When using a hydrometer to check a lead-acid battery's charge, at what electrolyte temperatures is a correction factor needed?

A correction factor is necessary when using a hydrometer to check a lead-acid battery's charge because the temperature of the electrolyte can significantly affect the specific gravity readings. At temperatures outside the optimal range, the readings may not accurately reflect the battery's state of charge.

For lead-acid batteries, the ideal temperature range for measuring specific gravity is around 70°F, as this is where the readings are standard. When the temperature falls below 70°F or rises above 90°F, the density of the electrolyte can change due to temperature variations, leading to readings that might suggest a charge level different from the actual state of the battery.

Therefore, to ensure that you are getting an accurate measurement of the battery's charge, a correction factor is applied when the electrolyte temperatures are either below 70°F or above 90°F. This adjustment allows for an accurate assessment of the battery's health and ensures proper maintenance and usage of the battery system.