Lucent Technologies Victory Class Batteries WP-93936
Issue 1 May 2000
Operation 7 - 5
Recharge and
Electrolyte
Stratification
When the battery is discharged the specific gravity of its electrolyte is
reduced. This is a result of the utilization of sulfate ions in the chemical
reaction with the active materials in the positive and negative plates.
The sponge lead in the negative plate and the lead dioxide in the
positive plate convert to lead sulfate, combining the sulfate ions of the
electrolyte with the lead compounds in the plates.
On recharge, lead sulfate in the plates is converted back to the original
compounds and the sulfate ions are released from the plates. The sulfate
ions combine to produce sulfuric acid with a density greater than that
of the electrolyte. As a result, the newly generated, concentrated (heavy
acid) falls to the bottom of the cell container. Specific gravity
measurements taken at the top of the cell will be lower than those taken
at the bottom. This physical condition is called electrolyte stratification.
Stratification does not materially inhibit the ability of a lead-acid
battery to deliver power. However, battery performance will be less
than optimum and specific gravity measurements must take into
consideration the non-homogeneity of the electrolyte. Specific gravity
measurements will not be accurate or reflect the true value.
There are two ways to eliminate stratification. The first is to provide
sufficient time for chemical “diffusion.” This can take several weeks at
float potential, depending upon the degree of stratification. A more
efficient method is to provide an equalize charge voltage that will mix
the electrolyte. Gases produced by an equalizing charge “stir up” the
electrolyte, causing uniformity throughout the cell. After a relatively
short time, the electrolyte will become mixed and homogenous. The
degree of gassing and the setting for the equalize potential are directly
associated with the time required for mixing.
