Well, if Tasmota isn’t giving you battery data, and you want battery data, something has to change on the Tasmota side so it gives battery data!
No idea if it’s possible though.
Beware that WiFi devices in general are quite power hungry, so you may find yourself changing batteries often. If you want to keep to batteries, I’d suggest have a look at the MySensors project. I’m using one with a DHT22 at the moment.