How is the charge of a storage battery measured?

The charge of a storage battery is primarily measured by assessing the specific gravity of the electrolyte. In lead-acid batteries, for instance, the electrolyte is a sulfuric acid solution, and the specific gravity indicates the concentration of the acid in the solution. As the battery discharges, the specific gravity of the electrolyte decreases because the lead sulfate is formed and the concentration of sulfuric acid diminishes. Conversely, during charging, the specific gravity increases as the acid concentration rises. By using a hydrometer, technicians can measure the specific gravity and determine the state of charge of the battery, providing an accurate assessment of its capacity and health.

While voltage level is also a useful indicator of a battery's charge, it can be misleading without considering the specific gravity, as the voltage may not significantly change until the battery is nearing a discharged state. Current flow is more indicative of the battery's performance during usage rather than its stored charge. Electrolyte composition could refer to the type of acid or solution used but does not provide a measure of charge directly. Understanding specific gravity is crucial for accurately gauging the charge in storage batteries, making it the correct answer in this context.