Make battery paste by mixing lead oxide powder with dilute sulfuric acid and a little water until it is as stiff as thick peanut butter, then press it firmly into both faces of a cast grid and smooth it flush. Negative-plate paste also takes 0.5 to 1% barium sulfate and a little carbon black so the sponge lead does not pack down during cycling. Keep the work wet, because lead oxide powder is easily inhaled and carried home.
Part of Battery Building and Maintenance
Preparing lead oxide paste and pressing it into cast grids to make unformed battery plates.
Paste Is Oxide Powder Wet Enough To Hold Its Shape
The active material starts as lead oxide. The carrier makes it by heating lead balls in air at 450°C in a rotating drum until they turn yellow-red; the lead-acid article describes red lead made by heating powdered lead in air at 450–500°C for several hours until orange-red. Those are different oxides under different names, and the corpus does not say which is better. If you can salvage oxide, test a small batch first.
The acid strength for the paste also differs. The carrier mixes the oxide with sulfuric acid at specific gravity 1.100; the lead-acid article mixes 100 g red lead with 7–8 mL water and 5–6 mL of acid it calls 1.4 specific gravity. Neither source explains why. Both aim at the same end state: a stiff paste that holds its shape when pressed. Mix a small test batch, and if it slumps add oxide, if it crumbles add a little water.
The negative paste differs from the positive. The carrier adds 0.5–1% barium sulfate as an expander and a small amount of carbon black; these keep the sponge lead from densifying during cycling.
What You Need
- Lead oxide — made or salvaged, handled as hazardous dust
- Dilute sulfuric acid — Electrolyte Preparation
- Distilled water — for adjusting stiffness
- Barium sulfate and carbon black — for the negative paste only
- A mixing vessel, a spatula or flat tool — non-metallic or lead only, since acid attacks other metals (Sulfuric Acid)
- Cast grids — Grid Casting
Method: Paste A Plate
Step 1: Wet the oxide before mixing. Add liquid to the powder rather than stirring dry powder, so no dust rises.
Step 2: Mix to a stiff paste. It should hold a ridge when you draw a tool through it and stick to the tool without running.
Step 3: Press into the grid. Lay the grid on a flat surface and press paste firmly into the openings from both sides, aiming for about 2–3 mm of paste on each face (carrier). The finished plate should be about 6–8 mm thick overall.
Step 4: Smooth. Scrape flush with a flat tool; the lead-acid article says to fill the voids completely, leaving no gaps.
Step 5: Label and set aside. Mark positive and negative plates clearly now, since they look alike until formed.
Worked Example: Paste Volume On The Faces
A grid 12 cm × 10 cm with 2–3 mm of paste on each face: face area 12 × 10 = 120 cm² per face, so the paste on one face is 120 × 0.2 = 24 cm³ to 120 × 0.3 = 36 cm³, and on both faces 48–72 cm³. That excludes paste inside the grid openings. The corpus gives no paste density, so do not calculate mass; instead weigh each grid before and after pasting and keep the paste mass of a batch consistent so that capacity is comparable from plate to plate.
Common Mistakes
- Making the paste too wet — it slumps off the grid and cracks on drying
- Too dry — it crumbles and leaves voids that never take part in the reaction
- Using the wrong metal tools — iron or copper dissolves and contaminates the plate
- Mixing positive and negative paste — only the negative takes the expander
Safety
Lead oxide dust is inhaled, swallowed and carried home
Dry lead oxide powder floats in the air and settles on hair, clothing and hands; lead is cumulative and the corpus holds that no exposure is safe. Keep the powder damp, work outdoors or under hood extraction, wear a respirator, wet-clean instead of sweeping dry, and change clothes before leaving the work area. The paste acid burns skin and eyes (Acid Handling). Keep children and pregnant people away (Lead Exposure Prevention).
What’s Next
- Learn next: Plate Curing and Formation — hardening and converting the paste
- Combine with: Plate Manufacturing — where pasting sits in the sequence