage that is the limiting factor whereas the chemicals in there 
can still deliver amphours, given  the right temperature and time 
later.  You can certainly see recovery take place when a battery 
warms up and/or operates on lighter loads.

One last time what happens to the chemicals (lead and lead oxide) 
that represent Amphours of charge in the battery plates?  For me this 
is a little bit like current of 10 amps entering one end of  a piece 
of wire and only 9 amps coming out the other end.  I understand that 
the volts go down due to voltage drop (in this analogy) but loss of 
current is entirely a different matter.

Thanks for any help with this rather obscure question.

Hugh

>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.
>
>----- Original Message -----
>From: <mailto:hugh at scoraigwind.co.uk>Hugh
>To: <mailto:re-wrenches at lists.re-wrenches.org>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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-- 
Hugh Piggott

Scoraig Wind Electric
Scotland
http://www.scoraigwind.co.uk
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