UNAUTH PENETRATION OF CLASS C AIRSPACE.

Narrative

DUE TO ALTERNATOR FAILURE (AGAIN) WAS TRYING TO MINIMIZE RADIO AND ELECTRICAL USAGE AND FLY BY PILOTAGE ONLY SO AS TO HAVE PWR AVAILABLE TO LAND AT ROCKFORD; IL; AND TALK TO TWR THERE. I WAS GOING TO SKIRT THE DES MOINES CLASS C SPACE ENTIRELY TO THE N BUT HVY RAIN SHOWERS THERE CAUSED ME TO TAKE A RTE TO THE S. OTHER SHOWERS FORCED ME TO GO TO THE VOR (AND TURN ON THE NAV/COM) AND STAY BELOW OUTER SPACE FLOOR OF 2200 FT. I WAS STAYING AT 2000-2100 FT WHILE IN THE AREA; BUT AN UPDRAFT LIFTED THE ACFT TO 2150 FT. SHORTLY AFTER THIS IT OCCURRED TO ME THAT SINCE I HAD THE RADIO ON ANYWAY; I SHOULD GET THE ALTIMETER FROM ATIS. THE ALTIMETER SETTING WAS RPTED AS 30.12 WHILE THE SETTING I HAD BEEN USING WAS 29.98 OR 30.00. THUS I HAD BEEN AT APPROX 2300 FT WHEN I THOUGHT I WAS STILL UNDER THE AIRSPACE FLOOR. CONTRIBUTING FACTORS: TRYING TO GET BY WITHOUT USING ATC NOT GETTING ALTIMETER SETTING BEFORE NEGOTIATING CLASS 'C' SPACE GET HOMEITIS. LESSON - - EVEN WITHOUT CHARGING WORKING; THE BATTERY OPERATED THE RADIO FOR A LONG TIME BEFORE IT WAS SERIOUSLY LOW. USE IT TO COM AND AVOID POTENTIAL PROBS.