| Battery Applications & Technology
One of the key parameters of battery
operation is the specific gravity of the electrolyte.
Specific gravity is the ratio of the weight of a solution to
the weight of an equal volume of water at a specified
temperature. Specific gravity is used as an indicator of the
state of charge of a cell or battery. However, specific
gravity measurements cannot determine a battery's capacity.
During discharge, the specific gravity decreases linearly
with the ampere-hours discharged as indicated in the
illustration below.
Changes in voltage and specific gravity
during charge and discharge
Therefore, during fully charged steady-state
operation and on discharge, measurement of the specific
gravity of the electrolyte provides an approximate
indication of the state of charge of the cell. The downward
sloping line for the specific gravity during discharge is
approximated by the equation below:
Specific gravity = cell open-circuit voltage
- 0.845
or
Cell open circuit voltage = specific gravity
+ 0.845.
The above equations permit electrical
monitoring of approximate specific gravity on an occasional
basis. As mentioned earlier, specific gravity measurements
cannot be taken on sealed lead-acid batteries. Measurement
of the cell open-circuit voltage has been used as an
indicator of the state of charge of a sealed battery. More
reliable methods for determining the state of charge of
sealed batteries are under development.
The specific gravity decreases during the
discharging of a battery to a value near that of pure water
and it increases during a recharge. The battery is
considered fully charged when specific gravity reaches it's
highest possible value.
Specific gravity does, of course, vary with
temperature and the quantity of electrolyte in a cell. When
the electrolyte is near the low-level mark, the specific
gravity is higher than nominal and drops as water is added
to the cell to bring the electrolyte to the full level. The
volume of electrolyte expands as temperature rises and
contracts as temperature drops, therefore affecting the
density or specific gravity reading. As the volume of
electrolyte expands, the readings are lowered and,
conversely, specific gravity increases with colder
temperatures.
The specific gravity for a given battery is
determined by the application it will be used in, taking
into account operating temperature and battery life. Typical
specific gravities for certain applications are shown in
Table 1.
Table 1
In the selection of a battery for a given
application, some of the effects of high or low specific
gravity to be considered are:
|
Higher Gravity |
Lower Gravity |
|
| More capacity |
Less capacity |
| Shorter life |
Longer life |
| Less space required |
More space required |
| Higher momentary discharge rates |
Lower momentary discharge rates |
| Less adaptable to "floating:
operation |
More adaptable to
"floating" operation |
| More standing loss |
Less standing loss |
A solution of higher specific gravity is
heavier per unit volume than one of lower specific gravity.
Therefore the more concentrated electrolyte created during
charging sinks to the bottom of the battery jar creating a
gradient in specific gravity. The gassing that occurs on
overcharge serves as a "mixer" and makes the
specific gravity uniform throughout the cell. To avoid
erroneous readings, specific gravity measurements should
only be taken after an equalizing charge and subsequent
float charge for at least 72 hours.
© Copyright 2000 - 2009, by Engineers Edge, LLC All rights reserved.
Disclaimer
|
