Page 297 - Introduction to Marine Engineering

Electrical equipment 271

used on a regular basis to provide a low-voltage d.c. supply to certain
equipment. To provide these services the appropriate size and type of
battery must be used and should be regularly serviced. Two main types
of battery are used on board ship: the lead—acid and the alkaline type,
together with various circuits and control gear.

Lead-acid battery
The lead—acid battery is made up of a series of cells. One cell consists of
a lead peroxide positive plate and a lead negative plate both immersed in
a dilute sulphuric acid solution. The sulphuric acid is known as the
'electrolyte*. A wire joining these two plates will have a potential or
voltage developed across it and a current will flow. This voltage is about
2.2V initially with a steady value of about 2V. A grouping of six
separate cells connected in series will give a 12V battery. The word
'accumulator* is sometimes used instead of battery.
Actual construction uses interleaved plates in the cell in order to
produce a compact arrangement with a greater capacity. The complete
battery is usually surrounded by a heavy-duty plastic, hard rubber or
bitumen case.
In the charged condition the battery contains lead, lead peroxide and
sulphuric acid. During discharge, i.e. the providing of electrical power,
some of the lead peroxide and the lead will change to lead sulphate and
water. The sulphuric acid is weakened by this reaction and its specific
gravity falls.
When the battery is charged, i.e. electrical power is put into it, the
reactions reverse to return the plates to their former material and the
water produced breaks down into hydrogen gas which bubbles out.

Alkaline battery
The basic cell of the alkaline battery consists of a nickel hydroxide
positive plate and a cadmium and iron negative plate immersed in a
solution of potassium hydroxide. The cell voltage is about 1.4V. A
grouping of five cells is usual to give about seven volts.
An interleaved construction is again used and each cell is within a steel
casing. This casing is electrically 'live' being in contact with the
electrolyte and possibly one set of plates. A battery consists of a group of
cells mounted in hardwood crates with space between each. The cells are
connected in series to give the battery voltage.
In the charged condition the positive plate is nickel hydroxide and the
negative plate cadmium. During discharge oxygen is transferred from
one plate to the other without affecting the specific gravity of the
potassium hydroxide solution. The negative plate becomes cadmium