Pair of Ring Video Doorbells - batteries drain even though hardwired

I have two Ring Video Doorbells. They have been installed and working for approx 5 months now. They are hardwired - one on the front door, one on the side door. They are both connected to the same electromechanical doorbell and transformer (that were there already). That existing transformer has a label that says it’s output is 16.5VAC and 37VA. They function fine in almost all respects.

Both Video Doorbells show as “hardwired” in the app. Unless I dial down the settings (e.g. turn live view “off”; reduce the number of alerts; etc), however, the battery level drops slowly over time on both Video Doorbells. I assume that this means I need a higher output transformer (ie 24v) but wanted to confirm that. Can someone confirm this?