Had something similar several years ago flying a C172 from Perth to Aberdeen at night, not long after take-off the low volt light came on and no amount of re-cycling / load shedding would put it out.
Fortunately only a short trip and there was enough juice left to power the flaps etc. Seem to remember it was caused by one or two of the alternator phases packing up hence the 'pulsing' of the light.