[RE-wrenches] discharging Rolls batteries

[James Surrette]

Hi Hugh, 

You are correct, my comments may have been too simplistic. 

However, one other key point we are missing is discharge rate. 

Note your comment, 400AH battery discharged at 10a for 16 (or 20) hours = 50% DOD, which is probably not the case. 

Use our S-530, which is 400AH @ the 20hr rate (http://www.surrette.com/pdf/pdf_surrette/s530.pdf), meaning it can support a 20a load for 20 hours (2.1VPC - 1.75VPC).  However, if the current is lowered to 10A it can support this load for nearly 50hrs.  This difference is as a result of the battery's ability to better utilize the available active material (Lead Oxide) during a slower discharge.  Deep cycle products prefer smaller loads for longer periods.  Once the loads increase, the reaction time is too long to support and available capacity plummets, ie, S-530 = 144AH @ 1hr.   So, place a 72A load on an S-530 for 30 mins and you've discharged 50%.  The preamble I've just written is similar to the temperature effect as it is the result of timeliness of converting lead oxide to lead sulfate. 

Take an S-530, cool to -5C and now the capacity drops by 20% as a result of the inability to convert at low temps.  You are correct, the expansion and contraction associated with converting lead oxide to lead sulfate (on the positive) is not occurring at the same value (20% less) but you are pushing the batteries at "a temp adjusted" higher discharge rate.  Thus, the battery cannot support the similar current load for as long a period and voltage drops.  SG values rise as temperature drops, so like for like SOC values will be different if using a hydrometer at -5c & 20C. 

Jamie

>>> Hugh <hugh at scoraigwind.co.uk> 1/15/2010 4:02 AM >>>

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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