At last I have found a description that seems reasonable to me - in
the UK Daily Mail no less. The Which feature on batteries was not
much help.

Rechargeables are NiCD Nickel Cadium or NiMH Nickel Metal Hydride.
NiCD are banned by EU from 2008 cos Cadium is hazardous. All mine
seem to be NiCD of course.

Charging time seems to be seven hours, but some chargers switch
themselves off more quickly. It seems difficult, perhaps with older
batteries, to get them up to the full 1.5 volts. Rechargeables lose
power at 1% per day so they will be completely flat without being
used in three months. No wonder the spare batteries I carry in my
camera case never seem much good.

They do say that you ought to discharge the batteries entirely before
recharging. I sometimes find this difficult, particularly because the
camera switches itself off after a few minutes. The razor is easier,
you can leave it running on the table. I worry about the mobile
phone, which I plug in every night.

So not useful for small load devices like smoke alarms, remote
controls. I don't use my camera very often so that rechargeable
batteries always seem to be flat, where as regular batteries hold
their charge better. If I used my camera or iPod every day I can see
that rechargeables would be much much cheaper - between 10 and 50
times better they say.

Regular batteries seem most useful for me, and I buy when the local
hardware shop has a discount offer, and have a vaste stock.