Battery Parasitic Drain Test
If a known good battery continues to die overnight or in a short period, suspect a parasitic drain. The drain could be caused by a short in the system, a small light, or an accessory drawing current overnight.
Allow the vehicle to rest for a certain period, then measure the voltage at each fuse. Set the multimeter to millivolts; each fuse should be 0.00 or within specifications. Circuits with voltage present require further inspection; the switch or actuator may be ON, causing the drain.
Connect a multimeter in series between the negative battery terminal and the negative battery post. If the draw is above 35-50mA, identify the circuit causing the drain by locating and removing each fuse until the reading on the meter drops to or below specifications.
Most likely, something like a light or an accessory was left on, but sometimes it's the component, wire, or connector. A circuit diagram may be necessary to identify and trace the circuit. Once found and repaired, measure the battery drain to ensure it's within specifications. Check with the manufacturer for specifications and any special procedures.