ACFT EQUIP PROB CRITICAL. ALTDEV EXCURSION FROM ASSIGNED.

Narrative

I WAS IMC IN MODERATE RAIN AT 9000 FT (FREEZING LEVEL WAS 9000-10000 FT ICE FORECAST ABOVE 12000 FT. DURING WHICH TIME; I WAS ASKING CTR FOR ADVISORIES AROUND THE HVY TSTMS AHEAD AND AT THE SAME TIME USING STORMSCOPE TO AVOID MOST OF IT. I WAS HAVING TROUBLE MAINTAINING ALT AND TOLD CTR. ALSO I INFORMED THEM I WOULD LIKE TO DEV AS NECESSARY WHICH WAS OKAY WITH THEM. I ONLY BUSTED MY ASSIGNED ALT FOR NOT MORE THAN 2-3 MINS PLUS OR MINUS 400 FT ON 2 OCCASIONS. ON THE OTHER RADIO I HAD FSS/HOU). TFC IT SEEMS; WAS TO BACKTRACK; FLY 50 MI E; THEN BACK S. (JCT-SAT). AFTER ADVISING CTR OF THIS; THEY ASKED ME TO DSND TO 8000 FT BECAUSE OF MY DIRECTION OF FLT; WHICH I PROCEEDED TO DO UNTIL MY ALTERNATOR FAILED AT WHICH TIME CONSIDERING: A) I NEEDED THE PITOT HEAT ON; B) I NEEDED NAV INFO; C) THE BATTERY WOULD DISCHARGE VERY RAPIDLY: I DECLARED AN EMER AND WAS GIVEN RADAR VECTORS BACK TO JUNCTION WHERE I HAD AN UNEVENTFUL LNDG. I DISCOVERED THE PROB BECAUSE MY ACFT IS EQUIPPED WITH A LOW VOLTAGE LIGHT. HUMAN PERFORMANCE CONSIDERATIONS. A) I HAD JUST FLOWN THAT SAME RTE 1 HR AND 45 MIN EARLIER AND I KNEW WHERE THE HVY WX WAS. B) I WAS DELAYED 1 HR AT JUNCTION TO AWAIT THE ARR OF ANOTHER ACFT WITH DETERIORATING WX. CONTRIBUTING FACTORS: POSSIBILITY OF ICE; HVY RAIN (WHICH I LATER FOUND OUT CAUSED THE PROB) AND TURB.