A B737-400 RPTED A 'BURNING PLASTIC' ODOR IN THE CABIN AND COCKPIT. AFTER QRH PROC; ODOR DISSIPATED. MAINT FOUND MAIN SHIP'S BATTERY VOLTAGE LOW. BATTERY FOUND VERY WARM AND SMELLED HOT. BATTERY REPLACED.

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UPON ARRIVAL AT THE GATE; THE CREW WROTE IN THE LOG THAT THEY HAD SMELLED A 'BURNING PLASTIC' ODOR IN THE CABIN AND COCKPIT AT THE TOP OF DSCNT AND THAT THE ODOR DISSIPATED RAPIDLY AFTER THEY ACCOMPLISHED THE QRH PROC. DURING TROUBLESHOOTING; IT WAS DISCOVERED THAT THE ACFT MAIN BATTERY VOLTAGE WAS LOW; THE BATTERY WAS VERY WARM; AND THAT THE BATTERY SMELLED HOT. NO EVIDENCE OF LEAKAGE WAS APPARENT DURING THE BATTERY CHANGE (BATTERY WAS DRY); BUT WHILE PROCESSING THE FAILED BATTERY FOR SHIPPING THE NEXT DAY; DRIED RESIDUE WAS NOTICED ON THE BACK SIDE OF THE BATTERY SUGGESTING THAT THE BATTERY MAY HAVE LEAKED WHILE INSTALLED IN THE ACFT. CONTACTED MAINT CTL AND SUGGESTED THAT THE AREA BE INSPECTED FOR CONTAMINATION AS SOON AS POSSIBLE.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT WHEN THE AIRPLANE ARRIVED IN ADDITION TO THE LOGBOOK ODOR RPT THE CREW RPTED THE APU WOULD NOT START AFTER LNDG. THE MAIN BATTERY VOLTAGE WAS CHKED AND FOUND TO BE 19 VOLTS. ON INSPECTION OF THE MAIN BATTERY IT WAS FOUND TO BE VERY WARM AND SMELLING HOT. THE BATTERY WAS REPLACED PER THE MAINT MANUAL AND CHKED OK AND STARTED THE APU. WHEN SHIPPING THE REMOVED BATTERY; RESIDUE WAS NOTED ON THE BACK SIDE OF THE BATTERY INDICATING BATTERY FLUID LEAKAGE HAD OCCURRED. MAINT CTL WAS ADVISED OF THE FINDING OF LEAKAGE AND MAY REQUIRE THE ACFT BATTERY SHELF BE CLEANED. THE BATTERY WAS OF THE NICKEL CADMIUM TYPE. THE BATTERY CHARGER WAS NOT REPLACED DUE TO NOT BEING IN STOCK.