I need suggestions or help thinking this problem through. I want to battery power a Raspberry Pi with multiple USB battery packs (for a remote sensing application) with no need to do anything manually. Solar re-charging of one battery is not really an option. I am talking about off-the shelf USB batteries sold by companies such as Anker.
Most USB batteries do not allow simultaneous charging while using a battery, so daisy-chaining multiple USB batteries (with one plugged into the RPi's USB) is tempting, but probably out.
I am wondering if I need to design a charge controller circuit that detects when a given battery is low, then switches instantaneously. Does this sound like my best bet? Is there an off-the shelf circuit or reference design that I should look into for this?
Another idea I had is maybe to connect all the batteries in series. This would of course produce a higher voltage than the original 5V. I could use a step-down buck converter to supply the 5V to the raspberry Pi.
Any ideas or strategies are welcome! Thanks for your help!