SMA PLT HAS ALTERNATOR FAILURE ON IFR FLT PLAN IN IMC WX. DISORIENTED; HE DSNDS BELOW ASSIGNED. BECOMES VISUAL; CANCELS IFR; LANDS.

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APPROX 40 MI OUTSIDE HUNTSVILLE; AL; CTR STATED THEY HAD LOST MY TRANSPONDER. TURNED ME OVER TO HUNTSVILLE APCH; WHO CLRED ME DOWN TO 4000. THEY ALSO WERE NOT PICKING UP MY TRANSPONDER. I THEN NOTICED MY ALTERNATOR NEEDLE INDICATED A DISCHARGE CONDITION. I TRIED TO RESET THE ALTERNATOR BY TURNING THE MASTER SWITCH OFF AND ON WITH NO RESULTS. I THEN NOTIFIED HUNTSVILLE APCH AND WOULD LIKE AN EXPEDITED APCH DUE TO LOSS OF ALTERNATOR. THEY CLRED ME FOR THE VOR APCH AND VECTORED ME TOWARD THE APCH. I SHUT DOWN THE TRANSPONDER; 1 RADIO; AND DME TO SAVE PWR AND THEN DISCOVERED THE ALTERNATOR MUST HAVE SHORTED AS IT NOW INDICATED A MAJOR DISCHARGE AND THE REMAINING RADIO WENT DEAD. I WAS AT 3000 FT AT THIS TIME. IN TRYING TO DEAL WITH ACFT SYS; I HAD LOST TRACK OF WHERE I WAS AND HAD LET HDG GET OFF. I SAW SOME LIGHT SPOTS BELOW SO I DROPPED DOWN TO 1500 WHICH WAS BELOW ASSIGNED ALT OF HUNTSVILLE APCH. AFTER DSNDING; I TURNED OFF THE ALTERNATOR AND REGAINED USE OF MY #1 RADIO OFF BATTERY ONLY. I CALLED HUNTSVILLE APCH ASKED WHERE I WAS. THEY DIRECTED ME TOWARD THE HUNTSVILLE ARPT BUT AT THAT POINT I SAW THE DECATUR ARPT BELOW ME SO I CANCELLED IFR AND LANDED AT DECATUR. BATTERY WAS DOWN TO THE POINT I HAD NO FLAPS AND HAD TO MANUALLY PUMP DOWN THE GEAR. FUTURE THOUGHTS -- I HAD A PORTABLE RADIO WHICH HAD A DEAD BATTERY (PART OF MY DISTR). NEXT TIME -- MONITOR HDGS AND ALTS WHILE TIMING THE APCH. MY DME WAS WORKING UNTIL ALTERNATOR DISCHARGE. IT WOULD HAVE GIVEN ME A BETTER IDEA OF HOW LONG TO HOLD ALT BEFORE DSNDING. MAKE SURE EQUIP IS CHARGED AND TAKE AN ALKALINE BATTERY PACK. MONITOR SYS BETTER. I HAD APCH PLATES IN A HOLDER WHICH OBSCURED THE ALTERNATOR GAUGE.