The pilot reported that he was building hours in the powered-glider to prepare for his glider instructor rating examination. He departed the airport with power and climbed to the west to chase thermals. After the powered-glider reached 4,000 ft above ground level (agl), he shut down the engine, left the battery power on, and continued to climb to 4,500 ft agl. After 30 minutes, he attempted to restart the engine, but the starter only cranked the engine briefly, and the propeller travelled one full revolution and then stopped. After another unsuccessful engine start attempt and with the glider too far away to glide back to the airport, the pilot located a field for a forced landing. During the landing roll, the main landing gear wheels dug into the soft field, and the powered-glider nosed over and then came to rest inverted, which resulted in substantial damage to the engine mount truss. Postaccident examination of the powered-glider revealed that the single battery was depleted to a level that was insufficient to power the starter during the engine restart sequence. The pilot reported that there were no preaccident malfunctions or failures with the airframe or engine that would have precluded normal operation.