Page 298 - Introduction to Marine Engineering

272 Electrical equipment

oxide and the positive plate is less oxidised nickel hydroxide. Charging
the battery returns the oxygen to the positive plate.

Battery selection

The choice between the lead—acid or alkaline type of battery will be
based upon their respective advantages and disadvantages.
The lead-acid battery uses fewer cells to reach a particular voltage. It
is reasonably priced but has a limited life. It does, however, discharge on
open circuit and requires regular attention and charging to keep it in a
fully charged condition. If left in a discharged condition for any period
of time a lead-acid battery may be ruined.
The alkaline battery retains its charge on open circuit and even if
discharged it can be left for long periods without any adverse effect.
Although more expensive it will last much longer and requires less
attention. Also a greater number of cells are required for a particular
voltage because of the smaller nominal value per cell.
Both types of battery are widely used at sea for the same basic duties.

Operating characteristics
When operating in a circuit a battery provides current and voltage and is
itself discharging. Depending upon the capacity, it will provide current
and voltage for a short or a long time. The capacity is measured in
ampere hours, i.e. the number of hours a particular current can be
supplied. Thus a 20 ampere-hour capacity battery can supply 2 A for 10
hours or 1 A for 20 hours. This is a reasonable assumption for small
currents. The ampere-hour capacity does depend upon the rate of
discharge and therefore for currents above about 5 A, a rate of
discharge is also quoted.
Having been 'discharged' by delivering electrical power a battery must
then be 'charged' by receiving electrical power. To charge the battery an
amount of electrical power must be provided in the order of the
capacity. Some energy loss occurs due to heating and therefore slightly
more than the capacity in terms of electrical power must be provided. By
charging with a low current value the heating losses can be kept to a
minimum.
The different methods of charging include constant current, constant
voltage and trickle charge. With constant current charging the series
resistance is reduced in order to increase the charging voltage. This may
be achieved manually or automatically. The constant voltage system
results in a high value of current which gradually falls as the battery
charges. The circuit resistance prevents the initial current from being
too high. Trickle charging is used to keep a battery in peak condition—a