And power it from a Lithium 3.6V battery, not a "9V". There are lots avaialable that are used in lights, will give you much longer run time.Use a 3.3v LDO voltage regulator.
Maybe, maybe not: the full charge voltage is 4.2V and the uP might not be able to handle that. Depends on the specs.In that case you don't need a voltage regulator.
The datasheet on Duracell's website is very brief and has only one page. It does not say the capacity but shows a graph with a load of 50mA when the voltage starts at 8V and it drops to 7V in about 15 hours when it is almost dead. It has a PTC current-limiter which might prevent a current as high as 300mA.How to know the mAh rating of the Duracell 9v lithium battery?
by Jake Hertz
by Aaron Carman
by Jake Hertz