What was found
WHILE PERFORMING PERIODIC INSPECTION, FOUND MAIN BATTERIES HAD CASE DAMAGE CAUSED BY OVERHEATING. ALSO, ELECTROLYTE MOSTLY BOILED OUT. AIRCRAFT ELECTRICAL SYSTEM TESTED OK. BATTERIES SERVICED AND CHECKED 97 HRS PREVIOUSLY, OK PER MANUAL. FAILURE POSSIBLY DUE TO AGE. SUBMITTER SUGGESTED TIME CHANGE REQUIREMENTS BE REQUIRED OR MORE DETAILED IN-SERVICE INSPECTION CRITERIA BE REQUIRED (I.E., LOAD TEST) IN ORDER TO DETECT PENDING FAILURES.