"Dave Plowman (News)" wrote in message
...
In article ,
The Medway Handyman wrote:
Recently got a catalogue from ITS London. On the page listing power
tool batteries is the following, verbatim;
----------------------------------------------- Are you confused about
the various battery types available? This information is a guide only,
only batteries suited to your charger and specific tool should be used.

NiCD (Nickel Cadmium). Entry level rechargeable battery which must be
fully discharged before recharging. Failure to do so can reduce
battery life.

That is total ********. *Totally* discharging a Ni-Cad battery - ie one
with more than one cell - is not a good idea. It should simply be charged
when the performance drops off.


Totally discharging a Ni-Cd battery is one of the two ways to kill it (the
other is of course poor charging) and the "battery conditioners" which were
popular at one time which did this were one of the worst things you could do
to them. The reason is that internally a battery consists of more than one
cell in series and these will initially have very slightly different
capacities. As the battery approaches total depletion the weakest cell will
become reverse biased by its friends and this is *very* bad news, weakening
it still further. Repeat this total discharge a few times and the weakest
cell will eventually fail.

From a longevity POV the best thing you can do is to charge the battery when
it's performace starts to drop off *using a high quality charger*. And
thereby lies the rub, if you recharge at this point using a poor charger it
will overcharge the battery and cause damage through that mechanisum
instead.

NiMH (Nickel Metal Hydride). These batteries have no memory effect
therefore can be 'topped up' or charged at any time without affecting
battery life. NiMH batteries also tend to out perform NiCD batteries in
high drain applications.

Memory effect is an urban myth.


There's an interesting back story to this which I've researched a bit and
believe to be true. NiCad's do suffer from memory effect but, as others
have said, only if repeatedly charged and discharged at the same rate for
the same time. There are good chemical reasons for this which I won't go
into here.

Now this effect is of course unobservable in normal daily life because they
are never used that way but when NASA started using them in space that's
exactly how they were treated. The charge/discharge cycle is dictated by
the orientation and trajectory of the spacecraft (ie. how its solar panels
align with the sun) and tends to be highly routine and the drain tends to
follow set patterns. NASA had to go away and discover how and why their
batteries were failing, which they did, and when they made this known the
manufacturers and the public saw it as the reason NiCad's died, missing the
detail and blaming memory effect rather than the crap chargers for killing
batteries.