OK a couple of comments on Nicad Batteries. I used to work in the
handheld medical device world, real big on battery technology. So here
goes:
NICAD - High current low impedance technology capable of delivering
large amounts of both burst and continuous energy. They are capable of
delivering extremely large continuous current draw hence there
popularity in the RC world (for example a sub-c sized cell (common in
tools and RC) are capable of delivering 40-80 Amps this drives the
current limitation to the wiring and motor rather than the battery, A
comparably sized LI ION can probably provide no more than 10 Amps).
Nicad is the lowest charge density (mAH per unit weight) chemistry
readily available on the market today. Nicad is designed to be a
complete discharge type of cell, meaning that it must be completely
discharged between every charge cycle.
Failing to do this causes passivation (accurately mentioned above as
the creation of a crystaline struction in the cell). Passivation causes
two primary problems first it decreases the total capacity of the cell,
making it difficult for the batteries to completely discharge. Second,
it causes a significant increase in the impedance of the cell. The
effect of increased impedance is decreased voltage stability. In the
case of fixed voltage batteries the ability to deliver a constant
voltage with the drawn current is what controls the total output power
of the battery. Last, Nicad cells are also very sensitive to
over-charging. Over-charging causes the maximum capacity of the
batteries to be radically reduced. This can be caused by the use of
poor quality chargers or negligence with a decent charger. Good
chargers have a shutdown that allows currrent flow to drop to zero once
the battery has achieved its full voltage state.
Passivation while very frustrating, is not a serious problem as is it
not irreversible. There are at least two ways to deal with passivation.
One as mentioned above is to shock the battery with a large amount of
current. This is a dangerous and very tricky way to address the
problem. The much easier and safer method is to completely discharge
the batteries. Passivation can be almost completely eliminated by
"fully" depleting the cells.
I try to do this about once a year on all my Nicad packs. This can be
done using the tools themselves. but you have to follow a few steps.
Notably for badly passivated batteries you may have to repeat the
process several times by recharging after the depletion process and
doing it again.
First, using the tool run the battery until they seem to be dead. It is
best to use a drill or saw as they don't burn out like the flashlights.

Then let the battery sit for a while (1/2 hour to a few hours usually
does it). During this time the battery will regain some available
energy, this is caused by the breakdown of the passivated material
chemistry
Repeat the first two steps until you get nothing after several hours of
resting.
Another additional step that you can take is to place a shorting wire
across the two power terminals on the depleted battery. (NOTE!!! NEVER
DO THIS WITH A FULLY CHARGED PACK. NICAD CAN PRODUCE ENOUGH CURRENT TO
ACT AS AN ARC WELDER AND FIX THE WIRE TO THE PACK WHICH MAY RESULT IN
AN EXPLOSION) . Done properly this method will result in completely
depleting the cells and a large reduction in passivation.
NiCad batteries should have a long and productive life. If you feel
like you haven't used your pack that much and you have not over charged
it you probably can fix it following this process.
I guess I should have said a bunch of comments on Batteries.
Hope it helps.

W