Ni-Cd batteries most definitely DO display a memory effect over time
if maintained improperly-- it is no myth. Airlines and industrial
facilities invest huge amounts of money in charging and maintenance
equipment to prevent this very thing from happening. NiCD cells can
get pretty expensive in larger sizes and proper maintenance equipment
saves them $$$ in the long run. Proper maintentance and
charging/discharging can virtually eliminate this phenomenon. Though
for avg. consumer purposes, the biggest practical decrease in cell
life is due to overcharging of the cells.

NiMh and NiCd batteries display similar performance characteristics
(i.e. current delivery and total capacity) however, NiMh batteries
generally can accept fewer total charging cycles (though this
characteristic is improving over time with advances in manufacturing
technology). Some of the advantages of NiMH are that they are lighter
and more environmentally friendly (no cadmium). They will eventually
replace NiCD in my opinion for general consumer use. Most likely they
will eventually replace NiCD in industrial applications (starting jet
engines, etc) when the technology catches up with NiCD and gets more
refined.

Charging the two types of batteries is similar, but definitely not the
same. The safest and cheapest way to charge BOTH types of batteries is
at a current rating of 10% of total capacity, and a voltage of at
least 1.4V (at 20 deg C) for no more than 15 hours, after which
dropping the current to 5% of total capacity to maintain the charge
without damaging the battery. Example: a 100mAh cell would need to be
charged at 10mA for no more than 15 hours, then the charger (usually
by a timer) would disengage the charging cycle or reduce the current
to 5mA to maintain the charge so as to not generate appreciable temps
on the cell.

You can't really damage a NiCD or NiMH battery by overcharging as long
as you keep the current 10% of total capacity or lower. When you
overcharge a battery, new chemical reactions occur to absorb the
excess current, which in most cases generates heat that leads to the
cell damage (in extreme cases venting of the battery-- basically
vaporizing the internals of the cell). All modern batteries have an
oxygen recycling catalyst integrated with them to handle overcharging,
but it is usually only can keep up with rates of 10% total capacity or
less.

The problem with leaving the charging current at 10% for longer than
15 hours or so is that the battery will heat up, and the heat will
eventually decrease the total life of the cell.

Probably your phone charger will by default deliver 5% total capacity
at 1.4V/cell or so. This should keep your cells in good working order
(albeit charging them a little slow-- it will probably take 18-20+
hours to get a full charge from a completely discharged state.)

Fast charging NiCd and NiMH batteries is similar but they need
different methods of detecting end of charge in order to prevent
overcharging. A voltage rise followed by a drop (called minus delta V
detection or -dV) can be reliably used to detect a NiCD end of charge
(in most practical cases), while with NiMH the cell must be
temperature monitored (dT or +dT detection) which requires a
temperature sensor on each cell (usually a more expensive gadget in a
charger). A good benchmark for determining end of NiMH charge for
dT/dt is a temp increase of 1 to 2 deg C per minute.

If you try to charge a NiMH battery with a -dV detection circut only,
you will most likely either undercharge or overcharge the battery.
This is due to the fact that NiMH batteries' characteristics (voltage
depressions, etc) are much more erratic and subject to wide variations
depending on cell temperature. For this reason, when many people stick
a NiMH battery in a NiCD charger, they normally either fry the battery
or consistently undercharge the cell-- both of which will result in
poor percieved performance.

You can charge a NiCd battery with a dT NiMH charger and get reliable
charge termination in just about all cases, but you can't charge a
NiMH battery reliably with most NiCd chargers (-dV). There are some
high-end NiCD chargers that use dT detection and can be used for NiMH,
but if you didn't pay at least $500 for the charger when you bought
it, it most likely will not display this characteristic.

Hope this helps.

wrote in message ...
Just had to replace the batteries in a pair of Vtech phones (model
20-2481).
Old were nicad. New are Nimh. Differences? Best way to charge, how
often?

Figure some of the kind folks in here will have knowledge.

TIA

LB