The Shelly door window sensor is great but the lithium batteries drain too fast. I converted it to use 4 AA batteries.

• use a shelly and remove the electronics
• either design your own battery holder or buy a cheap 4 battery holder (there are options here to print but need all the metal peaces to work)
• use my battery dummy or solder the ends of the battery holder directly to the shelly. With the dummy, there is no visible damage to the Shelly and can be returned to the original easily.
• the dummy uses two thumbtacks and solder the ends to it.