What happened
The pilot had operated the aircraft four days before the accident. Following that previous flight, the electrical master switch was inadvertently left in the ON position, which caused the battery to discharge.
To facilitate an engine start for the current flight, the pilot connected external electrical power to the exterior battery receptacle. After boarding the aircraft and starting the engine, the pilot set the engine RPM to a low setting. The pilot then exited the aircraft to remove the external power source and the wheel chocks. Immediately after the chocks were removed, the aircraft began moving forward. The pilot attempted to stop the movement, but was unsuccessful. The unoccupied aircraft continued to taxi across the airport and into a wet field, where it impacted a small ditch. There were no injuries reported.
Findings
The investigation determined that the battery had been discharged due to the electrical master switch being left on following a flight four days prior.