Table 5-1.--Specific Gravity Range of Batteries
Hydrometer
As a storage battery discharges, the sulfuric acid is
depleted and the electrolyte is gradually converted into
water. This action provides a guide in determining the
state of discharge of the lead-acid cell.
The specific gravity of the electrolyte in a lead-acid
battery is measured with a hydrometer. In the syringe
type of hydrometer (fig. 5-2), part of the battery
electrolyte is drawn up into a glass tube by a rubber bulb
at the top.
The hydrometer float has a hollow glass tube
weighted at one end and sealed at both ends. A scale,
calibrated in specific gravity, is laid off axially along the
body (stem) of the tube. The hydrometer float is placed
inside the glass syringe, and the electrolyte to be tested
is drawn up into the syringe. This immerses the
hydrometer float into the solution. When the syringe is
held approximately in a vertical position, the
hydrometer float will sink to a certain level in the
electrolyte. The extent to which the hydrometer stem
protrudes above the level of the liquid depends on the
specific gravity of the solution. The reading on the stem
at the surface of the liquid is the specific gravity of the
electrolyte in the syringe.
Figure 5-2.--Type-B hydrometer.
The Navy uses two types of hydrometer bulbs, or
floats, each having a different scale. The type-A
to mix the electrolyte before a hydrometer reading is
hydrometer is used with submarine batteries and has
taken.
three different floats with scales from 1.060 to 1.240,
1.200 to 1.280, and 1.228 to 1.316. The type-B
hydrometer is used with portable storage batteries and
CAUTION
aircraft batteries. It has a scale from 1.100 to 1.300. The
electrolyte in a cell should be at the normal level when
Flush hydrometers daily with fresh water to
the reading is taken. If the level is below normal, not
prevent inaccurate readings. Do not use storage
enough fluid will be drawn into the tube to cause the
battery hydrometers for any other purpose.
float to rise. If the level is above normal, the electrolyte
will be weakened and the reading will be too low. If a
Correcting Specific Gravity
hydrometer reading is taken immediately after water is
The specific gravity of the electrolyte is affected by
added, the reading will be inaccurate because the water
its temperature. When the electrolyte is heated, it
tends to remain at the top of the cell. When water is
expands and becomes less dense, and its specific gravity
added, the battery should be charged for at least 1 hour
5-3
