When you need a regular supply voltage - the Charge Pump module is there for you. Throw it anything from 2.2V to 5.5V and it will give you either 3.3V or 5V (switch selectable) with its magical buck / boost circuitry.  So your battery can decline and the Charge Pump will keep the right voltage for you.

Note that the initial run of these boards doesn't have silkscreen printing - but you'll figure it out by looking at the datasheet!