From Erich at Flytec in Switzerland writes:

We are actually testing and measuring a series of rechargeable batteries. We recommend so called precharged, or Ready to use NiMH batteries. These are batteries with a capacity of around 2100mAh and they have very little self discharging. This is a new technology.

We don't recommend NiMH batteries with high capacity over 2300mAh. This types are developed for cameras with high discharge rates. They don't bring this capacity to our application, because they self discharge very fast. We have tested the following batteries available in Europe:

Duracell DX1500, Active Charge 2000mAh
Varta Ready2 Use 2100mAh
Uniross Hybrio UO118675 2100mAh
We are testing Panasonic, but I don't have results yet.
Not suitable are Eneloop.