310T Engine And Supporting Systems Practice Exam

Specific gravity testing hinges on the fact that in flooded lead-acid batteries the electrolyte density changes with how charged the battery is. By using a hydrometer (and correcting for temperature), you can read the electrolyte’s density and infer state of charge. When fully charged, the sulfuric acid solution is more concentrated, giving a higher specific gravity (roughly around 1.25–1.28 at 25°C); as the battery discharges, the acid is consumed and the density drops, indicating a lower specific gravity. This direct link between density and charge also helps detect issues like electrolyte stratification or sulfation. The other battery types don’t fit this method because they don’t have a freely movable liquid electrolyte whose density reliably tracks charge. Lithium-ion and solid-state cells are typically sealed and use non-liquid or non-accessible electrolytes, so density isn’t a practical or meaningful SOC indicator. Nickel-metal hydride uses an alkaline electrolyte where density changes aren’t a dependable measure of state of charge.