Narrative:

When opening the ATR-42 aircraft for the morning flight ZZZ to ZZZ1; I found the main battery switch had been left on all night by the mechanic that had closed the plane the night before. Both the main and emergency batteries were completely discharged. I found the main battery switch on; and no power from the batteries. Here at ZZZ; I ask the flight crews to leave aircraft power 'on' when they leave the plane. I do this so that I can perform certain checks of systems after they leave; and so that I can set the plane up for the next morning. On this occasion; I was in ZZZ2 and mechanic X was covering the station for me. I believe mechanic X; being primarily a heavy maintenance technician working in ZZZ3; is unaccustomed to checking the cockpit for ensuring power is 'off' prior to closing the aircraft; and simply assumed the flight crew had shut it off.I was able to energize the ground power contactor to enable the batteries to be recharged by the ground power unit (gpu). By the time the plane was ready to depart; the batteries had sufficiently charged. They were replaced in ZZZ1 as a precautionary measure.stress the need to all technicians to do a cockpit check before removing ground power and leaving the aircraft. With other aircraft; airlines will require the beacon or navigation (nav) light switch to be left 'on' when the aircraft is departed. This provides an external visual indication of the power status of the aircraft. This doesn't work on the ATR's; however; as neither the navigation lights nor the beacon operate when on battery power. A modification that would allow some exterior lights to be on when the aircraft battery switch is 'on;' could help to prevent this sort of problem.

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Original NASA ASRS Text

Title: A Line Mechanic preparing an ATR-42 aircraft for a morning flight notices the Main Battery switch had been left on all night. Both the main and emergency batteries were completely discharged.

Narrative: When opening the ATR-42 aircraft for the morning flight ZZZ to ZZZ1; I found the Main Battery Switch had been left on all night by the Mechanic that had closed the plane the night before. Both the Main and Emergency batteries were completely discharged. I found the Main Battery switch on; and no power from the batteries. Here at ZZZ; I ask the Flight Crews to leave aircraft power 'on' when they leave the plane. I do this so that I can perform certain checks of systems after they leave; and so that I can set the plane up for the next morning. On this occasion; I was in ZZZ2 and Mechanic X was covering the station for me. I believe Mechanic X; being primarily a Heavy Maintenance Technician working in ZZZ3; is unaccustomed to checking the cockpit for ensuring power is 'off' prior to closing the aircraft; and simply assumed the flight crew had shut it off.I was able to energize the Ground Power Contactor to enable the batteries to be recharged by the Ground Power Unit (GPU). By the time the plane was ready to depart; the batteries had sufficiently charged. They were replaced in ZZZ1 as a precautionary measure.Stress the need to all Technicians to do a Cockpit Check before removing Ground Power and leaving the aircraft. With other aircraft; airlines will require the Beacon or Navigation (Nav) light switch to be left 'on' when the aircraft is departed. This provides an external visual indication of the power status of the aircraft. This doesn't work on the ATR's; however; as neither the navigation lights nor the beacon operate when on battery power. A modification that would allow some exterior lights to be on when the aircraft battery switch is 'on;' could help to prevent this sort of problem.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.