Apple - iPad being using in wall mount battery swollen

This always (eventually) happens if you use a Lithium battery long enough (and assuming nothing else breaks first).

It also happens prematurely and would be covered by AppleCare / Warranty even if you do everything right in less common circumstances.

To counteract this use newer hardware on iOS 11.3 or higher which have charge management features to optimize for always plugged in situations.

• https://support.apple.com/en-us/HT208710

Secondly, the cooler you can keep the battery, the less likely thermal fatigue will accelerate aging and eventual failure, but this expansion or a refusal to charge ultimately happens even if you keep the device permanently cool and properly managed and charged.

The main thing you can control is when the battery discharges and when it charges. A lower wattage charger is probably marginally safer since it will generate less heat when charging since the charge takes longer, so I would possibly consider very lower wattage chargers if you had a fleet of 100 of these and never needed a fast charge and could get away with trickle or low charge rates.

I wouldn’t change anything based on less than 3 or 4 failures since you’ll waste more time and resources swapping out chargers and not get much data in return. Once you’ve seen one iPad get a swollen battery, you’ll know how to gently touch the screen and detect the bulging before it separates the glass and get it in for diagnosis and a spare battery. Your failed unit or iFixit will show where on each screen to test for pressure / contact on the backside of the LCD panel.

I like the idea of powering them down somewhat regularly, but I don’t have evidence that in a population it will make any difference whatsoever. For people that need the battery to work, keeping it used in all ranges of the charge / discharge cycle can probably help marginally - but if you always run on power, it won’t really matter if you have an accurate measure of charge status and could get away with every 6 months draining and wiping and setting them up so you know which are starting to age.

Make sure you're using the latest version of iOS. iOS 11.3 and later on the iPad introduces a feature to preserve battery health for devices which are constantly plugged in, which is exactly your situation:

About iPad and iPhone Charge Management Feature

...when iPad or iPhone is connected to power for prolonged periods of time, such as when it is used in kiosks, point of sale systems, or stored in charging carts... When they remain at full charge for prolonged periods of time, battery health can be affected.

iPad with iOS 11.3 or later and iPhone XS, iPhone XS Max, or iPhone XR with iOS 12 or later include a charge management feature to help maintain battery health, which monitors these devices for use in these charging situations and, as required, reduces the maximum capacity of the battery...