Formation Charging
1. Definition
The initial electrochemical charging process that transforms cured paste into active battery materials. This is where a battery truly becomes a battery: positive plates convert to lead dioxide (PbO2 — dark brown), negative plates convert to sponge lead (Pb — metallic gray). Formation typically takes 18-48 hours and is the most energy-intensive manufacturing step.
2. Process
1. Pre-Formation Assembly
Cured positive and negative plates are stacked with microporous polyethylene separators between them. Like-polarity plate lugs are welded together by COS (Cast-On-Strap) to form plate groups. Plate groups are inserted into the battery case. Intercell connections are welded through the case partition (through-the-wall) or over the partition (external strap). The cover is heat-sealed to the case.
2. Electrolyte Filling
Dilute sulfuric acid (specific gravity 1.050-1.100 — weaker than operating strength) is filled into each cell through the vent openings. The low gravity reduces heat generation during initial charging. Vacuum filling ensures complete penetration into plate pores.
3. Formation Charging Profile
DC current is applied in a multi-step profile: - Step 1 (0-6 hrs): Low current (0.05-0.10 C-rate) — gentle start to distribute current evenly - Step 2 (6-20 hrs): Medium current (0.10-0.20 C-rate) — main conversion phase. Temperature rises to 40-55degC - Step 3 (20-30 hrs): Reduced current — top-of-charge. Specific gravity reaches target (1.270-1.290 for SLI) - Step 4 (30-48 hrs): Equalization charge — balances all cells
4. Water-Bath Cooling
Batteries are placed in water baths during formation to control temperature. Water circulation maintains battery temperature at 40-50degC maximum. Excessive temperature (>60degC) causes: grid corrosion, separator oxidation, and reduced capacity.
5. Post-Formation
Electrolyte specific gravity is adjusted to final specification (1.275-1.285 for SLI). Batteries are cooled, washed, and dried. Terminal posts are cleaned. Covers are inspected for seal integrity. Batteries proceed to final testing.
3. Quality Control at Chengguang
Formation efficiency (Ah output / Ah input — target >85%), final specific gravity (1.275 +/- 0.010 for SLI), cell voltage consistency (+/- 0.05V across 6 cells), plate color uniformity (PbO2 = dark brown, Pb = gray). Chengguang uses water-bath formation for precise temperature control and records formation data for every battery (stored 5 years per IATF 16949).
4. Why Grid Quality Matters for OEM Buyers
Every manufacturing step directly impacts the battery you receive. When sourcing OEM batteries, ask your manufacturer about their grid alloy, paste formulation, curing profile, and formation process — these are the four factors that determine real-world battery life in your market. Chengguang's IATF 16949 certification guarantees documented, audited, and traceable manufacturing at every step.
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Author: Chengguang Power Tech Engineering Team | Reviewed: IATF 16949 Quality Department | Battery Database