Page 83 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

(mass of given volume of liquid)
Specific gravity
(Mass of the same volume of water 4 at C)
Instrument for testing the condition of cells:
Hydrometer : The specific gravity of an electrolyte is measured with a hydrometer (Fig 2).
The charged condition of battery can be tested by means of a battery hydrometer. This instrument measures
the relative density of the battery electrolyte. Since the strength of the electrolyte varies directly with the state of
charge of each cell, you need only to find what specific gravity of sulphuric acid remains in each cell electrolyte to
determine how much energy is available.

Cell condition Hydrometer reading
Full charge 1.26
50% charge 1.20
Discharged 1.15

Voltage tests of lead-acid batteries, like primary cells, should be conducted under load. To make a simple light
load voltage test of a car battery, check the value of the battery output voltage with and without the headlights
on. A maximum load voltage test can be made by metering the battery voltage while operating the starting motor
(Fig 3). In the case of a 12V battery, a drop of battery output voltage below 7V indicates the battery is defective
or not fully charged.
High rate discharge tester: The internal condition of the cell is determined by this test. A low range (0-3V)
voltmeter is shunted by a low resistance (Fig 4). The two terminal prods are pressed on to the terminals of a cell
for testing. A fully charged cell which is in good condition reads in the range of full charge.

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CITS : Power - Electrician & Wireman - Lesson 13-19