[RE-wrenches] discharging Rolls batteries

[Joel Davidson]

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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