The Sage has
EIGHT magnets!
Two at button side (smaller than the other six).
One each top and bottom
Four on edge opposite buttons.
I pried open with plastic tool from button side with soft back overhanging table so it could bend back. Then sides. Finally the far vertical edge which maybe has a little glue.
The magnets are all glued in so it was only possible to prise out the two near the buttons as they are on plastic. The rest are in the alloy casting. IMO a stupid design as it picks up pins, paper clips etc. Can wake another reader in a sleep cover if set on top by "cancelling" out cover magnet; I've seen this. Can erase or corrupt magnetic strips on welfare or other cards or other magnetic tape.
A dumb way to add the "power back".
Adds weight.
The battery is fairly large and definitely 1200mAH! That's 1.2AH, so the less than approximately 50 minutes at 1.5A to charge makes sense.
The small PCB in the back is for the four connections to the "power" cover as well as the power/sleep button.
P.S.
Yes, still works. Some small soldering skill and any 1200mAH LiPoly that fits and has integral control PCB should do for replacement in 5 to 10 years time.