Battery keeps going bad in same device

Here’s the scenario: My customer is a pilot who flies small, private planes. He uses the iPad Air 2 to file flight plans and as a navigational aid. The device is always with him when he flies. It has had some bumps and bruises, the frame was bent at one point, but we were able to straighten it out. Overall, it functions just fine.

May 2018: I replaced a very swollen battery in the device. (The glass was separating from the frame.)

January 2019: The May battery was swollen and again, the glass was separating from the frame. I replaced that battery, straightened the frame, and sent him on his way.

October 2019: Battery is swollen again.

Any ideas as to what could be causing this device to go through batteries so frequently?