Narrative:

We were in a C-310 conducting BFR training flight approximately 15 nm northeast 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.

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Original NASA ASRS Text

Title: C310 pilot reported the electrical system had an unusual charge rate. Shortly thereafter all electrical power was lost.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.