I have a circuit that is currently powered by two 18650s connected in parallel via a TP4056 USB charging and protection module. The circuit uses very low power and therefore it lasts for weeks if not months.
The load is a number of RGB LEDs with some 8h on/16h off timer circuits repurposed from some cheap LED candles creating an effect of the lights being green during the day, turning amber in the late afternoon and then red in the evening before being off overnight. Setting this up involves using the remote that came with the LED candles to start the 8h on period for each timer at the correct time of day. Therefore when the batteries finally die setting it back up is a pain.
What I want to do is add a secondary power supply (made up again of two 18650s in parallel via a TP4056) but in a way so that it only kicks in once the primary power supply has turned off (which is at 2.5v due to the over discharge protection).
Ideally once the secondary power supply has taken over seamlessly a single warning LED would come on to remind me to charge the primary pack at which point it seamlessly takes over again.
I've searched quite extensively and so far only found circuit designs that either allow two independent battery packs to be toggled back and forth depending on which has the highest voltage at the time or are UPS style things where the primary power source is expected to be a constant voltage and cut off entirely, not slowly drop.
Is there a simple IC out there that will take two power sources to power one load but preferring one power source over the other, regardless of voltage? Or can/does this need to be somehow built from discreet components, hopefully with a minimal power consumption and a low parts count if so.