What was found
DURING PREFLIGHT, THERE WAS STRONG BURNING SMELL IN COCKPIT. TROUBLESHOT, NR 3 ACM MAKING A NOISE, DEFERRED. DURING CRUISE AFTER 2.5 HOURS OF FLIGHT, BURNING ODOR RETURNED IN COCKPIT. AFTER SEARCHING FOR SOURCE IT WAS NOTICED NR 1 GEN KVAR/KW METER WAS READING -10KW+/-15KVR. ELECTED TO TURN GEN OFF BY TRIPPING FIELD, ODOR DISSIPATED AND DID NOT RETURN. ALL CSD LIGHTS ON MAIN GEN ANNUNCIATOR PANEL WERE ON . AFTER 6 HRS AND 43 MINUTES THERE WAS A LOUD SQUEALING SOUND UNDER F/E PANEL AND STRONG ODOR AGAIN. BATTERY VOLTAGE DROPPED 30V-25V. PULLED C/B FOR BATTERY CHARGER. C/A: BAD SMELL COMING FROM OVERHEATED BATTERY. BATTERY SMOLDERING DURING CHARGE. REMOVED AND REPLACED MAIN BATTERY, OPS CHECKED GOOD.