On July 23, 2004, about 2003 Pacific daylight time, a Cessna 172RG, N5167R, was substantially damaged during landing rollout following the partial collapse of its main landing gear at the Montgomery Field, San Diego, California. The private pilot was not injured. The airplane was operated by San Diego Flight Training International, Inc., San Diego. Visual meteorological conditions prevailed at the time of the private pilot's solo instructional flight. An instrument flight plan was filed. The flight was performed under the provisions of 14 CFR Part 91, and it originated from San Diego about 1910. The pilot reported to the National Transportation Safety Board investigator that the purpose of the flight was to practice flying in furtherance of attaining a commercial pilot certificate. No anomalies were observed when the flight originated. Eventually, however, the pilot observed that the airplane's alternator was registering a discharge condition, and his radio communications became problematic. In the pilot's verbal and subsequent written statement he reported, in pertinent part, that approaching the Montgomery Field he cycled the airplane's master (electrical system) switch in accordance with instructions in the Cessna Pilot's Operator's Handbook. Because this did not completely restore electric power, he reduced the electric load to conserve energy. The pilot asserted that he lowered the landing gear and observed the corresponding green "gear down and locked" light illuminate. During the approach, no landing gear warning horn was heard. The pilot additionally indicated that he landed on runway 28R without use of the landing light. The touchdown was normal. During rollout the right main landing gear collapsed, directional control was lost, and the right wing struck the surface of the runway. The airplane veered right off the runway, and it came to rest in an adjacent open dirt field. At the direction of the Safety Board investigator, a Federal Aviation Administration (FAA) certificated airframe and powerplant mechanic with inspection authority examined the airplane. The mechanic reported that the airplane's battery power was low. An external power supply was connected to the airplane, and the landing gear was retracted and extended. The landing gear was retracted again, during which the aural warnings interconnected with the throttle position were checked. No anomalies were noted. The landing gear warning horn was functional and it operated properly. The emergency landing gear extension system was checked and was also found to be functional. No evidence of any mechanical malfunction was noted during the tests. The mechanic further reported that the airplane's battery was charged, and the airplane's engine was started. Thereafter, the ammeter showed a discharge and the low voltage light on the panel illuminated. The alternator circuit breaker was found popped. The breaker was reset and the alternator came on line, the low voltage light extinguished, and a positive charge was noted. All accessories and lights were turned on, and the alternator circuit breaker remained set. The FAA coordinator opined to the Safety Board investigator that during the flight the pilot had failed to properly perform procedures relating to ensuring that the alternator circuit breaker was engaged. Also, the pilot failed to ensure that the landing gear was fully extended by using the emergency gear extension system.