Agreed there’s a ‘low volts’ light on the annunciator panel, but that’s not what got my attention one summer morning climbing out of El Paso, Texas in a 172 ‘R’ model. What I did note was the OAT which was 27c even at this level. Even more puzzling, it was slowly decaying.
it was a minute or two before I realised I was reading battery voltage, not OAT! Maybe about then the low volt light came on. A quick 180 and a call to El Paso and time to investigate, I tried cycling the master. Bingo! Power restored. Back on course I exchanged emails with home base (a wonderful innovation back then) I was told it was nothing unusual and ok to continue. Certainly it didn’t happen again in the next 40 Hrs of my round trip to Florida.
When something similar happened in my PA28 in Sweden, the UK CAA mandated low volt light came on immediately. I diverted to Bromma and on final bingo! The alternator came back on, no amount of probing and fiddling on the ground revealed anything and after making suitable preparations continued to Helsinki expecting the power to die at any moment. It didn’t, and a friend came out with an alternator in his hand luggage (!). No more incidents for 30 Hrs or so, and then same again. Regulator, over voltage trip, etc changed out and another 30 Hrs once again the baleful glare of the LV light. This time, instead of just pressing on the alternator field breaker, for some reason I flicked it with my fingernail.
I still have that breaker, on my desk as I write this.