A two-minute voltage check tells you if your battery is healthy before it leaves you stranded.
Check after the car has sat for a few hours, with lights and accessories off, for a true resting reading.
About 12.6V is full, 12.4V roughly three-quarters, 12.2V about half. Below 12.0V means discharged.
Check for corrosion on the terminals, a swollen case or leaks. Any of these means a workshop visit.
With the engine running, roughly 13.7V to 14.7V shows the alternator is charging the battery.
A workshop load test shows how the battery copes under strain. Never touch both terminals with metal.
An inverter with a live voltage display shows your input voltage while you charge devices.