Meanwhile (if it were me) I think I would experiment with rechargeable PP-3's from *Tesco's.
Lastly, I have seen (on eBay) 8.4 V 880 mAh Lithium PP-3's with a mini-USB socket (that is, they can be recharged from a USB connector); something else to consider, perhaps.
Please remember you are using batteries with medical devices, which must meet the medical device regulations. Fitting a battery not specified or recommended by the manufacturer would be considered a modification of the device and therefore all liability for the CE marking would fall onto the shoulders of the person making the modification! Good luck with that one if things go wrong!
Any battery purchased must be traceable back to the manufacturer so buying from non-ISO certified companies who can't provide such traceability is out off the question.
