Hello Jeroen,
Sorry but we made a mistake, Li ion batteries do not exist in a AA size and thus cannot be fitted in a Myo Rxp. You can only use NiMh in size AA, take the most powerfull one, > 2300 ma/h.
info here : http://www.greenbatteries.com/libafa.html
Sorry again for this mistake.
Sincerely
Answer