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[OM] Re: OT: battery question

Subject: [OM] Re: OT: battery question
From: Tim Hughes <timhughes@xxxxxxxx>
Date: Mon, 14 Nov 2005 00:40:06 -0800 (PST)
Cc: jfwilcox@xxxxxxxxx
Joel wrote:
>>In a simple-minded way, typical of many of my passages through life, I
guess I am making the assumption that a 357 will do everything a LR44
needs to do, but also more.  Can anyone who knows battery lore bless
this assumption?
<<

In a low current slow discharge application like you describe, there are two 
main factors that affect battery life:  
a) The energy capacity of the battery (Watt-hrs or Joules)  
b) the self discharge or other loss of capacity (drying out etc) mechanisms, 
defined by "Shelf Life" or "self discharge curve".
 
In general the Energy density of most common primary batteries (except for Li 
anode and specialty cells) are in the 400-600mW-hr/mL range.   That means the 
theoretical energy available from many common cells of similar size but 
different chemistry, should not be wildly different, particularly at  low 
discharge rates.
 
 In reality two additional factors come into the picture:
a) Small cells may be much more inefficient due to wasted space from sealing 
and other  constraints. This may favour one chemistry over another. 
b) The elctronics may not run off low voltages, so chemistries with a sloping 
voltage discharge curve, may not allow nearly as much energy to be extracted 
before electronics cuts off.
 
In the case of LR44 cells the quoted mW-hr capacity of the cells is 50-60% or 
less, of that  of 357 cells. (Depends on vendor , and don't confuse mA-hr with 
mW-hr). 
But the actual life is likely less still than 50%, because the LR44 volage 
drops steadily as it discharges versus flat voltage curve of SR44's.  In 
addition the 357's have much better shelf life, so in a very slow discharge the 
LR44's cells lose capacity at >3x the rate of SR44's. (2-3% pa for SR44's vs 
10% for LR44's). So over 2years usage, the capacity of the LR44 will be ~15% 
lower, just from self discharge capacity loss. (self discharge varies quite a 
bit by vendor)
 
So you are correct, the 357 (SR44's) will always perform better than LR44's, 
especially in high current and low temperature use, but also in slow discharge 
use.
 
 In extreme low current use, the self-discharge will dominate battery life, 
making the SR44's look even better. The Energizer 357 (no longer available) 
aparently had demonstrated shelf-life of over 10years with about 20% loss of 
capacity. 
 
Tim Hughes
 




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