Make battery electrolyte by pouring sulfuric acid slowly into distilled water in a heat-resistant vessel while stirring, never water into acid, then measure with a hydrometer and trim toward a specific gravity of about 1.265 at 25°C for a charged cell. Plates are formed in a much weaker solution, and the strength you mix from a given concentrated acid is a mass balance you can compute before you pour.
Part of Battery Building and Maintenance
Diluting sulfuric acid with pure water to the strength a lead-acid cell needs, safely and measurably.
The Working Strength Is Set By The Fully Charged Cell
The carrier targets a specific gravity of 1.265 at 25°C for a fully charged battery. The lead-acid article gives a working electrolyte of 1.265–1.280, which it describes as approximately 37% sulfuric acid by weight; Rechargeable Batteries describes 1.26–1.28 as approximately 30% acid by weight. The two weight percentages disagree, so use specific gravity as the working measure and treat the percentage as approximate (Specific Gravity Control).
Plate formation uses weaker acid: 1.050 (carrier) to 1.100 and as high as 1.15 in other articles (Plate Curing and Formation). After formation the acid is raised to working strength.
Water must be distilled, deionized or condensed steam. Tap-water minerals such as calcium and chlorides contaminate the electrolyte and reduce battery life; the lead-acid article accepts clean collected rainwater only in an emergency (Distillation covers a still).
What You Need
- Sulfuric acid — salvaged or made (Sulfuric Acid Sources)
- Distilled water
- A heat-resistant, acid-proof mixing vessel — glass, or glazed ceramic with a known lead-free glaze (Chemical Vessels); never iron, steel, copper or aluminium
- A glass hydrometer — Specific Gravity Control
- Eye protection, gloves, an apron and a bucket of clean water — Acid Handling
- A stirring rod of glass or acid-resistant plastic
Method: Dilute The Acid
Step 1: Work out the mix. Decide the final mass and the acid fraction you want, then use the mass balance in the worked example below.
Step 2: Put the water in first. Measure the water into the vessel and stand the vessel somewhere it cannot tip.
Step 3: Add the acid in a thin stream. Pour slowly, stirring constantly. The vessel will warm. If it becomes too hot to hold, stop and let it cool. The lead-acid article says to mix in a heat-resistant container and cool before filling a battery.
Step 4: Cool, then measure. Let the mix return toward room temperature and float the hydrometer in it.
Step 5: Trim. If the reading is too low, add a little more acid (into the diluted mix, stirring); if too high, add distilled water. Make small changes and re-measure, since the effect of a spoonful is easy to overshoot.
Worked Example: Diluting Chamber Acid
Chamber acid comes at about 60–70% (Sulfuric Acid). For 1 kg of electrolyte at 37% acid by weight, the acid needed is 0.37 kg of pure sulfuric acid.
- From 60% acid: 0.37 ÷ 0.60 = 0.617 kg of acid solution, plus 1 − 0.617 = 0.383 kg (about 383 mL) of water.
- From 70% acid: 0.37 ÷ 0.70 = 0.529 kg of acid solution, plus 0.471 kg of water.
If the 30% figure is nearer your target: 0.30 ÷ 0.60 = 0.5 kg of 60% acid plus 0.5 kg of water. The difference between 0.617 and 0.5 kg shows how far apart the corpus targets are, which is why the hydrometer decides the final strength. The same sums apply to reclaimed battery acid: it is already near working strength, so filter it and trim.
Common Mistakes
- Adding water to acid — see the danger note
- Mixing in a metal vessel — the acid dissolves the metal, and for zinc or aluminium the reaction gives off hydrogen (Container Selection)
- Using a salvaged glazed pot of unknown glaze — acid leaches lead from the glaze (Chemical Vessels)
- Trusting the percentage target — the corpus’s two weight figures disagree, so trust the hydrometer
- Pouring hot electrolyte into the cell — cool it first
Safety
Water poured onto strong acid boils and throws acid
Mixing sulfuric acid with water releases a large amount of heat. Concentrated acid is far denser than water, so water poured onto it floats on top (general knowledge, not from the other articles), the heat is released in that thin layer and the water boils and spits acid. Acid poured slowly into a large volume of stirred water sinks and mixes into that mass, which absorbs the heat. Always add acid to water, wear splash goggles, and keep clean water close for flushing (Neutralization; Spill Response).
What’s Next
- Learn next: Sulfuric Acid Sources — where the concentrated acid comes from
- Combine with: Specific Gravity Control — measuring what you mixed