Cast battery grids by pouring molten lead-antimony alloy into a mold of iron, steel or soapstone that has been preheated to 150°C or more, so the metal fills every rib before it freezes. Reject any grid with cold shuts, holes, broken ribs or incomplete corners, because a defect concentrates stress and the plate fails early. The sources in this wiki disagree on pour temperature, so find the lowest temperature at which your mold fills completely.
Part of Battery Building and Maintenance
Casting the lead-antimony frame that holds a battery plate’s active paste and carries its current.
The Grid Holds The Paste And Carries The Current
The grid does two jobs at once: it keeps the active paste in contact with a metal skeleton, and it conducts current from every point of the plate to the lug and out to the bus bar. A grid that is badly cast is a plate that will lose paste or crack at a rib, so the details of geometry and temperature are worth the trouble (Plate Casting gives the full treatment; this article is the working method).
The grid is a rectangular frame with a mesh of ribs and a lug above the top edge. The carrier suggests plates 100–150 mm wide and 120–180 mm tall for hand-built cells with a thickness of 2–3 mm; the plate-casting article uses a 12 cm × 10 cm grid 3–4 mm thick with a 1.5 cm lug. Both are starting figures, not tied to a capacity.
What You Need
- Alloy — lead with antimony added (Lead-Antimony Alloy)
- A mold — cast iron is ideal; steel plate is adequate; soapstone or hardwood serves for prototypes, with the hardwood charring after 10–20 pours; an open sand mold is possible (Sand Casting)
- A ladle or crucible and a heat source — lead melts at 327°C (Crucibles)
- Tongs, a file, a knife, a scale — for handling, trimming and weighing
- A clamp or vice — to hold the two mold halves shut
Method: Cast A Grid
Step 1: Preheat the mold. The carrier gives 150°C; the plate-casting article gives 150–200°C, “just too hot to touch”. A cold mold freezes the metal before it fills the ribs.
Step 2: Bring the alloy to pouring heat and skim it. The sources give 380–400°C (carrier) and 420–450°C (plate-casting and the lead-acid article). Too cool leaves cold shuts and short shots; too hot gives turbulence, porosity and shrinkage cavities. Skim the dross from the surface before each pour.
Step 3: Pour in one steady stream into the sprue until metal rises slightly in it, which shows the cavity is full.
Step 4: Let it set, then open the mold. The carrier says 30 seconds; plate-casting says 2–4 minutes undisturbed. The sources conflict, so open when the plate has frozen hard enough to lift out without bending. Never quench in water, because that cracks the grid.
Step 5: Trim and inspect. File off flash and sharp edges, which can pierce separators, and hold the grid up to the light for holes and thin spots. Plate-casting asks for flatness within 0.5 mm and for every rib complete.
Worked Example: How Much Metal Per Grid
General knowledge, not from the other articles: lead has a density of 11.34 g/cm³. A solid plate 12 cm × 10 cm × 0.35 cm has a volume of 12 × 10 × 0.35 = 42 cm³ and a mass of 42 × 11.34 = 476 g. A ribbed grid holds less metal than a solid plate, so 476 g is an upper bound. A ladle holding 1–2 kg (the plate-casting figure) then holds at least 1,000 ÷ 476 = 2.1 grids’ worth, which is why the batch can be poured grid after grid from one melt. Weigh every cast grid; a spread of more than ±5% in a batch shows an inconsistent pour.
Common Mistakes
- Pouring into a cold mold — gives lines where the metal froze and incomplete ribs
- Quenching to speed things up — cracks the grid
- Leaving sharp flash — a thin fin punches through a separator and shorts the cell
- Flattening a warped grid by force — lead cracks if bent sharply; press gently between flat plates
Safety
Molten lead throws metal and gives off fume
Lead melted for casting emits toxic fume, especially above 450°C, and a drop of water on the mold or in the ladle turns to steam and flings molten metal (general knowledge, not from the other articles). Cast outdoors or under a strong hood, keep every tool and mold bone dry and warmed first, and wear eye protection, leather gloves and sleeves. Do not eat, drink or smoke where lead is worked, and wash before touching food (Lead Exposure Prevention).
What’s Next
- Learn next: Plate Pasting — filling the grid with active material
- Combine with: Lead-Antimony Alloy — the metal that goes into the mold