It is a common way to measure battery charge through a hydrometer in automobiles batteries (not the sealed type): https://en.wikipedia.org/wiki/Hydrometer http

It is a common way to measure battery charge through a hydrometer in automobiles batteries (not the sealed type): https://en.wikipedia.org/wiki/Hydrometer https://en.wikipedia.org/wiki/Hydrometer >Battery hydrometer The state of charge of a lead-acid battery can be estimated from the density of the sulfuric acid solution used as electrolyte. A hydrometer calibrated to read specific gravity relative to water at 60 °F (16 °C) is a standard tool for servicing automobile batteries. See also here: https://deepcyclebatterystore.com/use-battery-hydrometer/ https://deepcyclebatterystore.com/use-battery-hydrometer/