Refer to https://hackaday.io/project/191791-stop-watch-x10-from-smart-response-pe/log/220989-visual-notes

I decided to not use a traditional switch.  Rather, I thought having the calculator automatically turn on when picked-up and used then power-off completely when put down.

The use of a single rechargeable, LiFePO4 cell provided the space needed to construct the dummy-cell with a mercury switch.  

Conversion back to 2xAA alkaline cells only takes seconds.