Recombination Technology - EnerSys Nexsys 12NXS26 Technical Manual

RECOMBINATION TECHNOLOGY

How gas recombination works:
When a charge current flows through a fully charged conventional lead acid cell, electrolysis
of water occurs to produce hydrogen from the negative electrode and oxygen from the
positive electrode. This means that water is lost from the cell and regular topping up is
needed.
However, evolution of oxygen gas and hydrogen gas does not occur simultaneously,
because the efficiency of recharge of the positive electrode is not as good as the negative
electrode. This means that oxygen is evolved from the positive plate before hydrogen is
evolved from the negative plate.
At the same time that oxygen is evolved from the positive electrode, a substantial amount of
highly active spongy lead exists on the negative electrode before it commences hydrogen
evolution.
Therefore, provided oxygen can be transported to the negative electrode, conditions are
ideal for a rapid reaction between lead and oxygen:
i.e. This oxygen is electrochemically reduced on the negative electrode according to the
following scheme,→
2e- + 2H+ + 1/2 O H O
2
and the final product is water.......
The current flowing through the negative electrode drives this reaction instead of hydrogen
generation which would occur in a flooded cell.
This process is called gas recombination. If this process was 100% efficient no water would
be lost from the cell. By careful design of the constituents within the cell, gas recombination
up to 99% is achieved.
Conventional
cell
Oxygen and
hydrogen escape
to the
atmosphere
RECOMBINATION EFFICIENCY
Recombination efficiency is determined under specific conditions by measuring the volume
of hydrogen emitted from the bloc and converting this into its ampere hour equivalent. This
equivalent value is then subtracted from the total ampere hours taken by the bloc during the
test period, and the remainder is the blocs recombination efficiency and is usually expressed
as a percentage.
2
Principle of the oxygen Reduction Cycle
Electrolyte
H
2
о
2
Separator
4
NexSys Technical Manual
Oxygen evolved
from positive
plate transfers
to negative and
recombines to
form water.