Page 294 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Electrical systems 277

H 2 H 2
O O
G G

Figure 3.41 Chemical changes reverse Figure 3.42 Discharge of hydrogen and Figure 3.43 Construction of a
when current is applied (see text) oxygen (see text) hydrometer

and, therefore, the performance. Only distilled or specially produced topping-
up water should be used. Tap water is not suitable for topping up a battery.
Acid should never be used, as this would strengthen the acid solution and
alter the chemical reactions. Most modern batteries, however, include vapour
traps and other features to minimize water loss and therefore need little or no
attention.
The electrolyte chemical composition changes with the state of charge. It is
possible to measure this change using a hydrometer. Sulphuric acid is denser
and provides greater buoyancy than water. This property is called specifi c gravity
or relative density. Water, which is used as the base for measurement of all
liquids, is given a value of 1 for readings at 15°C (60°F).

The dilute sulphuric acid of the electrolyte of a fully charged battery cell has a

reading of 1.280. The reading for a half-charged battery cell is 1.200 and for
a fully discharged battery is 1.150. A reading below 1.140 may indicate a cell
that can no longer be recharged. It is common to write these values with three
decimal places but to just say the signifi cant digits (e.g. twelve eighty).

A hydrometer consists of a calibrated fl oat in a glass cylinder ( Fig. 3.43 ). A bulb
on the top of the cylinder is depressed so that it acts as a vacuum pump when Key fact
it is released. A small rubber tube is attached to the bottom of the cylinder and The relative density reading for a fully
is inserted into the electrolyte in the battery cell. A sample of the electrolyte can, charged battery cell is 1.280, for a
half-charged cell 1.200 and for a
therefore, be drawn into the cylinder.
fully discharged cell 1.150.
The sample of electrolyte in the hydrometer lifts the fl oat in proportion to the
buoyancy of the liquid. The higher it fl oats, the greater the relative density.
Calibrated marks on the fl oat align with the top of the liquid to give the actual
reading ( Fig. 3.44 ). This is compared with standard data and all cells are
compared with each other to check the general condition of the battery. There
should be very little difference between the cells.
Key fact
However, as most batteries are sealed the voltage of the battery can be used to
indicate battery condition. A fully charged battery will give a reading of 12.6 V and A fully charged battery will give a
reading of 12.6 V and a discharged
a discharged battery 12.0 V.
battery 12.0 V (when not under load).
Measurement of battery condition by heavy-duty discharge should only be
carried out on a fully charged battery. Heavy-duty discharge is carried out with