The first source of battery acid is the electrolyte in salvaged lead-acid batteries, which is already near working strength once it is drained, filtered and trimmed. Making acid from scratch means burning sulfur or roasting pyrite into sulfur dioxide for the lead chamber process, or heating green vitriol above 480°C in a sealed clay retort. The chamber process gives acid at about 60 to 70%, stronger than a cell needs, so diluting rather than concentrating is the usual final step.
Part of Battery Building and Maintenance
Where the sulfuric acid for battery electrolyte comes from, from salvage to the historical production routes.
Salvage Comes First Because Making Acid Is Hard And Hazardous
The carrier lists three salvage sources: car batteries (drain, filter and adjust the concentration), industrial chemical stores, and fertilizer plants. A salvaged flooded battery’s acid has already done its job as electrolyte and needs only filtering and a hydrometer check (Specific Gravity Control). Making acid from scratch is, in the words of Sulfuric Acid, “genuinely difficult and hazardous”, so do it only when salvage runs out.
Three Production Routes Appear In The Corpus
Lead chamber process. Burn sulfur or roast pyrite for sulfur dioxide, add nitrogen dioxide from heated saltpeter as a catalyst in a fully sealed chamber with water vapour, and collect “chamber acid” at about 60–70% (Lead Chamber). Both the sulfur dioxide and the nitrogen dioxide are highly toxic.
Dry distillation of green vitriol. Heat iron sulfate crystals above 480°C in a sealed clay retort with a tube outlet; the released sulfur trioxide is led into water and dissolves to sulfuric acid. It is a batch process with limited yield, and the acid is moderately concentrated rather than fuming.
Contact process. The carrier says burning sulfur to sulfur dioxide and oxidizing that to trioxide over an iron oxide or vanadium catalyst “is feasible at small scale but requires careful temperature control”. The Sulfuric Acid article disagrees: it calls a simplified catalyst version “theoretically possible but difficult” and beyond the scope of early rebuilding chemistry. Treat the contact route as a later goal.
What You Need
- Sulfur or pyrite — sulfur from volcanic deposits, pyrite (“fool’s gold”) from ore-bearing rocks, or sulfur recovered from smelting
- Saltpeter — for the chamber route
- A sealed chamber or retort — pottery, clay-rendered brick or lead-lined
- Glass or glazed ceramic vessels — for collecting and storing; not iron, steel, copper, aluminium or wood
- A way to dilute and measure the product — Electrolyte Preparation
Method: Judge And Prepare Acid From Any Source
Step 1: Measure the strength. Float a hydrometer in a cooled sample; do not guess.
Step 2: Filter solids. Pass salvaged acid through a clean cloth or glass wool into glass.
Step 3: Adjust to working strength by adding acid to water or distilled water to acid in the way shown in the electrolyte article, a little at a time.
Step 4: Test a single cell first. The corpus does not list the impurities that damage plates. General knowledge, not from the other articles: iron, chlorides and nitrogen oxides carried over from the chamber process are harmful to lead-acid cells. A trial cell, watched for unusual gassing or self-discharge, tells you whether the batch is acceptable.
Worked Example: Sulfur Needed For A Batch
Each mole of sulfur gives one mole of sulfuric acid. The molar masses are general knowledge, not from the other articles: sulfur 32.06 g/mol and sulfuric acid 98.08 g/mol, so 32.06 ÷ 98.08 = 0.327 kg of sulfur per kilogram of acid at 100% yield. For 1 kg of electrolyte at 37% acid (the lead-acid article’s figure) the acid is 0.37 kg, needing 0.37 × 0.327 = 0.121 kg of sulfur at perfect yield. The vitriol route is stated to have a limited yield, so budget for substantially more.
Common Mistakes
- Concentrating chamber acid unnecessarily — 60–70% is already stronger than the 30–37% a cell needs (the two corpus figures, see Electrolyte Preparation)
- Boiling acid in an iron pot — it corrodes rapidly
- Leaving concentrated acid open — it is strongly hygroscopic and absorbs water from the air
- Skipping the trial cell — impurities show up only in use
Safety
Making acid releases toxic gas and the acid itself burns
Sulfur dioxide and nitrogen dioxide are toxic even in small amounts, so the chamber must be fully sealed with the operator never in contact with the gases. Concentrating acid by evaporation must be done in an open vessel, never a closed one, and not beyond the point where the acid appears thick and oily because concentrated acid decomposes above 337°C. Store in glass or glazed ceramic in a cool, ventilated place, and read Acid Handling before opening any container.
What’s Next
- Learn next: Electrolyte Preparation — diluting what you have to working strength
- Combine with: Sulfuric Acid — the full production routes