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March 16, 2019

Nickel Cadmium Battery Chemistry FAQ

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Nickel/Cadmium Cells

Anode: Cadmium

Cathode: Nickel oxyhydroxide Ni(OH)2

Electrolyte: Aqueous potassium hydroxide (KOH)

Applications: Calculators, digital cameras, pagers, lap tops, tape recorders, flashlights, medical devices (e.g., defibrillators), electric vehicles, space applications

The cathode is nickel-plated, woven mesh, and the anode is a cadmium-plated net. Since the cadmium is just a coating, this cell's negative environmental impact is often exaggerated. (Incidentally, cadmium is also used in TV tubes, some semiconductors, and as an orange-yellow dye for plastics.) The electrolyte, KOH, acts only as an ion conductor and does not contribute significantly to the cell's reaction. That's why not much electrolyte is needed, so this keeps the weight down. (NaOH is sometimes used as an electrolyte, which doesn't conduct as well, but also doesn't tend to leak out of the seal as much). Here are the cell reactions:

Reaction V vs SHE
Cd + 2OH- —> Cd(OH)2 + 2e- 0.81
NiO2 + 2H2O + 2e- —> Ni(OH)2 + 2OH- 0.49
Cd +NiO2 + 2H2O —> Cd(OH)2 + Ni(OH)2 1.30

Advantages include good performance in high-discharge and low-temperature applications. They also have long shelf and use life. Disadvantages are that they cost more than the lead-acid battery and have lower power densities. Possibly its most well-known limitation is a memory effect, where the cell retains the characteristics of the previous cycle.

This term refers to a temporary loss of cell capacity, which occurs when a cell is recharged without being fully discharged. This can cause cadmium hydroxide to passivate the electrode, or the battery to wear out. In the former case, a few cycles of discharging and charging the cell will help correct the problem, but may shorten the lifetime of the battery. The true memory effect comes from experience with a certain style of NiCad in space use, which were cycled within a few percent of discharge each time.

An important thing to know about "conditioning " a NiCd battery is that the deep discharge spoken of is not a discharge to zero volts, but to about 1 volt per cell.

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