I'm looking to use a solar + lithium ion battery combination in order to be able to continuously power a Raspberry Pi.
I have found an MPPT solar battery charger, which will output a fixed voltage. From my understanding, however, a fixed voltage output isn't the best way to charge l-ion batteries. So my thought is to feed this fixed voltage into a l-ion battery charger like this one, which will then charge the battery.
Once the battery is charged, it should provide current to the RPi and some other motors via a step-down regulator or two.
The thing I can't wrap my head around is how to make a circuit that will always give the RPi/load the power it needs, but charge the battery if there is any juice left over. If the panels are only getting a little bit of power, the battery should also augment them in order to power the load.
What is the most energy-efficient and reliable way to do this?