How can you test a vehicle battery's state of charge?

The most effective way to test a vehicle battery's state of charge is by using a voltmeter or a hydrometer. A voltmeter measures the voltage across the battery terminals, which indicates the state of charge. A fully charged battery typically shows around 12.6 volts or more, while a reading below 12.4 volts indicates a discharged state.

A hydrometer measures the specific gravity of the battery electrolyte, providing insight into the charge level based on how well the electrolyte conducts electricity. This method is particularly useful for lead-acid batteries, as it directly correlates the density of the electrolyte with the battery's state of charge.

In contrast, measuring the battery size does not provide any information about its charge level or condition, nor does checking the battery warranty indicate its current state of charge. Observing the battery color can give hints about lead corrosion or surface condition but has no direct correlation with the charge state. Thus, utilizing a voltmeter or hydrometer is the established and reliable method for accurately assessing a battery's state of charge.