I'm currently building a portable USB charger. It uses 2 Li-ion batteries in series connected to a USB port via a 5V regulated buck converter. While a single Li-ion battery would suffice for most applications, this USB charger needs to be able to output 1A at 5V; a single Li-ion battery of the type I have is only able to output 1A at 3-4V, or a max of 3-4W - therefore stepping up the voltage would reduce the maximum current output. The converter itself is not an issue other than requiring more than 5V to operate (hence wiring the batteries in series to produce an input voltage of 6-8V), since it has a maximum current output of 3A.
For the purposes of charging, I have a relatively cheap TP4056 charging micro-board specifically designed for charging 3.6V nominal batteries such as the ones I have. It's capable of CC-CV charging and automatic shutdown when current drops too low (indicating saturation). However, it's only designed for operating with one battery - as such, it can only detect whenever the battery it is connected to is full. Is there anything I could set up that would allow the charger to switch between batteries whenever one is full, and cease to charge either whenever both are full?
I imagine a relatively simple solution of a manual switch for operation after one battery is full might work, but this introduces micromanagement that I believe can be fairly easily avoided.