Cessna 310/T310C
Flight Crew Commercial; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 4; Flight Crew Total 1619; Flight Crew Type 183
Troubleshooting
Aircraft Equipment Problem Less Severe
Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem
Aircraft
We were in a C-310 conducting BFR training flight approximately 15 nm NE of ZZZ at 6;500 MSL practicing Single Engine maneuvers on a northeasterly heading when I noticed an unusual charge rate of 28 amps.  At this point we divided duties and as the switches were in my position I tried to troubleshoot the fault and the CFI flew the airplane. Shutting down the left and right generators separately and together and reset the battery master switch in different combinations to eliminate the charge did not stop the charge and all electrical power was soon lost.We lowered the gear and visually confirmed they were down but could not confirm they were locked with no electrical power.  Since there was the possibility of other issues we decided to divert to ZZZ as they have a longer runway. We called via cell our home airport manager requesting they call ZZZ Tower and advise we were inbound with no radio and to provide our cell number. ZZZ tower called us and advised to continue for a low pass to confirm the gear was down and return to land. Tower confirmed the gear was down; gave us a green light and we landed on Runway XX and off at the taxiway to parking.Contacted the Mechanic who determined it was a failed left starter solenoid/left starter. It's with an abundance of caution I make this report as I do not feel we were in a serious situation or incident and believe we managed the systems problem successfully while communicating with ZZZ Tower.Recommendation: I've noticed other C-310s having similar solenoid problems resulting in electrical power loss due either to the starter solenoid or starter switch. This might be worth determining whether this is systemic and advise other operators.
The reporter stated they were in flight for about 30 minutes when they noticed an unusual charge rate of 28 amps (normal is about 14 amps). Attempting to isolate systems failed and they were unable to resolve the issue. They lost all avionics; flap control; and had to manually lower the landing gear. After landing; reporter had a Mechanic checkout the aircraft's electrical system and he found the starter solenoid contacts were 'welded' together; thus shorting out the battery. The reporter stated experiencing a similar incident in another Cessna 310 where a starter switch caused a similar problem.
C310 pilot reported the electrical system had an unusual charge rate. Shortly thereafter all electrical power was lost.