How can you determine a battery's state of charge?

A battery's state of charge can most effectively be determined by using a hydrometer or a digital multimeter.

Using a hydrometer allows you to measure the specific gravity of the electrolyte in a flooded lead-acid battery, which correlates to the charge level. A higher specific gravity indicates a fully charged battery, while a lower value shows that the battery is discharged.

Similarly, a digital multimeter can measure the voltage of the battery. By knowing the nominal voltage of the battery type (for example, 12 volts for a standard automotive battery), you can assess whether the battery is charged or needs recharging based on its voltage reading. Generally, open-circuit voltage readings provide a good estimation of the state of charge: for instance, a reading of around 12.6 volts generally indicates a fully charged state.

While a visual inspection can provide insight into physical damage or corrosion, it does not give an accurate measurement of the charge. Measuring the temperature of the battery does not directly relate to its state of charge; different conditions might affect the temperature without indicating the charge level. Checking the battery's weight is not a reliable method for determining its charge, as weight can vary widely among battery types and does not correlate well with the charge level.