Perry said:
> It is most certainly possible.
>
> The only difficulty is that the charger in the current unit is designed t=
o
> manage NiCad batteries, which have a different charging characteristic
> from
> that required by Lithium ion.
>
>  It would be relatively straight forward to design and construct a 'smart=
'
> charger for Li-ion, the chief requirements being tight control upon the
> charge voltage (4.2V/cell, +/- 50mV) and accurate current monitoring,
> terminating charge when current has dropped to <3% of rated charge curren=
t
> specified for the cell.

A simplistic but usually [tm] effective LiIon charger can be made by
ensuring max current is below allowed max. (often 1C) and clamping the
pack to not more than say 4.1V x N for N cells. Providing a clamp per
cell is better still. While Perry said 4.2V +/- 0.05V, you MUST NOT
err on the high side but may safely use a lower voltage per cell.
At 4.1V or lower you do not need to ever cut off charging - the cell
will trickle out to about 90% of max capacity and happily sit there.

The main thing that this simplistic charger does not do is allow for
cells that are overdischarged.
Stopping at 3V/cell is wise. You can go lower but there is so little
capacity left that you can drive one or more cells into a danger
region where they should be slow trickled to regain voltage before
going to full charge. Applying 1C charge to a cell at say 2V is liable
to lead to "vent with flame". There is a lower limit below which cells
with protection boards will refuse to even thing about trying to try
charging. In some cases the cells involved can be returned to life by
very very slow trickloe but this may need open heart surgery on the
wrapped cell to bypass the protection board and is obviously not
standard practice.

Brother Bob has apparently had substantial experience in using LiIon
in place of NiCd in battery packs and can probably provide good advice
on how he charges them. I'm in the process of acquiring some
notionally dead LiIon packs to experiment with and to try to follow
Bob's lead. (Watch for magic smoke (if the Cyanide and Mercury don't
get me first*)).



               Russell


* Gold reclaiming.
--=20
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