I’ve treated a lot of NiCad batteries this way,
but haven’t needed to do any NiMH cells yet.
Such treatment works if the cause of failure
is crystalline growth in a cell that shorts it out.
The zap melts the crystal hairs, at least to the
point where the battery will work, but doesn’t
keep them from growing back eventually.
I used it to get radios back on the air while we
waited for new batteries to arrive, but it
isn’t a long term solution.
Other failure modes, however, won’t recover.
I used a current-limited (0.5 A) adjustable
power supply. A standard 12V supply will work,
but will want a resistor in series to limit the
current. Probably 100ma might be a reasonable
max current, which could be a small light bulb.
And higher voltage may work better for a
nominal 9v battery, since all the cells are in
series.
The first step is to put the battery on the power
supply with the current limiting resistor and see
if the voltage recovers. I generally use clip leads,
and connect my multimeter across the battery to
monitor the voltage. For a single cell, if I see
more than about 0.1 V then it isn’t a dead short,
and probably can’t be recovered this way. For
a multiple-cell battery, you might have some
shorted cells and others good.
In fact, if you just want enough voltage to get
your charger to work, this may do the job all
by itself. Just don’t leave the battery on too
long - at 50-100mA you should see any voltage
within a minute or two. (You might try this
first with about a 1 K resistor, which will
allow the battery to charge slowly without
overcharging.)
To try to recover a shorted AA cell, I used a
large capacitor, like 25,000 uF, with a voltage
rating above that of my supply. For a 9V
battery, 1000uF might be enough. You just
want to make a good spark when you short
it out.
I put the current limiting resistor between
the power supply and capacitor. A lamp
actually works well here, as it gives a visual
indication of when the capacitor is charged.
Then a separate set of leads go to the
battery: I had some brass bolts that I used
as contacts at the ends of my clip leads,
otherwise the clips can get pitted.
Let the capacitor charge up through the
resistor, then, observing polarity, touch the
leads to the battery terminals. There
should be a spark as you do so. The burst
of current from the capacitor hopefully will
eliminate the short circuit, then the
residual current (through the resistor) will
provide charge for any recovered cells.
So the power supply voltage, and capacitor
size, determine the amount of zap, and the
current limiting resistor sets the charging
current once the capacitor is discharged.
This may take a few tries, and some
experimentation. It’s easier with
individual cells, as you don’t have to worry
about overheating the good cells while
trying to recover the last couple, but may
be worth a try if the cells are shorted
internally. If they are open instead, or just
won’t take a charge, then it won’t help.