Datasheet

A sealed NiMH cell requires that towards the end
of charging, oxygen which is generated at the positive
electrode must be recombined to avoid pressure build-up.
The extra charge-reserve capacity is re sponsible for this
process. Additionally a discharge reserve is necessary to
prevent degradation of the negative electrode at the end of
discharge. In general the negative electrode is overdimen-
sioned compared with the positive. The positive electrode
determines the useable cell capacity (Fig. 16).
FIG. 15
Schematic view of a NiMH
High Rate Button Cell from VARTA Microbattery
A precision seal, with long diffusion path, ensures excel-
lent sealing properties. The cup of the casing acts as the
positive terminal and the lid as the negative terminal. The
punched positive sign with precisely predefined rest-wall
thickness on the cell serves as a safety device which
opens smoothly at predetermined internal pressure, in
case of gross abuse. The new Multi-Electrode technology
is the reason for more power.
Rechargeable Button Cells
3.1 CONSTRUCTION AND ELECTROCHEMICAL PROCESSES
OF NIMH HIGH RATE BUTTON CELLS
Lid
Separator
Negative Electrode
Positive Electrode
Sealing Ring
Can with Pressure Relief Vent
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)
Chemical Process of Charging/Discharging
MH-Metal –
NiOOH/ Ni(OH)
2
+
Useful capacity
Positive electrode
Negative electrode
Discharge reserveCharge reserve
FIG. 16
Schematic representation of the elec trodes,
demonstrating useful capacity, charge reserve and
discharge reserve