All 4 of my EDS OX derailleurs have the removable batteries.   To date, the removable batteries last a long time, over 600 miles of riding for me with lots of shifting.

OTOH: Weird things have been happening.  Yesterday, I went out on a snow ride on a bike I haven't ridden it about a month.  The bike was shifting OK, but I though that I would swap to a charged battery to be sure that the battery on the bike might be low on charge.  I swapped to a supposedly fully charged battery.  It wouldn't shift. No lights.  Swapped to another battery.  This battery didn't shift the derailleur either.  Put the original one back one and rode, leaving the other 2 on their chargers.   Then swapped one of the freshly charged battery onto the derailleur and it worked.  Now I am wondering why 2 fully charged batteries were drained of power after sitting around on a shelf for a month after last being charged. 

Regarding the fixed battery.  I am sure you can put a routine in place where you use your Wheeltop app on your phone to check the % charge / # of shifts left.   If you see the battery is below your charge comfort level, you can top it off. 

I wonder too, if the magnetic charging cable charges the battery while the derailleur is in operation.  One could secure the charge cable to the frame and use a battery bank to charge the battery while riding.  (Anyone test this?)