We worked on a new power delivery system for the Raspberry Pi. Jack designed this schematic before finding a simpler PCB that had all the functionality required.
This is the device we will use:
This is the PowerBoost 500 Charger. With a built-in battery charger circuit, it can keep the Pi running even while recharging the battery.
The Raspberry Pi and other boards with low power draw will be powered by a 3.7v PKCELL (or similar) battery. The servos will use a Lithium Polymer battery.