PLT OF AN SMA SEL ACFT WANDERED OFF ASSIGNED ALT WHILE IN A TCA DUE TO A DISTR OF AN ACFT ELECTRICAL SYS PROB.

Narrative

I WAS CRUISING (VFR) WITHIN THE BOS TCA WITH AN APPROPRIATE TCA CLRNC AT 4500 FT MSL. I RECEIVED INSTRUCTIONS TO 'RESUME OWN NAV' (STILL WITHIN THE TCA WITH CLRNC) TO MY DEST OF NASHUA; NH (ASH). IN THE VICINITY OF BEDFORD AND LOWELL; MA; APPROX 18 NM FROM BOS ARPT; I ENCOUNTERED A PROB WITH MY ACFT ELECTRICAL SYS AS INDICATED TO ME BY THE ACFT VOLTAGE MONITOR. THE VOLTAGE MONITOR SUDDENLY EMITTED AN AUDIBLE AND VISIBLE ALARM THAT INDICATED THE ACFT ELECTRICAL CHARGING SYS (ALTERNATOR) WAS FAILING. I IMMEDIATELY BEGAN TO ADDRESS THE PROB; FOLLOWING THE PLT OPERATING HANDBOOK PROCS FOR HANDLING IMMINENT ELECTRICAL SYS FAILURE. DURING MY HANDLING OF THIS PROB; I ALLOWED (UNINTENTIONALLY) THE ACFT TO DRIFT IN ALT FROM MY ASSIGNED 4500 FT MSL TO APPROX 4900 FT MSL WHILE STILL IN THE BOS TCA (STILL WITH TCA CLRNC). I CORRECTED BACK TO 4500 FT IMMEDIATELY. ALSO DURING THE HANDLING OF THE PROB; I HEARD THE BOS APCH CTLR ADVISE ME TO CONTACT MANCHESTER (MHT) APCH CTL FOR FURTHER VFR ADVISORIES. I DID NOT ACKNOWLEDGE THE BOS CTLR; BECAUSE I WAS STILL HANDLING THE ELECTRICAL SYS PROB. AFTER HANDLING THE PROB (I ULTIMATELY HAD TO SHUT DOWN THE ALTERNATOR AND REDUCE ALL ELECTRICAL LOAD TO ESSENTIALS ONLY); SEVERAL MINS HAD PASSED AND I HAD EXITED THE BOS TCA. I DID NOT RECONTACT BOS APCH; AS I WAS THEN WELL OUTSIDE THE TCA AND I WAS STILL MONITORING CLOSELY THE ELECTRICAL SYS FOR FURTHER SIGNS OF TROUBLE. I LANDED AT NASHUA WITHOUT INCIDENT. FOR ME; ONE LESSON LEARNED WAS TO INCLUDE A MORE REGULAR SCAN OF THE INST PANEL AND FOR TFC; EVEN WHILE ADDRESSING A COCKPIT EMER OR NEAR EMER. DOING SO WILL PREVENT UNINTENDED ACFT MOVEMENT.