On October 15, 2022, about 1448 central daylight time, a Diamond Aircraft DA62, N84LT, was substantially damaged during a forced landing near the Dallas Executive Airport (RBD), Dallas, Texas. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. A review of ADS-B data revealed that the airplane departed Winston Field Airport (SNK), Snyder, Texas, about 1448, and climbed to a cruise altitude of 11,500 ft mean sea level (msl). On arrival, the pilot descended to fly a visual approach to Runway 13 at RBD, extended the landing gear, and contacted the tower controller for landing clearance. During the controller’s response, the pilot heard a “pop” and observed the avionics display screens lose power. A few seconds later, he observed that both engines had lost power. The pilot executed a forced landing on to a road, during which the airplane struck a power line in the descent, and after touchdown, two road signs, which resulted in substantial damage to the right wing. A review of downloaded engine and flight data indicated the airplane lost electrical power to the avionics system and both engines nearly simultaneously while at 2,000 ft msl and about 5 miles west of RBD. The airplane was equipped with two Austro Engine E4P-C liquid-cooled, in-line four-stroke four-cylinder diesel engines. All engine components are controlled by an electronic engine control unit (ECU) system. Each engine can only be operated with the engine master switch ON. Each engine has its own ECU which receives its electrical power from the alternator when at least one engine is running. When both engines are off, the ECU receives its electrical power from the main battery or the back-up ECU batteries. The engine master switch must be set to ON to activate the engine ECU. To support the alternator electrical power supply to the ECUs in case of a malfunction of the main battery, additional sealed-lead-acid batteries are connected to the left and right engine ECU buses. These batteries are designed to provide 30 minutes of engine operation in case of a complete airplane electrical failure. Both engines may stop if the 30 minutes have elapsed. Postaccident examination of the airplane revealed the backup ECU batteries for both engines had been wired in parallel, rather than in series, and the in-line fuses to the ECU batteries were blown. The main battery capacitance was tested and found to be 11.04 Amperes-hours (Ah), or 81.2% of its rated capacity. The battery capacitance requirement was 85% or better. Additionally, the alternator relays were wired such that the alternator would not disconnect the alternator power from the main electrical system. The relays would cut power to the glow plugs for the respective engines. The Garmin G1000 NXi electronic flight instrument system (EFIS) recorded data indicated that the accident flight had a duration of 1 hour 25 minutes and 49 seconds. The Austo Engine ECUs contained non-volatile memory used to store 16 channels of data signals. The right engine data contained 23 sessions of recording to include the accident flight. The left engine data contained 11 sessions to include the accident flight. However, while the right engine event flight recording had one hour 25 minutes 3 seconds of recording, the left engine data ended earlier and had only 18 minutes and 46 seconds of recording. The ECU right engine data of the event showed the right engine circuitry bus voltage was at 27 volts until 1447:41, when it suddenly dropped to 11.7 volts, after which the recording stopped. Airplane and component testing did not reveal a definitive root cause of the initial power failure. Testing could not duplicate the actual conditions of the flight when the electrical system anomaly occurred or the anomaly itself. According to the airplane’s maintenance records, as part of the annual maintenance inspection on June 24, 2022, the 12-month ECU battery backup replacement check was completed, and 4 new ECU battery backup batteries were installed. The 1000 hour/12-month Concorde battery capacitance check was also completed, and the battery had 92% capacitance. On September 21, 2022, the airplane’s main battery was replaced with a serviceable one as the maintenance facility waited for a new battery that was on order. The airplane had flown about 15 hours from the time the serviceable battery was installed to the accident. A review of the aircraft maintenance manual showed it did not provide a procedure to verify the ECU back-up batteries were functioning correctly after replacement. The last step in the ECU backup battery installation was to run engines and verify that the electrical system operated correctly. However, this step does not verify that the ECU backup batteries were installed correctly and were ready to provide power to the ECUs. In addition, there was no ECU backup battery operational test. Diamond Aircraft issued a service information letter requiring inspection of ECU backup batteries wiring installation on all DA 62 aircraft.