Yes, 4 AA rechargeable. There's a picture floating around here somewhere on this site of the inside of the controller.
Slight correction, 3 x AA batteries, not 4.
Funnily enough I was looking at those threads the other day (my HC-2 soon needs new batteries, they are 6 years or so years old and I won't be sending it back for the change out).
There was two threads about this. First thread (
https://rpug.pdc.ca/index.php/topic,24.0.html) had pictures and some battery size discussion. Unfortunately the pictures of the battery compartment side of the circuit board were external links which the pics are now dead links. Second thread (
https://rpug.pdc.ca/index.php/topic,170.0.html) no pics but more battery discussion.
The take from the threads is that Ring uses a variety of battery capacities, most likely what he gets the cheapest at a given time of purchase. Known capacities used are 2300 mAH, 2400 mAH and 2500 mAH across different brands of battery used. I don't see why larger capacities could not be used but I'm no expert on these things by all means.
As to voiding the warranty by opening it up and changing the batteries, that would be an interesting situation to watch. I don't know how things work in the US but here such a claim by Ring would be laughed out of the room by the consumer law authorities. "What, you can't change the batteries in your consumer device without sending it back?!!!"
- Tim