The
Nickel cadmium accumulator was a improvement of
the
nickel iron accumulator and was
developed in 1932, but
wasn't put into common use before the 1960's. It consists
of a positive
electrode of nickel
foil and a negative of
cadmium. Between the electrodes a
porous plastic layer soaked
with an
potassium hydroxide electrolyte. This
assembly is
rolled up and put inside a metal or plastic container.
The foil buildup is used to get a high
surface area.
The NiCd accumulator is usually
charged by a constant
current
charging circuit. A standard charger uses a
current of 0.1C,
this means 0.1 x cell
capacity (a 1 Ah battery would be charged
with about 0.1A). Usual charging time is 14-16 hours.
A rapid charger uses between 0.5C and 1.5C, but this type of
charger must
monitor the battery to prevent overcharging. Since
the battery
temperature rises rapidly when the cell is fully
charged, this is usually done with a
temperature sensor that
cuttes off the charging current when the temperature reaches
45°C.
In
stand-by applications the battery is constantly charged
at 0.05C. This type is used in backup power for memory and clock
circuits.
The NiCd cell can supply up to 100C(100x capacity) for short
periods, but constant discharge should not be higher than about
10C. The NiCd cell has a selfdischarge of about 0.041%/h.
The NiCd accumulator is one of the most used rechargeable
batteries, but since it contains the highly toxic element
cadmium it is in the process of being replaced with the
NiMH
accumulator.