A fully charged lead-acid battery will not freeze until extremely low temperatures because

• A. Most of the acid is in the solution.
• B. The acid is in the plates, thereby increasing the specific gravity of the solution.
• C. Increased internal resistance generates sufficient heat to prevent freezing.
• D. The electrolyte is freeze-safe at most temperatures.

Answer: (A)

The key idea is that freezing point is lowered by the presence of dissolved solute. In a fully charged lead-acid cell, the electrolyte becomes rich in sulfuric acid, so most of the acid is in the liquid solution rather than bound in solid compounds on the plates. That concentrated electrolyte has a much lower freezing point than pure water, so the battery’s electrolyte won’t freeze until extremely cold temperatures. The plates themselves aren’t where most acid resides in this charged state, and while some heat can come from resistance, it isn’t the main reason it won’t freeze. The electrolyte isn’t simply freeze-safe at normal temperatures.