On June 23, 2012, at 0830 mountain daylight time, an experimental amateur-built Hoyle model Fighter Escort Wings airplane, N2051P, was substantially damaged during a forced landing near Pueblo, Colorado. The commercial pilot was not injured. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91. Day visual meteorological conditions prevailed for test flight, which was operated without a flight plan. The local area flight departed Pueblo Memorial Airport (KPUB), Pueblo, Colorado about 0810. The accident occurred during a phase-one test flight of the 2/3 scale P-51 Mustang airplane replica. The pilot reported that there was an electrical system failure during cruise flight, which resulted in insufficient voltage to maintain engine operation using the primary or secondary battery circuits. Following the total loss of engine power, the pilot elected to perform a wheels-up landing on a gravel road, during which the airplane was substantially damaged. The non-certificated engine, a 350-horsepower Chevrolet model LS1 automobile engine, serial number 12550592, was equipped with a computer-controlled electronic ignition system and high-pressure fuel pumps. According to the pilot/builder, the airplane incorporated two 12-volt batteries wired in parallel to supply voltage to the main power bus. A postaccident examination revealed that the primary battery had an internal short and would not take a charge. The secondary battery was found below normal service voltage, but was able to be recharged. No anomalies were found with remaining electrical system components or wiring paths. The alternator functioned as designed during a bench test. The two battery circuits were protected by contactors and bridge diodes on the main power bus; however, postaccident testing revealed that they were not isolated from each other, and as such, an internal short of one of the two batteries could draw-down the charge of the other battery. The closest weather observing station was at KPUB, located about 8 miles north of the accident site. At 0853, the KPUB automated surface observing system reported the following weather conditions: wind 220 degrees at 3 knots; visibility 10 miles; clear sky conditions; temperature 29 degrees Celsius; dew point -2 degrees Celsius; altimeter setting 29.91 inches of mercury.