What was found
FLT 7378 - MSY - DURING CRUISE CREW STARTED THE APU FOR BACKUP AFTER NOTICING THEIR BATTERIES WERE DISCHARGING. AFTER THE APU WAS STARTED THE FIRE WARNING ALARM WENT OFF. CREW ELECTED TO DISCHARGE FIRE BOTTLE. AIRCRAFT LANDED IN MSY WITHOUT INCIDENT. MAINTENANCE INVESTIGATION REVEALED COOLING DUCT FOR HOT SECTION OUT OF ALIGNMENT. ALIGNED DUCT, APU OPS CHECKED GOOD. PLACARDED PER MEL 26-22-3 CAT C DUE TO LACK OF SPARE FIRE BOTTLE. AIRCRAFT RETURNED TO SERVICE.