Datasheet

Rechargeable Button Cells
Charging
Ni(OH)
2
+ Metal NiOOH + MH
Discharging
Charge product of the positive electrode: Nickel (III) oxyhydroxide – NiOOH
Charge product of the negative electrode: Metal hydride
Discharge product of the positive electrode: Nickel (II) hydroxide – Ni(OH)
2
Discharge product of the negative electrode: Metal alloy
Electrolyte: Alkaline solution (KOH)
2.1 CONSTRUCTION AND ELECTROCHEMICAL PROCESSES
OF NIMH BUTTON CELLS
A special sealing design maximizes the diffusion path
and guarantees optimal protection against leakage. The
cup of the casing acts as the positive terminal and the
lid as the negative terminal. The punched positive sign
on the cell is used as a safety device which opens at
predetermined internal pressure, in case of gross abuse.
Some cells are interchangeable with 1.5 V primary cells
of identical dimensions.
A sealed NiMH Button Cell requires that towards the end
of the charging process, oxygen which is generated at
the positive electrode must be consumed to avoid pres-
sure build-up (charge reserve). Additionally a discharge
reserve is necessary to prevent degradation of the nega-
tive electrode at the end of discharge. In general the
negative electrode is overdimensioned compared with
the positive, which determines the usable cell capacity
(Fig. 2).
FIG. 2
Schematic view of a
NiMH Button Cell
CUP – Nickel-plated steel, acting as positive terminal
POSITIVE ELECTRODE (NICKEL HYDROXIDE) – Mainly nickel
hydroxide, enclosed in wire mesh
WIRE MESH
SEALING RING
SEPARATOR – Non-woven material having excellent electrical
insulation characteristics retaining a suitable amount of electrolyte
for ion transport
LID – Nickel-plated steel, acting as negative terminal
NEGATIVE ELECTRODE (METAL HYDRIDE) – Metal hydride,
a hydrogen storage alloy, enclosed in wire mesh
MH-Metal –
NiOOH/ Ni(OH)
2
+
Useful capacity
Positive electrode
Negative electrode
Discharge reserveCharge reserve
FIG. 3
Schematic representation of the elec trodes,
demonstrating useful capacity, charge reserve
and discharge reserve
Chemical Process of Charging/Discharging