Page 129 - Offshore Electrical Engineering Manual

116 CHAPTER 11 Power Electronics (Semiconductor Equipment)

Although able to endure longer periods of inaction without charge, they may require
more skilled maintenance than lead–acid batteries for the following reasons:

1. There is a memory effect associated with repetitive cycling of the battery which
may cause the battery voltage to be depressed below the value expected from its
discharge characteristic. The effect is usually temporary and can be reversed by
reconditioning cycles.
2. Sustained boost charging may have the effect of depressing the discharge volt-
age slightly. As mentioned earlier, this can be reversed by reconditioning cycles.

BOOST CHARGING FACILITIES
It is common to find a boost charge mode available on battery chargers, the intention
being to enable the operator to reduce the battery recharge time.
With the electronic chargers now available, battery recharge durations are short
enough for most purposes without having to recourse to a manually initiated boost
charge mode. Boost charge facilities are not recommended on any battery systems
because of the excessive gassing they can cause. If they must be fitted, some form of
timer should be incorporated to prevent sustained overcharging. They should not be
fitted to the chargers of sealed lead–acid cells to avoid permanent cell damage.

VENTILATION AND HOUSING OF VENTED BATTERIES
Special provisions must be made for the housing of batteries offshore to minimise
the risks due to electrical ignition hazard, evolved hydrogen and electrolyte spillage.
The battery room provided should be lined on the walls and floor with a rubber-
based or similar electrolyte (either sulphuric acid or potassium hydroxide)-resistant
material. Owing to the risk involved in any contact between the two forms of electro-
lyte, it is not advisable to house a mixture of lead–acid and nickel–cadmium cells in
the same room. Inadvertent topping up with the wrong type of electrolyte would also
be extremely hazardous for the personnel concerned.
Both types of cell evolve hydrogen, and this must not be allowed to build up to
explosive levels. A ventilation system must be provided that is sufficient to dilute
the hydrogen well below this level at all battery states. A hydrogen detector is usu-
ally provided, and any increase in hydrogen concentration would be indicated at a
manned control room for the installation. Cells should be racked in tiers so that each
cell is easily accessible for maintenance or replacement.
To cater for major gas leak conditions, an explosion-proof isolator must be pro-
vided for each battery. These double-pole isolators would be remotely operated
through the emergency shutdown system should such a serious gas leak occur.

SEALED CELLS
Although very little hydrogen is emitted from lead–acid recombination cells, it is still
advisable for batteries of significant size to be located in a dedicated purpose-built