>> FWIW best storage point for LiIon is just at the constant current /
>> constant voltage changeover knee.

> IIRC they suggest about half charge, though I'm fairly sure it's non
> critical so long as you don't get too near fully charged or fully
> discharged. Given you can charge LiIon using a variety of different
> charge currents during the constant current phase, where exactly is
> this knee point in the absolute charge curve?!

The "normal" practice up until people started producing new
better-faster-dearer variants was to charge at 1C until you reached
desired max voltage and then charge at constant voltage and let the
current taper as it wished and then truncate charge when current
dropped to some %age of 1C. The lower the cutoff current the greater
the charge % AND the lower the cycle life. Doing this at say 4.2V and
say 0.25C cutoff I understand the knee occurs at about 2/3 of charge -
I said 60-70 % previously. Real value will vary and I think if
anything may be above this.

Let's think and try a wild guess. Slower current to constant V almost
certainly puts more charge in. eg if your cutoff was AT 0.2C and you
charged at 0.2C then as soon as you hit   Vmax you'd be at 100% charge
by definition. Whether this was lower or higher in absolute terms than
the 1C version is tbd. Probably higher going slowly as you get less
internal delta-V,less heat ... . Maybe not.

If you charge at say 2C or 4C (if the battery and manufacturer allow)
then the prior argument suggests that the cell will have less absolute
charge when it hits constant voltage.

Changing Vmax from say 4.2V to 4.1 V gives  a significant reduction in
capacity and a much larger improvement in cycle life. I've never heard
of people going to only 4.0V but that would presumably be even lower
capacity and long er cycle life.

As well as cycle life you have to allow for absolute calendar life -
the cells just die of old age. That said you hear many accounts  of
people getting long lives from laptop batteries with still an OK % of
original capacity.

 Russell




>
>> LiIon are actually easier to charge properly than NimH and to know
>> where you are in the charging cycle.
>
> Indeed - you can do a pretty good job using just a regulated bench
> power supply. Disclaimer - charging LiIon cells can be dangerous,
> don't do this at home unless you're sure you know what you're doing.
>
> Chris
>
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