Daniel Bull has written up some excellent instructions for wiring up and soldering an Adafruit PowerBoost LiPo charger onto a Raspberry Pi. He’s used a Zero for this initial hack but you should be able to use any Raspberry Pi (the lower-powered the better). The instructions are available on GitHub and to talk to Daniel, get advice on implementing it and to discuss his approach, you can use this G+ post. It’s a brilliant hack, especially as the PowerBoost has a charging circuit built in to give you a self-contained system.