Hydrometer

A hydrometer is a simple tool that measures the density (specific gravity) of a liquid. For motorcycles it is used to check the state of charge of a conventional flooded lead-acid battery by drawing a sample of electrolyte from each cell: the more charged the cell, the denser the acid.

Comparing cells also reveals a weak or failing cell that a voltmeter on the whole battery might not show. Similar hydrometers are used to check antifreeze strength in coolant.

Sealed AGM, gel and lithium batteries cannot be opened, so a hydrometer is useless on them; use a multimeter or battery tester instead. Battery acid is corrosive, so wear eye protection and gloves.