Cessna 310 Flight Instructor reported loss of electrical system during cruise. Flight diverted and landed uneventfully.

Narrative

Enroute on a time building/training flight with a student from ZZZ2 to ZZZ1 I began to see a loss of buss voltage displayed on the AV30 flight instrument. I cycled the alternator switches but no change at all on the amp gauge or buss voltage. I needed to shed the load to conserve electrical power. We were cruising over an overcast ceiling and night conditions were approaching. The threat management I undertook was to try to have enough electrical power to extend the landing gear; have a landing light; and extend flaps when we reached ZZZ1. All these system are powered electrically. I continued the flight to the destination because a precautionary landing was not an option with the solid overcast below; I needed to remain VFR. Weather forecasts predicted no ceiling near the destination and Foreflight weather from the Stratus showed the same on METAR and TAF. So I turned off all non essential equipment. Specifically; the Number 2 NavCom; taxi and landing lights; strobe lights; autopilot; cockpit lighting. I grabbed my flashlight. Volts were still dropping; just slightly slower. I needed to shed more load. So I confirmed I was not in airspace that required a transponder or two way communication. Before turning off that equipment I terminated flight following with ATC. I did not report any problem to the controller because I was not in an urgent or emergency. Also I would have no comms and they would not have me on radar. So near ZZZZZ intersection I turned off Number 1 NavCom and transponder. Only the beacon and nav lights remained on because the beacon (anticollision light) is required at all times and the navigation (position) lights are required at sunset; which was now upon us. As the flight continued the electrical power continued to decrease and I could see I had no navigation lights or landing light and about 45 minutes of flight to the destination remained so there was no point in leaving that equipment on. Hoping it would save just a pit more volts I turned off all remaining electrical devise switches. I flew a route to the west of the ZZZ3 Mode C veil and stayed to the south and east of the ZZZ4 Class C. In other words I ensured that I maintained a course that would not put me in airspace requiring a transponder or two way communication with ATC. As expected; the overcast layer we had been cruising over since the area was behind us and the conditions were now CAVU.When I reached ZZZ1 I attempted to extend the landing gear but nothing. I then manually extended the gear with the hand crank. That worked uneventfully. I attempted to extend flaps but nothing. I turned on the landing light switch but nothing. I could see a very faint gear down light but even with a flashlight I could not confirm gear down in the mirror on the left engine cowling. I called staff on my cell and asked them to observe my gear position during a low pass. They confirmed it appeared gear was down.I decided landing at ZZZ1 with no flaps and no landing light was not the best option so we diverted to ZZZ with its longer and wider runway. I turned the comm radio on to see if there was enough electrical power to turn on the pilot controlled lighting. That was unsuccessful so I turned the radio back off. I circled the field thinking someone may turn on the lights. We also had an aircraft inbound to pick us up that we could talk to on the cell phone so they would turn on the runway lights upon arrival in a few minutes.After a few circles overhead the field the runway lights came on. I executed a good no landing light and no flap landing. We taxied to the ramp uneventfully. I determined the next day when I examined the engine compartment that the alternator belt on each engine failed.I think I handled the flight well but upon reflection I may have done a few things differently. 1. Before terminating flight following advise the controller of my situation so he could have at least alerted controllers and pilots along my route. 2. Try to make use of my handheld navcom in my flight bag that I had not used in years so I forgot it was there. 3. Attempt more load shedding sooner. I kept the anticollision light and position lights on until they failed on there own due to lack of electrical power because I did not want to violate rules regarding lights. But my threat management was to try to preserve power for gear; landing light and flaps so that should have taken priority. It probably did not matter because even though I left the required light switches on it appeared out the window they were not illuminating. 4. Observe the decreasing volts and amps sooner so as to take action sooner.