Air taxi Captain reported an alternator malfunction during cruise flight at night. The flight diverted to an alternate airport and landed.

Narrative

On Day 0 we were returning to ZZZ1 from ZZZ2 after dropping two passengers at about XA:00. Our departure for ZZZ1 was delayed until about XC:00 awaiting the termination of a TFR at ZZZ1. About10 minutes after reaching our cruise altitude of 8;000 the left Alternator Fail light came on. We attempted to reset it per the checklist; but it did not reset. We then turned off the left side of the master switch per the checklist. The voltage indicator was still showing 28 volts; the normal output for each alternator. We reduced the electrical load as much as possible and the amp meter for the right alternator was showing about 22-24 amps (well below the rated amperage of 70 for each alternator); The left alternator was still showing 2-3 amps; even though it had been shut down.I contacted maintenance and he said that as long as we continued to get 24 volts or more on the meter we should be able to continue to ZZZ1 where it could be repaired the next day. As we approached the ZZZ; the voltage began to slowly waiver; dropping a few tenths and then recovering some of the loss; but with a net loss over time. It was reading about 25 volts as we passed over ZZZ. The voltage continued slowly dropping with occasional increases. I again contacted maintenance and he agreed with our decision to return to ZZZ. We contacted ZZZ Center and began a descent and return to ZZZ. A few minutes later the voltage began to drop more quickly; dipping below 24 volts. It then began to decrease at a faster rate and we requested priority handling. As it dropped below about 20 volts; the right Alternator Fail light illuminated. We were below the cloud deck by then; at about 4500 - 5000' MSL I believe; and could see ZZZ. We were cleared for a visual approach to runway XX. We experienced a brief loss of radio contact with ZZZ approach control. We regained contact and turned on the additional power switch to ensure continued radio communication. After landing; the tower controller asked if we needed any assistance and I replied that we did not and was given clearance to taxi to the FBO. After shutting down on the ramp we met with a member of the ground crew and answered his questions and thanked him for their response.