The pilot intended to depart his home airport for another airport to perform an owner-assisted annual inspection. While loading the airplane with supplies, the pilot placed a portable ADS-B receiver on the top of his airplane. The ADS-B receiver utilized a commercially available portable lithium-ion battery pack for power, and the battery was attached to the receiver via a hook and loop fastening. The receiver and battery pack fell from the top of the airplane onto the ramp which resulted in damage to the receiver and battery pack. The pilot attempted to power up the receiver but noted that the receiver was no longer working; he then placed the receiver and battery pack behind the front seats and underneath an airplane cover. The pilot subsequently taxied and began his runup of the engine, during which the pilot noted smoke coming from behind the front seats. The pilot lifted the airplane cover and saw that a fire had started. The pilot exited the airplane and unsuccessfully attempted to remove the airplane cover, which was on fire. The fire eventually consumed most of the cockpit area (including the battery pack itself) and substantially damaged both wings. Based on this information, it is likely that the portable lithium-ion battery pack sustained damage to at least one of the battery cells when it was dropped onto the ramp, resulting in thermal runaway of the battery pack and subsequent fire.