This is the process of charging the
initially filled cells. This converts the unformed plate active materials (Pb,
PbO, and PbSO4) to sponge Pb at the negative plate and
PbO2 at the positive. These are the fully charged forms of the
negative and positive plate, respectively.
The formation charge can be in a
current controlled mode, voltage controlled mode, or variations and
combinations of the two. Most frequently, current control is used, and the
current is often varied or stepped as formation progresses.
Some battery designs require formation
in two different steps, with dumping of the initial electrolyte residue after
the first formation step, followed by refill with a different electrolyte and
either mechanical or electrochemical agitation (by continued charge) to
equilibrate the electrolyte in the bulk with the electrolyte in the plates and
The specific gravity and amount of the
sulfuric acid solution will depend upon the previous treatment of the plates.
Usually, it is in the range of 1,050 to 1.150 specific gravity on a 2-step
formation process. It ranges from 1.230 to 1.250 specific gravity in the
1-step formation process. Increasing specific gravity of the acid increases
the time requires for forming the positive plates.
Completion of formation is indicated
The color of the active materials (i.e.,
the plates are uniform in color)
Reference electrode readings are
The proper charging current will
depend on the thickness and type of plates and on the temperature. For the
more common sizes of automotive plates, forming time is about 45 to 50 hours.
The 2-step formation process requires between 160 and 170 ampere-hours per
pound of positive active material. The 1-step formation process requires about
55 to 60 ampere-hours per pound more than the 2-step method. Foaming and
excessive gassing must be avoided. When either of these occurs, the current
should be reduced. In addition, electrolyte temperature during formation
should be controlled within prescribed limits.
Capacity test (as required)
Formed plate analysis (periodic checks by
wet chemistry or instrumental analysis)
High rate discharge (as