Nickel-Cadmium Battery (NiCd or NiCad)

Techopedia explains Nickel-Cadmium Battery (NiCd or NiCad)

Two or more NiCd battery cells combine to form a battery pack. Because they are often sized like primary cells (non-rechargeable batteries), NiCds may have lower terminal voltage and less ampere-hour capacity. However, NiCds provide a nearly constant terminal voltage during discharge, unlike primary cells, which results in nearly undetectable low charges. During discharge, NiCd batteries transform chemical energy into electric energy. During recharge, NiCds retransform electric energy into chemical energy.

NiCd battery advantages are as follows:

• Tolerates deep discharges over long periods
• More charge/discharge cycles than other rechargeable batteries for longer battery life
  
• Higher energy density, lighter and more compact than lead-acid batteries. NiCd is preferable when size and weight are key factors, such as in airplanes.
• Lower self-discharge rate than nickel-metal hydride (NiMH) batteries (20 percent per month versus 30 percent per month)

NiCd batteries are extremely toxic. Additionally, nickel and cadmium are expensive metals.

Unlike lead-acid batteries, NiCd batteries heat excessively, go into thermal runaway mode and self-destruct if charged with a dynamo - even in over-current cutout systems. However, NiCd battery packs are usually equipped with an interior thermal charger cutoff, which is signaled if a battery heats and/or reaches maximum voltage.