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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