Page 128 - Offshore Electrical Engineering Manual

Batteries 115

BATTERIES

TYPES OF BATTERIES
The following three types of batteries are used commonly offshore.

Lead–Acid Planté Cells
This type of cell is appropriate for most applications where a suitable ventilated
battery room can be provided. It should not be used in a hot vibrating environment
such as adjacent to engines. This is because the electrolyte will tend to evaporate and
require frequent topping up owing to the heat, and the vibration will tend to cause
the oxide coating to fall off the plates. With careful maintenance, these batteries may
last in excess of 20 years.

Lead–Acid Recombination Cells
These cells are a relatively recent development and provide several advantages over
the Planté cells. The cell contains no free electrolyte, the entire electrolyte being
contained in an absorbent blotting-paper-like material.
Except for a small safety vent, the cells are sealed for life; unlike the Planté cells,
they normally emit only molecular quantities of hydrogen. Because of their recent
availability, no operational life expectancy figure is available but from present expe-
rience it is known to be in excess of 8 years.
Excessive charging of recombination cells will permanently damage them by
electrolysing the small amount of electrolyte they contain. Once this has been vented,
it cannot be replaced. Therefore it is vital that, with the chargers used, there is a very
low probability that a fault will occur which allows charging voltages greater than the
maximum recommended by the manufacturer. For the same reason, a boost charging
facility must not be fitted and engine-starting batteries of this type must not be con-
nected to an engine-driven charging alternator, as this will more than likely exceed
the permitted charging voltage.
If, on discharge, the cell voltage falls below a certain minimum value, it will no
longer be possible to recharge the cells affected, making it necessary to replace them.
Therefore, it is important that during the installation and commissioning period, cells
are fully recharged at least every 6 months.
Despite these considerations, these batteries are becoming very popular, mainly
because of the saving that can be obtained from the reduction in maintenance
required, as well as because reliable electronic chargers are available with good volt-
age regulation and current limiting facilities.

Nickel–Cadmium Cells
These cells are generally more capable of withstanding heat, shock and vibration than both
of the lead–acid types. A nickel–cadmium battery is 35% lighter than the equivalent lead–
acid one. However, nickel–cadmium batteries are considerably more expensive, espe-
cially in comparison with recombination cell batteries of the same capacity and voltage.