A COMMERCIAL PLT FLYING A PIPER AZTEC TO TPA; WHO WAS EXPERIENCING ELECTRICAL PROBS; TURNED DOWN ASSISTANCE OFFERED FROM ATC AND ACTUALLY BELIEVED THAT HIS BATTERY WOULD LAST THE ADVERTISED 30 MINS. HE WENT LOST COM AFTER BEING TURNED OVER TO TPA TWR. THE RPTR BROKE OFF HIS APCH; AND LANDED AT X16; A NEARBY UNCTLED FIELD.

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ON AUG/THU/99; I WAS SCHEDULED TO MAKE A FLT FROM FORT LAUDERDALE EXECUTIVE ARPT TO TAMPA BAY INTL ARPT IN A PIPER AZTEC; REGISTRATION NUMBER 123. I STARTED THE ACFT AT ABOUT XA35 HRS AND TAXIED THE ACFT TO THE RWY AND DID A NORMAL RUN-UP. THE ACFT AMMETER SHOWED THAT THE ELECTRICAL SYS WAS CHARGING ADEQUATELY. I DEPARTED FT LAUDERDALE EXECUTIVE ARPT AT ABOUT XA50 HRS. THE FLT PROCEEDED NORMALLY UNTIL ABOUT THE TIME I REACHED BRIDGE INTXN SE OF TAMPA BAY. AS I APCHED BRIDGE INTXN I NOTICED THAT THE INTERIOR LIGHTS WERE BECOMING DIMMER AND THE DISPLAY ON MY ADF WAS BEGINNING TO BLINK. I SUSPECTED THAT I HAD ELECTRICAL PROBS AND I CHKED THE AMMETER. THE BATTERY WAS SHOWING A SLIGHT DISCHARGE AND THE AMMETERS WERE READING ZERO. I TURNED OFF ALL UNNECESSARY ELECTRICAL EQUIP AND I ADVISED THE APCH CTLR THAT I WAS HAVING ELECTRICAL PROBS AND THAT IT WAS POSSIBLE THAT I MIGHT LOSE RADIO COM. THE CTLR ASKED IF I NEEDED ASSISTANCE. I TOLD HIM THAT I DID NOT NEED ASSISTANCE AT THE TIME AND I EXPECTED MY BATTERY PWR TO LAST UNTIL I REACHED MY DEST. THE CTRL GAVE ME A DIRECT VECTOR TO TAMPA INTL TO SAVE TIME. SHORTLY THEREAFTER; BOTH OF MY CDI'S DISPLAYED NAV FLAGS AND I TURNED THEM OFF. ON MY LAST CONTACT WITH ATC; THE CTLR TOLD ME TO SWITCH TO TWR CTL FREQ. I SWITCHED FREQ AND CALLED THE TWR BUT COULD NOT UNDERSTAND THE REPLY. SEVERAL MINS LATER THE RADIOS WENT OUT COMPLETELY. HAVING RECEIVED NO LIGHT GUN SIGNAL FROM THE CTL TWR; AND BECAUSE I WAS IN VFR CONDITIONS; I DECIDED TO DISCONTINUE MY APCH TO TAMPA INTL. I INSTEAD DECIDED TO LAND AT NEARBY VANDENBURG ARPT; AN UNCTLED FIELD. I LANDED WITHOUT INCIDENT AND ADVISED FLT SVC OF MY SIT. BECAUSE OF THE ELECTRICAL FAILURE; I WAS FORCED TO DEVIATE FROM MY IFR CLRNC.