C210 pilot reported an alternator failure occurred during a test flight that led to lost communications. Some of the communications returned but out of caution the landing gear was manually retracted for landing.

Narrative

I was asked to observe the operation of the aircraft. The owner/pilot thought that the aircraft was not performing to expectation. He thought that the main landing gear may be drooping below the aircraft - a relatively common occurrence in Cessna 210 aircraft. He agreed to have a person on the ground observe the aircraft and report if any gear legs were protruding. We departed the airport and remained in closed traffic for a low pass with the landing gear up.Upon completion of the low pass; we remained in closed traffic for landing. Upon selecting the landing gear down; all electric power was lost. Apparently; the primary alternator failed. The standby alternator was incapable of providing sufficient power so automatic load shedding took place. All communication was lost.Once the pilot reduced the load on the standby alternator; some communication capability was restored. However; out of an abundance of caution; the pilot decided to lower the landing gear with the emergency pump. After completing the required number of pump cycles; the landing gear down light illuminated. The Tower was contacted and a normal landing followed. The aircraft was taxied to the parking area and turned over to Maintenance. After landing and in consultation with the owner/pilot; I learned that the aircraft had intermittent electrical issues since the installation of new avionics. Had I known; I probably would have declined to take the flight.