Which statement best explains why a fully charged lead-acid battery resists freezing?

• A. The acid is in the plates, thereby increasing the specific gravity of the solution.
• B. Increased internal resistance generates heat.
• C. Antifreeze additives are present.
• D. Most of the acid is in the solution.

Answer: (D)

The key idea is that the freezing point of a lead‑acid battery’s electrolyte depends on how concentrated the sulfuric acid is in the solution. When the battery is fully charged, most of the sulfuric acid is in the electrolyte, not locked up as lead sulfate on the plates. This makes the electrolyte more concentrated (higher specific gravity), which lowers its freezing point, so it resists freezing in cold conditions. If the battery were discharged, sulfuric acid would be converted to lead sulfate and water, thinning the electrolyte, raising the freezing point, and making it more susceptible to freezing. The other statements don’t explain this effect: antifreeze additives aren’t used in standard batteries, internal resistance heating isn’t the main factor for freezing resistance, and having most acid in the plates would actually mean less acid in solution.