Duracell Ni-MH - Manual (Page 4)

The sealed nickel-metal hydride cell uses the
“oxygen-recombination” mechanism to prevent a build-
up of pressure that may result from the generation of
oxygen towards the end of charge and
overcharge .
This mechanism requires the use of a negative electrode
(the metal hydride /metal electrode) which has a higher
effective capacity than the positive (nickel oxyhydrox-
ide/nickel hydroxide electrode) electrode. A schematic
drawing of the electrodes is shown in Figure 3.3.1.
During charge, the positive electrode reaches
full charge before the negative electrode which causes
the evolution of oxygen to begin:
2OH- _____> H
2
O + 1
2
O
2
+ 2e-
The oxygen gas diffuses through the separator
to the negative electrode, a process which is facilitated
by the “starved-electrolyte” design and the selection of
an appropriate separator system.
At the negative electrode, the oxygen reacts
with the metal hydride and oxidizes or discharges the
metal hydride to produce water:
2MH + 1
2
O
2
_____> 2M + H
2
O
Thus, the negative electrode does not become fully
charged and pressure does not build up.
The charge current, however, must be con-
trolled at the end of charge and during overcharge to
limit the generation of oxygen to below the rate of
recombination. Thus,
charge control is required to pre-
vent the build-up of gases and pressure. Duracell rec-
ommends that continuous overcharge not exceed C/300
for optimal performance.
As shown in Figure 3.3.1, the nickel-metal
hydride cell is designed with a discharge and charge
reserve in the negative electrode. The discharge
reserve minimizes
gassing and degradation of the cell in
the event of overdischarge . The charge reserve ensures
that the cell maintains low internal pressure on over-
charge.
The negative electrode has excess capacity
compared to the positive electrode and is used to
handle both overcharge and overdischarge. Thus,
the useful capacity of the battery is determined by
the positive electrode.
3
Ni-MH Rechargeable Batteries
Charge
Reserve
Schematic representation of the electrodes, divided
into useful capacity, charge reserve and discharge
reserve.
NiOOH/Ni(OH)
2
Positive Electrode
FIGURE 3.3.1
Negative Electrode
Useful Capacity
Discharge
Reserve
MH/Alloy
Composition and Chemistry (cont.)