@garethjwells It seems possible, but NOT necessarily easy. This thread (What is the expected battery lifetime of Phonak Roger Select? - #12 by hold4triple) might be of help. If you do a search within the thread, there are numerous comments about PEN and battery.
I do not have PEN, but am looking at used Roger Select. For the latter, HAs repair places even that do onsite work, seem to send KS10/Paradise OR ROger mics back to Phonak for the battery replacement and cost is $200-225 per item - insanely expensive.