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I've been searching everywhere for a solution that will allow me to charge 2 Li-On batteries (3.7V) in parallel with a USB port without taking off the load/run (mechanically) of the rest of the circuit. If possible with charge balancing as well, although I would ensure similar charging levels before connecting of course. To be clear, the first part of the load is a boost circuit taking the 3.7V up to about 6.5V. I can't use a larger capacity battery due to their size (the batteries I'm using are really small).
I was thinking of using two chargers connected to a single USB port, but that might take too much current from the port, although I could adjust for that. It would also require a switching circuit that switches from running mode to charge mode if the voltage would drop below a threshold (2V?) for either battery for example, which would need some voltage regulator IC, but I have no idea where to look for this. This would also require some type of limiting circuit for simultaneously charging and using, which is what I'm aiming for. Is this setup a good idea? I imagine there might be simpler solutions.