Battery grids are cast from lead hardened with roughly 3 to 6% antimony, which resists the slow growth that pushes a grid through its separator, but the antimony raises self-discharge over time. The practical source is scrap that already contains it, mixed with soft lead by mass balance to reach the target, because a settlement has no analytical lab to measure the result.
Part of Battery Building and Maintenance
Mixing a lead-antimony casting alloy from scrap and checking it without a laboratory.
Antimony Hardens The Grid At A Price
Pure lead is very soft and, in a cycling cell, suffers grid growth: the grid slowly expands, pushes through the separator and shorts the cell (Plate Casting). Adding 4–6% antimony hardens the lead, reduces growth and improves mold filling. The carrier says 3–6%; the plate-casting and lead-acid articles say 4–6%. The cost is that antimony migrates to the negative plate over time and raises self-discharge. The same article accepts pure lead for simple cells that only need to last 2–3 years. If you need a battery that holds charge for long standby, a lower antimony content is a trade you can make deliberately; this wiki gives no figure for it.
Where The Antimony Comes From
The corpus names four sources:
- Old battery grids — typically 4–8% antimony, the ideal source
- Type metal — used in printing, very high at 15–25% antimony
- Some lead cable sheathing — contains antimony, content not stated
- Stibnite ore — the carrier mentions it, but reducing it to metal is a smelting job of its own and this article does not cover it
Scrap has an unknown exact content, so treat any figure from scrap as a range, not a measurement.
What You Need
- Soft lead — pipe, sheet or ingot with no antimony
- Antimonial scrap — grids or type metal
- A crucible, ladle and skimmer — Crucibles
- A scale — for the mass balance
- A bar mold — to cast test bars
Method: Mix By Mass
Step 1: Choose the target inside 4–6% antimony, for example 5%.
Step 2: Compute the mix with a mass balance: the antimony in the scrap must equal the antimony wanted in the batch.
Step 3: Melt the soft lead first, then add the scrap, hold the melt hot and stir thoroughly, as the carrier says. Antimony melts at a much higher temperature than lead (general knowledge: about 631°C), so lumps of pure antimony dissolve slowly in lead at casting heat and scrap or type metal is the easier route.
Step 4: Skim and pour into bars, then cast a test grid.
Step 5: Check hardness. The plate-casting article gives the scratch test: antimony-lead resists a fingernail where pure lead dents easily. Make this useful by casting bars of the lead you know to be pure and of the mix, then comparing indentation under the same blow.
Worked Example: Two Mixes For A 10 kg Batch At 5%
Target antimony: 5% of 10 kg = 0.5 kg.
- From type metal at 20% (the middle of the stated 15–25%): scrap mass = 0.5 ÷ 0.20 = 2.5 kg, so 2.5 kg type metal + 7.5 kg soft lead. This assumes the rest of the type metal is lead; real type metal also contains tin (general knowledge), which the mass balance ignores.
- From old grids at 8% (the top of the stated 4–8%): scrap mass = 0.5 ÷ 0.08 = 6.25 kg, so 6.25 kg grids + 3.75 kg soft lead.
- From old grids at 4%: no soft lead can be added; 4% is already under the 5% target, so use them alone or find stronger scrap.
If the scrap’s content is unknown, the actual result can lie anywhere in the range: scrap at 4–8% mixed in the 8% ratio gives 2.5–5% in the batch. This is why the hardness comparison matters.
Common Mistakes
- Treating “typically 4–8%” as a measured value — it is a range for a class of scrap
- Assuming more is better — extra antimony cuts growth but raises self-discharge
- Skimming too late — dross in the pour appears as inclusions in the grid
Safety
Alloying lead gives off fume and splatters
Lead fume is toxic and rises with temperature, especially above 450°C, and antimony fume is also harmful (general knowledge, not from the other articles). Melt outdoors or under a strong hood, keep scrap bone dry because wet or oily scrap spits molten metal, and never use a vessel you cook in. Wash your hands before eating, and keep children and pregnant people away (Lead Exposure Prevention).
What’s Next
- Learn next: Grid Casting — pouring the alloy into the grid mold
- Combine with: Plate Casting — the alloy’s effect on plate life