What would be the appropriate method to determine the state of charge in a battery?

The most appropriate method to determine the state of charge in a battery is to use a voltmeter on an open circuit. When a voltmeter measures the voltage of a battery that has been disconnected from any load (an open circuit), it provides a direct reading of the battery's terminal voltage. This voltage level can then be compared to established voltage benchmarks corresponding to specific state of charge levels.

For instance, a fully charged lead-acid battery at rest will typically show a voltage of about 12.6 to 12.7 volts, while a battery that is significantly discharged may show a voltage around 12.0 volts or lower. This reading gives a clear and immediate indication of the battery's state, allowing for easy assessment of whether the battery needs charging or is in good condition.

Using a hydrometer, while also a valid method, is more specific to flooded lead-acid batteries and requires physical access to the electrolyte, making it less universal than a voltmeter. An amp meter measures current flow, which doesn't directly indicate the state of charge without knowing the rate of current discharge or charge over time. A thermal camera, on the other hand, detects temperature differences but does not provide any information related to the charge level of a battery. Therefore