Sulfuric Acid H₂SO₄ in Lead-Acid Battery production — Electrolyte technical standards

TL;DR: H₂SO₄ 98% is diluted with purified water to form an electrolyte solution at ~37% w/w concentration, density 1.28 g/ml (25°C), used in Lead-Acid Batteries. Loc Thien supplies H₂SO₄ at various concentrations as required for battery manufacturing plants — delivery to industrial parks by 5–30 ton tanker, with batch COA/MSDS.

Role of H₂SO₄ in Lead-Acid Batteries

Sulfuric Acid (H₂SO₄, CAS 7664-93-9) serves as the electrolyte in Lead-Acid Batteries — the most common type of battery for electrical energy storage in automobiles, forklifts, UPS systems, solar power systems, and telecommunications stations.

In a Lead-Acid Battery cell, the positive electrode is PbO₂, the negative electrode is Pb, and the electrolyte is dilute H₂SO₄ solution. During discharge, PbO₂ and Pb react with H₂SO₄ to form PbSO₄ and water, decreasing the H₂SO₄ concentration. During charging, the reverse occurs — PbSO₄ converts back to Pb and PbO₂, and the H₂SO₄ concentration increases. The relationship between solution density and battery state of charge is the basis for measuring remaining capacity with a hydrometer.

Electrolyte concentration and density standards

Electrolyte for Lead-Acid Batteries is prepared from H₂SO₄ 98% (d > 1.82 kg/L) diluted with purified water meeting IEC 62877-2 standards.

Standard density

Per IEC 62877-1:2023, the electrolyte after preparation has a common density range of 1.24–1.30 kg/L at 25°C. At a density of 1.28 kg/L (25°C), the corresponding H₂SO₄ content is:

Source: IEC 62877-1:2023 Table 2 — Acid electrolyte density at 25°C versus percentage of sulfuric acid

A density of 1.28 g/ml at 25°C corresponds to ~37.8% w/w — this is the standard density for most SLI (starting, lighting, ignition) and stationary Lead-Acid Batteries. Traction batteries typically use 1.26–1.28 density; stationary batteries typically use 1.24–1.26.

Preparation from H₂SO₄ 98%

Dilution formula to obtain V liters of 1.28-density electrolyte:

Mass of H₂SO₄ 98% needed = V × 1.28 × 0.3781 / 0.98

Example: To prepare 1,000 liters of 1.28-density electrolyte, approximately 494 kg of H₂SO₄ 98% is needed, mixed with ~590 kg of purified water.

Safety rule: Always slowly pour concentrated H₂SO₄ into water (never the reverse) to avoid violent heat generation causing boiling and splashing.

Purity requirements per IEC 62877-1

Not all technical-grade H₂SO₄ is suitable for battery electrolyte. Metal impurity content must be strictly controlled to prevent self-discharge and electrode plate damage.

Impurity limits — Filling electrolyte

Source: IEC 62877-1:2023 Table 3 — Filling electrolyte impurity limits

For operating electrolyte, the Fe limit is relaxed to 100 mg/L, and halogen to 30–200 mg/L depending on battery type.

Documents supplied with H₂SO₄ for battery plants

Every lot of H₂SO₄ for battery production supplied by Loc Thien ships with a COA and MSDS/SDS and is traceable by lot code; supporting documents can be provided for customer audits. If your end brand requires ZDHC documentation, contact us to confirm the documents currently available for each product.

Electrolyte preparation process for batteries

Step 1 — Incoming material inspection

• H₂SO₄ 98%: check COA — content ≥98%, Fe impurity ≤20 ppm, colorless
• Purified water: conductivity ≤1 µS/cm per IEC 62877-2

Step 2 — Dilution

• Use acid-resistant mixing tanks (PP, FRP, or lead/ceramic-lined steel)
• Add water first, then slowly add H₂SO₄ with continuous stirring
• Control solution temperature not to exceed 60°C

Step 3 — Quality inspection

• Measure density at 25°C (or measure at actual temperature and correct using factor 0.00075 kg/L·K)
• Analyze Fe, Cl, As impurities per IEC 62877-1

Step 4 — Cooling and filtration

• Allow solution to cool to room temperature
• Filter through 5–10 µm membrane to remove mechanical impurities

Density correction by temperature

The density of H₂SO₄ solution changes with temperature. When measured at a temperature other than 25°C, correction is required:

d₂₅ = dₜ + f × (t − 25)

Correction factor f depends on the density range:

Source: IEC 62877-1:2023 Table 1

Example: Measured density 1.285 at 35°C → d₂₅ = 1.285 + 0.00075 × (35−25) = 1.2925 — significant deviation if uncorrected.

Effect of H₂SO₄ concentration on battery performance

Research by Pavlov et al. (ResearchGate, 2000) on 12V/32Ah batteries showed the relationship between H₂SO₄ concentration and battery lifespan:

• Region H (H₂SO₄ < 1.24 g/ml): initial capacity lower than nominal, but cycle life exceeds 100 cycles due to reduced plate sulfation
• Region P (H₂SO₄ > 1.24 g/ml): initial capacity higher than nominal, but shorter lifespan (maximum 100 cycles)
• Density of 1.24 g/ml is the balance point between capacity and lifespan

Commercial VRLA batteries currently use density > 1.30 g/ml (Region P), while H-type (traction, deep-cycle) batteries prefer lower density to extend cycle life.

H₂SO₄ storage and transport

• Packaging: PE/PP plastic tanks, 316L stainless steel tanks for concentrated H₂SO₄; do not use carbon steel with dilute H₂SO₄
• Supply formats: 5–30 ton tanker trucks, 20–35 kg plastic jerry cans, 250 kg drums
• Delivery: Delivery in HCMC and industrial parks in Dong Nai, Binh Duong, Long An
• Accompanying documents: COA (batch analysis), MSDS, CO if required

Frequently asked questions

What temperature is density 1.28 at?

25°C per IEC 62877-1 standard. If measured at a different temperature, correction is required using factor 0.00075 kg/L·K.

Can tap water be used for electrolyte preparation?

No. Tap water contains Cl⁻, Ca²⁺, Mg²⁺ ions exceeding IEC 62877-2 limits. Purified water (DI/RO) with conductivity ≤1 µS/cm must be used.

How can battery charge level be determined from density?

Measure the electrolyte density of each cell. A fully charged battery reads ~1.27–1.30; a fully discharged battery reads ~1.10–1.15. A difference of >0.05 between cells indicates a faulty cell.

Which H₂SO₄ grade is suitable for battery production?

High-purity H₂SO₄ 98% — Fe content ≤20 ppm, colorless, meeting impurity limits per IEC 62877-1 Table 3.

How to dilute H₂SO₄ 98% to 1.28?

Approximate ratio: 1 part by mass of H₂SO₄ 98% to 1.2 parts by mass of purified water. Always add acid to water, never the reverse.

Quote request / contact

🛒 Need H₂SO₄ for battery production? Request quote for Sulfuric Acid H₂SO₄ 10%–98% — batch COA/MSDS, nationwide IP delivery. Hotline 0979 891 929.

Loc Thien Chemical supplies H₂SO₄ in bulk at various concentrations — from H₂SO₄ 98% raw material to pre-diluted electrolyte solutions at requested densities (1.24–1.30). 6 warehouses across 3 regions (Ba Ria – Vung Tau, Dong Nai, Binh Duong, Da Nang, Can Tho and Bac Ninh), 5–30 ton tankers, nationwide delivery. Tax code 0313650856 — direct official import.

See also: Sulfuric Acid H₂SO₄ complete guide — properties, production, applications.

Quality control: QC Specialist Phan Cam Thuy inspects and Vo Thi Nhu Hoa approves each H₂SO₄ batch. Batch COA — ensuring concentration and impurities meet battery-grade standards.