The Question –
Is a sealed battery airworthy if it is bulging?
The Answer –
Yes, this is normal when the internal gas pressure is higher than the outside atmospheric pressure, it’s a GOOD sign as each cell pressure relief valve is not leaking.
The cell pressure relief valves remain closed normally but do burp occasionally to relieve excess internal pressure while in service. The recombinant gas battery requires a positive internal pressure to operate normally.
The case will go concave if the aircraft makes a rapid descent from a high altitude.
ALL rechargeable batteries of any type are called “secondary” and ALL have pressure relief valves to prevent the battery case from rupturing or bursting due to high gas pressure that may be caused by overcharging with higher than recommended voltage from either internal or external power. Sometimes this happens to batteries that are shipped in a charged state that are of normal shape at the time of shipment and are subject to either higher or lower atmospheric pressure during transit.
See Page 9 of Document Number 5-0324 “ CONCORDE RG SERIES AIRCRAFT BATTERY OWNER/OPERATOR MANUAL “ packed with each battery.