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[OM] C-8080 Battery Charging

Subject: [OM] C-8080 Battery Charging
From: Tim Hughes <timhughes@xxxxxxxx>
Date: Tue, 1 Feb 2005 02:54:46 -0800 (PST)
Cc: OlySimian@xxxxxxxxxxxxxx
In an old thread, Cy asked about Li-ion battery trivia, after I had mentioned 
how cells may loose capacity if stored fully charged:

Cy:  >If a rechargeable cell has any amount of self discharge, a fully-charged 
cell will all by itself become a partially-charged cell when stored. So if it 
is better to store cells partially charged, one could store them fully charged 
and simply let nature take its course.

Or are you saying that lithium-ion cells have zero self discharge (which might 
be true for all I know)?

<<

I have also scratched my head over this issue, when I first read about it, but 
my questions were different.  

I will respond in two ways: Answer your question and make a speculation as to 
why I think it may happen. 

In any battery recomendation (and design) you need to understand your 
("typical") user's usage pattern(s). 

We can postulate a number of patterns:

1) You use the camera a lot and so the battery gets discharged every few days. 
Here, it should be fully charged to get best cycle life and energy extraction.  
The battery loses a small % of capacity (due to end of charge conditions) each 
cycle showing a steady decline in capacity as it is used, but you largely get 
the vendors specified cycle life from the battery. The slope of this decline 
depends heavily on the charger's design. Here you wear out the battery by 
cycling (and some chargers are worse than others).

2) You use the camera once every two months and if you fully charge the battery 
each time of use, it effectively is "stored" fully charged for extended 
periods, since self discharge is something like a few % per month.  This is the 
condition where you may lose capacity by "storing fully charged". Here you 
could charge for only part of the charge time or charge just before you were 
going to use it, thus always "storing it partially charged" to extend life. 

3)  You store the battery away for a year. Here the battery is stored fully 
charged but after some period like 6 months maybe (a guess), the battery has 
self discharged to a level where it is no longer "damaging itself" and 
**thereafter** your assumption becomes correct. In this case if you knew ahead 
of time, it might  pay to not fully recharge it, or partially discharge it, by 
say exercising the flash at full dump a number of times. 

4) You use your camera a lot, but only take a few shots each usage.  Here you 
may be better off not recharging *each use* as the charging damage reduces 
battery capacity every top up charge.  Very deep discharges are also usually 
not good, but cells have overdischarge protection circuits, and if the camera 
has a capacity guage (that actually works!), then you have a pretty good idea 
when is a good time to fully charge it. 

My take on the capacity loss when stored fully charged, is an educated guess:

The "cell charge damage" occurs mainly at the end of charge where the voltage 
is limited by the charger to between 3.05 and 3.2V (exact value is important 
design choice ,depends on cell type and cycle life-capacity trade-off). At this 
stage current drops off and charge is terminated (in a good design) typically 
when current drops to 10% of initial charge rate. Cell capacity loss is caused 
by the lithium being converted to a form where it is no longer available in 
future cycles. The higher the voltage and the longer it is applied, the greater 
the capacity loss.  At this stage off charge, the cell may have an unloaded 
voltage between ~3.05 and 3.15V. My *guess* is the cell's own voltage may be 
high enough to slowly convert Li to the unuseable form when stored fully 
charged, thus leading to idea of rather storing partially charged. Additional 
complications occur with multi-cell batteries and cell matching, but this is 
already too much technical information for a list like this!

Regards,

Tim Hughes 

 



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