3V AA sized rechargeable battery???

[Phil Jessop]

It is probably a Lithium Thionyl Chloride which have a terminal voltage of

3.6V. You can get them in both 1/2 AA and full AA sizes. Try Budget batteries

[Ian Stirling]

How heavy is this? NiCd and NiMH batteries are considerably heavier than lithium of any sort. I would - as others have suggested suspect a 3V lithium non-rechargable battery - simply as rechargable lithium are more expensive.