BECOMING AWARE HE IS LOSING HIS ELECTRICAL SYS A C172 PLT DECLARES AN EMER BEFORE COMPLETE LOSS. DSCNT IS MADE TO VFR CONDITIONS BASED ON FAMILIARITY WITH THE AREA IN SPITE OF BROKEN CLOUDS BEING THE RESTR TO VISIBILITY.

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WAS BEING VECTORED FOR THE VOR RWY 36 APCH IN LA CROSSE; WHEN MY ELECTRICAL SYS FAILED. I WAS AT 5000 FT IN IMC CONDITIONS. I WAS ABLE TO DECLARE AN EMER WITH CTR BEFORE I LOST COM WITH THEM. LA CROSSE WX WAS APPROX 1500 FT BROKEN; 2900 FT OVCST. AFTER REVIEWING MY CHARTS AND WITH LAST KNOWN POS AND KNOWLEDGE OF THE AREA; I DECIDED TO DSND TO BELOW THE OVCST LAYER. MY PLAN WAS TO SEE ENOUGH OF THE TERRAIN THROUGH THE BROKEN LAYER OF CLOUDS AND THEN DSND THROUGH THAT LAYER TO VMC CONDITIONS. I WAS ABLE TO DO THIS AND FLY VISUALLY TO THE ARPT AND LAND. I BELIEVE THE VOLTAGE REGULATOR FAILED AT SOME POINT DURING MY 2 HR FLT; WHICH PREVENTED THE ALTERNATOR FROM CHARGING THE BATTERY. I WAS JUST ON TOP OF A CLOUD DECK AT 9000 FT MSL. IT WAS EXTREMELY BRIGHT AND WITH SUN GLASSES ON; COULD HAVE CAUSED ME TO MISS THE LOW VOLTAGE LIGHT. THIS IN CONJUNCTION WITH A VERY SLIGHT DISCHARGE RATE ON THE AMMETER CAUSED ME TO NOT BE AWARE OF THE PROB UNTIL MY BATTERY WAS ALMOST COMPLETELY DISCHARGED. I PLAN TO INSTALL A REDUNDANT ELECTRICAL SUPPLY; AND CARRY A BATTERY OPERATED PORTABLE GPS SYS WITH ME IN THE FUTURE.