Page 299 - Introduction to Marine Engineering

Electrical equipment 273

very low current is continuously passed through the battery and keeps it
fully charged.

Maintenance
To be available when required batteries must be maintained in a fully
charged condition. Where lead—acid batteries are used this can be
achieved by a constant trickle charge. Otherwise, for both types of
battery, a regular charge-up is necessary.
A measure of the state of charge can be obtained by using a
hydrometer. This is a device for measuring the specific gravity of a
liquid. A syringe-type hydrometer is shown in Figure 14.13. A sample of
electrolyte is taken from each cell in turn and its specific gravity is
measured by reading the float level. All specific gravity values for the
individual cells in a battery should read much the same. The specific
gravity reading can be related to the state of charge of the battery. The
specific gravity reading must be corrected for the temperature of the
electrolyte. The value for a fully charged lead-acid battery is 1.280 at

Glass
tube
Float
stem
1
r-f
•1200
; 1— - 1.25
:1250 '
• I
:1300 ,*.
*- Electrolyte
„
Reading the float scale
• Float

PJ

^_ Rubber
tube

Figure 14.13 Syringe-type hydrometer