PA28 pilot reported Alternator failure and diversion to a nearby airport.

Narrative

General Aviation flight; light aircraft; flying IFR above the clouds; setting up for initial approach into ZZZ; planning on ILSXX into landing. While in contact with ZZZ [Approach] while adjusting lean mixture the pilot noticed indications on the onboard EGT of low voltage from the Alternator; now the aircraft was using battery power; pilot initiated appropriate checklist for reset of alternator system. A few minutes passed after reset; with no change to the battery warning light present in the onboard EGT. Pilot initiated turning off unnecessary electrical systems to preserve battery power if the situation didn't resolve from a second; this time full reset of electrical systems. While turning off systems for another reset; ZZZ advised my mode C temporarily disappeared; they had just reacquired it after vocalizing - pilot looked at onboard EGT and noticed a system flicker indicating imminent failure of that system. Not wanting to take any chances with the Mode C in/around clouds; instead of performing a second aerial reset; pilot advised ZZZ of intentions to land at nearest airport and perform operation on ground.ZZZ gave pilot direction and vectors towards nearest airport - ZZZ1 - and advised they'd be [requesting assistance] for the aircraft. ZZZ handed us off to tower which provided immediate landing and taxi to a nearby FBO to perform the ground reset. No injuries - No Damage caused/received. Cause unknown. System began draining from battery in flight. Didn't take initial reset and required full electrical system reset (avionics and master). Aircraft was airworthy upon that reset and the alternator returned to properly charging the system.