What Are Ni-Cd Batteries?

Ni-Cd batteries are rechargeable electrochemical cells that use nickel oxide hydroxide as the positive active material and metallic cadmium as the negative active material, with a potassium hydroxide (KOH) aqueous solution as the electrolyte. The abbreviation Ni-Cd reflects the chemical symbols for nickel (Ni) and cadmium (Cd). Waldemar Jungner, a Swedish scientist, invented the basic cell in 1899, making it one of the oldest rechargeable battery chemistries still in engineering use. The nominal cell voltage is 1.2 volts, and the cells maintain that voltage with relative stability across most of the discharge curve before falling sharply near depletion.

The technology is distinguished by its ability to deliver high discharge currents, tolerate deep discharge without irreversible damage, and operate reliably across a wide temperature range, from approximately -30°C to +60°C. These properties place Ni-Cd cells in a category distinct from lead-acid and lithium-ion chemistries, each of which imposes different constraints on operating conditions and cycling behavior.

Electrochemistry

The discharge reaction at the positive electrode reduces NiOOH to Ni(OH)2, while the negative electrode oxidizes metallic cadmium to Cd(OH)2. Both processes occur in the alkaline KOH electrolyte, which acts as the ionic conductor but is not consumed in the overall reaction: the KOH concentration remains nearly constant through charge and discharge cycles. This electrolyte stability contributes to the long service life of Ni-Cd cells, which can sustain more than 1,500 charge-discharge cycles under proper conditions.

The ScienceDirect overview of nickel-cadmium battery chemistry describes how the flat 1.2 V discharge plateau results from the two-phase solid-state reaction at each electrode, in contrast to lithium-ion cells where the voltage profile slopes continuously during discharge. A recognized drawback is the memory effect: if cells are repeatedly discharged to only a partial depth before recharging, the effective capacity gradually decreases. Proper conditioning cycles can reverse this effect in most cases.

Cell Construction and Variants

Ni-Cd cells are manufactured in two principal forms. Sealed cells use a starved-electrolyte design with an oxygen-recombination mechanism that prevents pressure buildup and allows hermetic sealing; these are the cylindrical or prismatic cells found in portable equipment. Vented (flooded) cells contain excess electrolyte and are equipped with resealable vents that release gas during overcharge; these are used in stationary industrial installations where the larger size and required maintenance are acceptable.

The electrode plate design further subdivides the market. Sintered-plate cells provide high energy density and high-rate discharge and are used in aviation and military applications. Pocket-plate cells use a steel mesh pocket to contain the active material, giving them exceptional mechanical robustness and service lives measured in decades. Wiley's chapter on Ni-Cd rechargeable batteries provides a detailed comparison of these construction variants and their respective performance characteristics.

Environmental regulations have affected Ni-Cd production and use. Cadmium is classified as a hazardous material, and the European Union's Battery Directive of 2006 restricted the sale of Ni-Cd cells in portable consumer applications, while allowing continued use in industrial and emergency systems where alternatives are inadequate.

Electronics Notes coverage of NiCd technology summarizes the practical engineering tradeoffs that sustain demand for Ni-Cd in specific niches despite the growth of lithium-ion alternatives.

Applications

Ni-Cd batteries have applications in sectors requiring reliable high-rate discharge and wide operating temperatures, including:

• Aviation battery systems for engine starting and emergency power
• Stationary industrial UPS and switchgear backup
• Emergency lighting systems in commercial buildings
• Cordless power tools and professional handheld equipment
• Space satellite power systems requiring long cycle life at low temperatures