Nickel cadmium, universally abbreviated to NiCad or NiCd, is a rechargeable battery technology that uses nickel oxide hydroxide as the positive electrode, cadmium as the negative electrode, and an alkaline solution - typically potassium hydroxide - as the electrolyte. NiCad batteries were among the first practical and widely available rechargeable battery technologies for consumer and professional use, and became the standard rechargeable power source for photographic equipment including cameras, motor drives, electronic flash units, and portable lighting systems throughout much of the 1970s, 1980s, and 1990s.
Compared to the primary non-rechargeable alkaline batteries they were designed to supplement and replace, NiCad batteries offer several significant practical advantages. Their rechargeable nature allows them to be used, recharged, and reused hundreds of times over their working life, reducing the ongoing cost of powering photographic equipment and the environmental impact of disposing of large quantities of spent primary batteries. NiCad cells also deliver a relatively flat and consistent discharge voltage throughout most of their capacity, maintaining more stable performance as they deplete compared to alkaline cells whose voltage drops progressively during discharge, and they are capable of delivering high current bursts suited to the demands of motor drives and electronic flash recycling.
The most significant practical limitation of NiCad batteries is their susceptibility to a phenomenon commonly known as the memory effect - a reduction in effective capacity that develops when the batteries are repeatedly recharged before they have been fully discharged. The memory effect occurs because repeated partial discharge and recharge cycles can cause the formation of cadmium crystal structures within the negative electrode that reduce the electrochemically active surface area available for the discharge reaction, causing the battery to behave as though its capacity ends at the point to which it was previously discharged rather than at its true full capacity. The practical consequence is that a NiCad battery affected by memory effect will appear to run flat sooner than expected, even though its total stored energy has not significantly diminished. The standard remedy is to fully discharge the battery - running it completely flat before recharging - on a regular basis to break down the accumulated crystal structures and restore the battery's full effective capacity, a maintenance practice that NiCad users were advised to follow consistently to preserve battery performance over the long term.
NiCad batteries also suffer from a relatively high self-discharge rate compared to some other rechargeable technologies, losing a significant portion of their stored charge during storage even when not in use, which can be a practical inconvenience for photographers who charge their batteries in advance of a shoot and then find them partially depleted by the time they come to use them. The cadmium content of NiCad batteries is also an environmental and health concern, as cadmium is a toxic heavy metal that requires careful disposal through appropriate recycling and hazardous waste channels rather than general waste disposal.
NiCad batteries have been largely superseded in most photographic applications by nickel metal hydride and lithium ion battery technologies, both of which offer higher energy density, lower self-discharge rates, reduced or absent memory effect, and in the case of lithium ion, significantly lighter weight - all of which represent meaningful practical improvements for photographic use. Nevertheless, NiCad batteries continue to be used in certain specialist and industrial applications where their high current delivery capability, robustness, and wide operating temperature range remain advantageous.