Question

Write a short paragraph and discuss what the main differences are between reversible and irreversible battery failure in NiCd batteries

Answer 1

A fully charged NiCd cell contains:

• a nickel(III) oxide-hydroxide positive electrode plate (Cathode)
• a cadmium negative electrode plate (Anode)
• a separator, and
• an alkaline electrolyte (potassium hydroxide)

• Anode reaction - at the cadmium electrode during discharge are:

  

  Cathode reaction - at the nickel oxide electrode are:

  

  The net reaction during discharge is

  

  During recharge, the reactions go from right to left.

Reverse battery failure

When a NiCd cell is reverse-charged, a strange thing happens: Conductive metallic "hairs" (often called dendrites) begin to form and they "grow" from one electrode to another. Eventually, this dendrite forms a short across the cell - one that can have a range of resistance from high to low, depending on the severity of the damage.

Once this dendrite has formed in the NiCd cell it is permanent and cannot be "dissolved" by charging the cell correctly. Furthermore, this dendrite can form a leakage path that can cause the cell to run down by itself - the rate at which can vary depending on the resistance of the dendrite. The effect can range from a cell that just doesn't "hold a charge as long as it used to" to, in extreme cases, the dendrite may be big enough that the cell won't even seem to take a charge at all and results in battery failure.