Fundamentals of Electricity & Electronics
What is the most reliable method for determining the state of charge of a lead-acid storage battery?
Answer
Measuring the specific gravity of the electrolyte with a hydrometer
State of charge in a lead-acid battery is determined by measuring electrolyte specific gravity with a hydrometer. As the battery discharges, sulfuric acid combines with the plates and the electrolyte becomes more like water, so specific gravity falls; a fully charged cell reads roughly 1.275–1.300. Terminal voltage is only a rough indicator, and electrolyte level or weight does not reveal charge state.
Source: FAA-H-8083-30B, Ch. 12 (Electricity & Electronics), 'Storage Batteries — Lead-Acid Cell / Specific Gravity'
Not the answer
- Measuring the open-circuit terminal voltage
- Weighing the battery
- Checking the electrolyte level in each cell
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