I am developing a project that has access to intermittent 0, 9-15 V DC external power, and want to augment that with an on-board battery to cope with multiple hour power outages from the supply. I am drawing less than 1 amp at 9 V from a step-up step-down regulator in the device, however input voltage can vary 7-12 V if necessary. I had imagined using one or several Li-Ion or Li-Po cells to step up to around 12 V, and include a circuit to handle charging the battery when power is available and discharging the battery when not, all while the device is active.
I thought it would be a simple case of putting a battery in line with supply, much like a lead acid battery in a car. However it seems modern batteries need a lot of nursing. I was put on to PMICs however there seem to be such a massive variety of them with different functionalities, I don't know what terminology would describe what I am after.
A belt and suspenders solution I could have the device monitor battery power and shut down gracefully below a level, but that is not absolutely necessary.
What strategies are used to integrate batteries with devices and power supplies, and thus what functionalities do I need to complete the project I have in mind?
I can see many answers to similar questions here, however some appear to accept that either the battery will discharge with no power, or be switched out when power is available, or switched to dedicated charging mode or something clunky like that. I'd like something more graceful.