Plate Casting

Part of Energy Storage & Batteries

Casting lead grid plates for lead-acid batteries requires a simple mold, a small crucible, and basic metalworking — the most labor-intensive but essential step in battery construction.

Why This Matters

The lead grid plate is the structural and electrical backbone of a lead-acid battery cell. It serves two functions simultaneously: it holds the active paste material (lead oxide or sponge lead) in intimate contact, and it conducts current from every point on the plate to the terminal connection. Getting the grid geometry right — thickness, spacing of ribs, lug shape — directly determines how long the battery lasts and how much current it can safely deliver.

Plate casting is entirely achievable with basic foundry equipment. Lead melts at 327°C, well within the capability of a simple charcoal forge and small cast-iron crucible. Grid molds can be cast from iron or machined from hardwood for initial use. This article covers the complete process from mold design through finished plates ready for pasting.

Understanding plate casting also covers alloy selection — pure lead is too soft and grows dimensionally during cycling, while antimony-lead alloys are harder, more stable, and easier to cast.

Lead Alloy Selection

Pure lead (Pb): Very soft, casts beautifully, but suffers from grid growth (gradual dimensional expansion during charge-discharge cycling). Growth causes grids to push through separators and short-circuit the cell. Pure lead is used in some modern batteries with special designs; for simple homemade construction, pure lead is adequate for short-service batteries (2–3 years).

Antimony-lead (Pb-Sb): Adding 4–6% antimony significantly hardens the lead, reduces growth, and improves castability (sharper mold filling). Antimony-lead alloys were standard for decades. The downside: antimony migrates to the negative plate over time, increasing self-discharge. Still the best practical choice for most rebuilding scenarios.

Sources of antimony: Antimony metal or antimonial lead can be found in:

• Old lead-acid battery grids (typically 4–8% antimony — ideal source)
• Type metal (used in printing — very high antimony, 15–25%)
• Some lead cable sheathing

Alloy preparation: Melt pure lead, add antimony metal (or re-melted old battery grids) and stir thoroughly. Test alloy hardness by scratch test — antimony-lead resists scratching with a fingernail; pure lead dents easily.

Mold Design

A good plate mold produces plates that are flat, uniform in thickness, and have clean ribs and lugs.

Grid geometry (typical for a 6 Ah cell):

• Overall plate size: 12 cm × 10 cm
• Thickness: 3–4 mm (thinner = lower capacity but lower resistance)
• Border frame: 4 mm wide, full thickness
• Internal ribs: 2–3 mm wide, spaced 8–10 mm apart, running parallel across the short dimension
• Cross ribs: 1.5–2 mm wide, connecting the parallel ribs
• Lug: rectangular tab extending 1.5 cm above the top border, 2 cm wide — this is where the connection strap welds

Mold materials:

• Cast iron: ideal — preheat to 200°C before casting, longest mold life
• Steel plate: adequate, moderate thermal mass
• Hardwood (oak, maple): for prototype molds only — charred after 10–20 pours, needs resurfacing

Mold construction:

1. Machine or carve the grid pattern into one face of a steel or iron block (the cavity face)
2. A flat backing plate clamps against the cavity face to form the mold
3. A small pour channel (sprue) runs from the top edge into the cavity
4. Vent channels at the top corners allow air to escape as lead fills

Dimensions: The mold cavity depth is the plate thickness (3–4 mm). Rib channels are 1.5–2 mm wide and 1.5–2 mm deep (the ribs will be recessed, so active material sits in the inter-rib spaces).

Casting Process

Crucible and furnace: A cast-iron ladle or small crucible holds 1–2 kg of lead. Heat over charcoal or propane until lead is fully molten and reaches 420–450°C (a dull orange glow on inserted iron rod indicates approximately 400°C).

Lead temperature: Too cool: cold shuts (lines where lead froze before mold filled), incomplete corners. Too hot: excessive turbulence, porosity, shrinkage cavities.

Pre-heat the mold: Cold molds cause premature solidification. Heat to 150–200°C (just too hot to touch) before each pour.

Pour sequence:

1. Clamp the mold securely in a vertical frame or vice
2. Ladle molten lead and pour steadily into the sprue in a thin stream
3. Fill until lead rises slightly in the sprue (indicating the cavity is full)
4. Allow to cool undisturbed for 2–4 minutes (do not quench in water — this causes cracking)
5. Open mold, remove plate with tongs
6. Trim sprue, clean flash (thin overflow metal) with a knife

Plate inspection:

• Hold plate up to light — no holes or thin spots visible
• Check flatness — both surfaces flat to within 0.5 mm
• Verify ribs are complete with no breaks
• Weigh — variation above ±5% between plates in a batch indicates inconsistent pouring

Post-Casting Treatment

Deburring: Remove any sharp projections with a flat file. Sharp edges can puncture separators.

Cleaning: Rinse in dilute sulfuric acid (5%) to remove oxide film. Rinse with water and dry.

Flattening: Warped plates (common if cooled too fast) can be gently pressed flat between flat metal plates. Do not force — lead cracks if bent sharply.

Batch sizing: For a 12 V, 100 Ah battery, you need:

• 6 cells in series
• Each cell: 9 positive and 10 negative plates (or adjust for desired capacity)
• Total: 54 positive + 60 negative = 114 plates

Casting a battery’s worth of plates in a single session is practical — about 3–4 hours of work for an experienced caster.

Quality Control

Consistent plate quality directly determines battery life. The most common defects and their causes:

• Porosity (small holes): Mold too cold, lead too hot, or turbulent pour. Remedy: slow pour, preheat mold.
• Cold shuts (surface lines): Lead partially solidified during pour. Remedy: increase mold and lead temperature.
• Short shot (incomplete filling): Lead cooled before reaching extremities. Remedy: increase temperatures, ensure sprue is large enough.
• Warping: Non-uniform cooling. Remedy: open mold slowly, avoid drafts.

Reject plates with porosity or cold shuts — these defects concentrate stress during cycling and cause early plate fracture.