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?
Correct Answer: the electrolyte is a mixture of 64% distilled water and 36% sulphuric acid 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:
Relate state of charge to specific gravity (hydrometer reading).Use standard tables mapping 1.26–1.28 SG to approximately 36% H2SO4 by weight.Identify the option that matches this proportion.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:
the electrolyte is a mixture of 64% distilled water and 36% sulphuric acid by weight