[seltzered_]

I did similar with a galaxy S4 (third party battery w/special back case to accommodate the larger size). Worked nice but eventually the battery bulged.

The tradeoff with replaceable batteries is if when you swap the phone loses track of time until finding a cell tower. Fine if in range but a risk when hiking far away, and might also propagate the wrong time to your smartwatch.

[bombolo]

It should be able to sync its clock slowly from the GPS signal. But I have no clue if the feature is actually implemented.