Hi Jinx

You'll probably get lots of ideas on this one and I'll add my 2 cents here
now.

A common theory to explain the problem is that chemical dendrites grow
between the electrodes on the nicads. You didn't say nicad but you did say
power drill which up to lately have mostly used nicads.

In any case, when left 'dead' for a while, the dendrites form and keep a
given cell from charging because they effectively short it out.  The
solution I use often is as follows.

Dump a lot of current through the cell and fry the dendrites off. Monitor
the problem cell with a volt meter.  Charge up a big cap (say 100,000 ufd)
to 10, 15 or 20 volts. Using a low impedance delivery method, dump it into
the cell. At first, perhaps, the meter doesn't budge. Repeat several times
and usually, the cell voltage will suddenly jump up to over 1 volt. It will
continue up to 1.2 or 1.3 or so.  Burning out the dendrites can be done
with either polarity but forwards is best.

I usually hit the cell afterwards with one or two amps for one or two
minutes to get it well up the charging curve. At this point, you can charge
it with the usual nicad chargers.

On stuff you want to keep, try to keep the cells charged up.  Either use a
low trickle current or remember to charge them once every week or two.
When the cells get old enough to begin shorting out like this, they gladly
will short out again quickly.

It's best to apply this directly to each cell that needs it rather than to
the pack as a whole.

Good luck.

All the usual precautions apply.

At 12:30 PM 8/18/01 +1200, you wrote:
>A couple of years ago I was given a cordless drill that
>wouldn't charge. I found that one of the cells in the pack was
>dead flat and was stopping the others charging up. At the
>time I was told how to break down the high resistance
>inside that flat cell, which I did, and the drill has been as
>good as new since. But I can't remember the details of
>what I did. I seem to recall applying 12 or 15V to it for a
>few seconds, keeping an eye on when the meter swung
>from 0 to a rapid charge (indicating that the insulating
>dendrite barrier had been punched through) and then
>removing said 12 or 15V and putting the pack into its
>normal charger. Have I got it right ? There are one or two
>appliances around here with what I think are dead cells
>and I'd like to get them operational. As always, the cells
>are an odd size and would be expensive to replace with
>the manufacturer's "recommended" spare part
>
>TIA
>
>--
>http://www.piclist.com hint: The PICList is archived three different
>ways.  See http://www.piclist.com/#archives for details.
>
>
>

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.