Alpha Technologies Industrial Ni-Cd Batteries Standard Range - Manual (Page 16)

16 EN-ALPHA-TMSR-001 (10/09)
PR
EL
IM
IN
AR
Y
4.9 Water consumption and gas evolution
At the fi nal stage of the charging procedure of a NiCd battery the provided electrical energy cannot be fully
absorbed but is absolutely necessary to reach the fully charged state of the cells. The difference between
absorbed and provided energy leads to a break down of the electrolyte's water content into oxygen and hydrogen
(electrolysis). This loss has to be compensated by topping up the cells with pure distilled water. The water loss
depends on the current used for overcharging. A battery on stand by operation will consume less water than a
battery that is cycled constantly, i.e. which is charged and discharged on a regular basis. In theory, the quantity
of water used can be found by Faraday's equation that each ampere hour of overcharge breaks down 0.336 cm3
of water. However, in practice, the water usage will be less than this, as the overcharge current is also needed to
counteract self-discharge of the electrodes. The overcharge current is a function of both voltage and temperature,
so both have an infl uence on the consumption of water. The table below gives typical water consumption values
over a range of voltages.
Example:
A cell KM 110 P is fl oated at 1.41 V/cell.
The electrolyte reserve for this cell is approx. 400 cm³.
From the table below an Alpha cell at 1.41 V per cell will use 0.25 cm³/month for 1 Ah of capacity.
That means a KM 110 P will use:
0.25 cm³/month x 110 Ah = 27.5 cm³/month,
and the electrolyte reserve will be used in:
400 cm³ / 27.5 cm³/month = 14.5 months.
The gas evolution is a function of the amount of water electrolised into hydrogen and oxygen and is predominantly
given off at the end of the charging period. The battery does not give off any gas during a normal discharge.
During electrolysis the amount of 1Ah produces 684 cm³ of gas mixture and this gas mixture is in the proportion of
2/3 hydrogen and 1/3 oxygen. Thus 1Ah produces about 456 cm³ of hydrogen.
4.0 Operating Features, continued
Graph 4-5, Relationship between water loss and charging voltages (app.) 20°C