C182 PLT HAS AN ALTDEV DURING A GPS APCH TO DYL AFTER AN ALTERNATOR FAILURE.

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IFR FLT FROM ASH TO DYL. IMC THE ENTIRE TRIP WITH LIGHT TO MODERATE RAIN AND OCCASIONAL LIGHT TURB. 40 KT HEADWINDS RESULTING IN 3 HRS OF INST FLYING WITHOUT AN AUTOPLT. IN THE VICINITY OF RBV; I WAS HANDED OFF FROM MCGUIRE APCH TO PHL APCH AT 4000 FT. UPON MAKING CONTACT WITH PHL; I REQUESTED AND RECEIVED RADAR VECTORS TO THE FINAL APCH COURSE FOR THE RNAV RWY 23 AT DYL. I WAS ALSO GIVEN A DSCNT TO 3000 FT. DURING THE DSCNT; I NOTICED MY JPI ENG ANALYZER FLASHING A BATTERY WARNING. A QUICK LOOK AT THE AMMETER CONFIRMED THAT THE ALTERNATOR WAS OFF-LINE AND THAT I HAD ONLY THE SHIP'S BATTERY FOR ELECTRICAL PWR. I NOTIFIED THE CTLR OF THE SITUATION AND ASKED FOR EXPEDITED HANDLING FOR THE APCH. HE ASKED IF I NEEDED ASSISTANCE ON THE GND AT DYL AND I REPLIED IN THE NEGATIVE. AS THE DSCNT CONTINUED; I BEGAN SHEDDING ELECTRICAL LOAD AND DIGGING OUT THE EMER PROCS CHKLIST. IN THE PROCESS; I ISSUED THE LEVELOFF AT 3000 FT AND CONTINUED TO DSND. I CAUGHT THE ERROR AT 2700 FT AND ARRESTED THE DSCNT AT 2600 FT. I QUICKLY ZOOM-CLBED THE ACFT BACK TO 3000 FT WHILE SIMULTANEOUSLY CONFIRMING WITH THE CTLR THAT MY ASSIGNED ALT SHOULD BE 3000 FT. THE CTLR DID NOT MENTION THE DEV BUT DID ASK IF I'D LIKE TO DIVERT TO TRENTON (TTN). BECAUSE I ALREADY HAD THE WX AT DYL AND HAD THE APCH LOADED INTO THE GPS; I FELT THAT THE DIVERSION WAS NOT NECESSARY. I WAS WORKING THE EMER PROCS CHKLIST AND DID NOT NEED THE ADDITIONAL WORKLOAD INVOLVED IN CHANGING AND RE-BRIEFING THE APCH. ASIDE FROM THAT; THE TIME SAVINGS WOULD NOT HAVE BEEN THAT SIGNIFICANT. AFTER COMPLETING THE LOAD-SHED; I HAD THE GPS/COM; THE XPONDER; THE JPI; AND THE TURN AND BANK INDICATOR STILL RUNNING. THE AMMETER SHOWED MINIMAL DISCHARGE AND THE BATTERY VOLTAGE WAS JUST UNDER 12 VDC. I THEN CYCLED THE ALTERNATOR SIDE OF THE MASTER SWITCH IN AN ATTEMPT TO GET THE ALTERNATOR BACK ON-LINE. THIS SUCCEEDED AND I SO INFORMED THE CTLR. I THEN JOINED THE FINAL APCH COURSE AND WAS HANDED OFF TO THE CTAF FOR LCL ADVISORIES. DURING THE APCH; THE ALTERNATOR ONCE AGAIN TRIPPED OFF-LINE AND I SHUT IT DOWN FOR GOOD. I BROKE OUT ABOUT 400 FT ABOVE APCH MINIMUMS AT DYL AND LANDED UNEVENTFULLY THEREAFTER. MECHS AT DYL LATER DIAGNOSED THE PROB AS A BAD ALTERNATOR GND AND REPAIRED IT RATHER EASILY.