Suggest you use a higher voltage battery, then use that to power both motor and a voltage regulator for the PM board.
I agree with yendis.
Not many servos will work properly with a voltage supply from a single LiPo/Li-Ion cell.
I can think of three options to get your project working.
As yendis suggests, use a higher voltage supply and a regulator to power the Arduino. The problem with this option is you might need a regulator for the servo. Most servos aren't happy with voltage levels from either a single LiPo cell or the voltage from two LiPo cells. If you use a 2S LiPo/Li-Ion you'll likely need a regulator for the servo.
Purchase a servo which will operate powered from a single cell.
(Probably my favorite for this application.) Use a boost converter for the servo. There are lots of boost converters on ebay. Depending on how much current your servo draws, you may get a way with using one like this. If your servo is a full sized servo, you'll probably need a boost converter capable of higher current.
Whenever I purchase cheap ebay parts, I usually get a couple extra in case one of the parts doesn't work.
Adafruit sells several nice boost converters.
If you want to find a servo capable of being powered from a single cell, let us know and we can try to help you find one.
- a servo motor (low energy)
A link to the servo you're using would be helpful.
I don't think I'm the only one who thinks "a servo motor (low energy)" is an oxymoron.
Have you measured the force required to push the lock mechanism?