Lead-acid battery service — composition of electrolyte when fully charged For a fully charged automotive lead-acid battery at room temperature, which statement best describes the approximate composition of the electrolyte by weight?

Introduction / Context:
Electrolyte concentration determines battery open-circuit voltage, internal resistance, and freezing/boiling characteristics. Technicians often relate acid strength to specific gravity to assess state of charge.

Given Data / Assumptions:

• Standard flooded lead-acid battery in automotive use.
• Room temperature and fully charged condition.
• No additives or gel/AGM variants considered.

Concept / Approach:
When fully charged, electrolyte specific gravity is typically around 1.26–1.28 at 25°C, which corresponds to roughly 36% sulphuric acid and 64% water by weight. As the battery discharges, sulphuric acid is consumed and water fraction increases, lowering specific gravity.

Step-by-Step Solution:

Verification / Alternative check:
Service manuals and battery datasheets list the same nominal composition for a fully charged state.

Why Other Options Are Wrong:

• Pure water or pure acid: neither exists in a working flooded cell.
• 90/10 or 50/50: do not match the specific gravity required for proper voltage and capacity.

Common Pitfalls:
Ignoring temperature corrections on hydrometers; confusing volume percent with weight percent.

Final Answer: