In article ,
default wrote:

On Fri, 16 Dec 2005 14:58:20 -0000, "N Cook"
wrote:

When that white crystalline material forms on th ends of the cells and the
impedance goes high and they become useless. What is that substance and how
harmfull is it ?
Is it the same process, in effect, that renders lead-acid cells uselss - a
build up of an insoluble salt so the ions cannot pass to/from the lead to
the electrolyte?

The cells tend to leak if left discharged for a long period. How
harmful would depend on what you plan to do with it. Its not good to
eat and may kill you, pollute the environment, etc.. Cadmium is very
poisonous, nickel is poisonous, they may contain small/trace amounts
of mercury as well.

I don't know the chemistry of nicads, but do know they are radically
different from lead acid, so a different failure mechanism might be at
work.

Most of the cells I have experience with (back in the day) short
internally, If left in that condition, over time, they leak, or just
corrode from the inside out. Cells that have an internal short can
often be rejuvenated by burning the short out with a discharge of
heavy current through it. We used to build power supplies that would
charge a cap to dump energy into the short for that purpose.

Throw them out and get new rechargeable's. Once they leak, they've
had it.

BAD IDEA! The cadmium in them is very toxic. That's why you should be
careful about those crystals from a leaky one, and also why you should
never "throw them away", but dispose of them properly as toxic waste.

NiMH use a different chemistry, and are nowhere near so bad.

Isaac