Just to add further detail, I travel with a POC myself. I have found flights where either the sockets are not functioning or the device draws too much for the socket to support and so have had to use batteries for the full trip.

it is always recommended to take batteries for 150% of trip length in case of diversions etc.

As mentioned above you also have to send a MEDIF form, which must have the second section competed by a medical doctor. Do allow time for this to be done.

Also BA only will cover you for BA and AA metal, if the flight is code shared onto a different operating airline you will need to do their MEDIF procedure instead which may be different.