The PE battery seems far more reliable than the removable EE battery, so chances are the device would wear out before the battery anyway, if a dead battery is what you're worried about.

If you want a spare for backup during a long day, the simply closing the PE when you're not using it reduces power consumption to zero thanks to the Ermine release. You get 5-6 hours of run-time, which is when the PE is open. That means if you have it closed 4 hours and open 5 hours, you have a total time span of 9 hours where previously you'd get just 5.