Definition
A maintenance procedure in which the specific gravity of the sulfuric acid electrolyte in a lead-acid storage battery is measured using a hydrometer. The reading indicates the state of charge of the battery: a fully charged cell typically reads about 1.275 to 1.300, while a discharged cell reads near 1.150. The hydrometer is a sealed glass tube with a weighted float; electrolyte is drawn into the tube with a rubber bulb, and the depth to which the float sinks shows the specific gravity on a calibrated scale.
Plain English
Checking how charged a lead-acid battery is by drawing some of its acid into a small glass tool that floats higher in stronger acid and lower in weaker acid. The reading tells you whether the battery is full, partly used, or flat.
Context Anchor
Seen during aircraft battery servicing, especially when checking a flooded lead-acid battery during maintenance or troubleshooting.
Derivation
Electrolyte comes from Greek elektron (amber, the original source of static electricity) and lytos (loosened or dissolved), meaning a liquid that carries electric current by dissolved ions. Hydrometer comes from Greek hydro (water) and metron (measure), literally a water-measurer -- it measures how dense a liquid is compared to water.
Why Pilots Care
A low reading warns that the battery may not deliver reliable starting power or support electrical systems in flight.
Analogy
It is like using a simple tester to check the strength of a mixture, rather than guessing by how it looks.
Intuition Check
Do not assume this means checking the liquid level. A hydrometer checks the liquid’s density, which tells you about battery charge.
Example Sentence 1
During the 100-hour inspection, the mechanic tested the electrolyte with a hydrometer and found one cell reading well below the others.
Example Sentence 2
A reading of 1.265 confirmed the electrolyte was at full charge when tested with a hydrometer.