# Charging (via USB) and using 2 Li-Ion batteries in parallel simultaneously? [duplicate]

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.

## marked as duplicate by Ignacio Vazquez-Abrams, Bence Kaulics, Voltage Spike, Daniel Grillo, Dave Tweed♦Aug 24 '16 at 19:31

• You need an OR gate, with two diodes. Figure 1.71, Horowitz The Art of Electronics, 3rd Edition. – user2798 Aug 24 '16 at 12:17
• Balance charging only makes sense in the context of charging in series. If you're charging the batteries in parallel, the parallel configuration itself will take care of the balancing for you. Are you discharging the batteries in series or in parallel? – Dampmaskin Aug 24 '16 at 12:41
• The question of current depends on the size of the batteries in question. The max (for the 5.2V 2A Gen. 2 usb chargers) battery space that you could theoretically have and still charge at peak effecienc is about 1000mAh, while the max (for gen.3 5.2V 3A charging, only used by google and samsung, at the moment.) would be about 1250mAh. – user86234 Aug 24 '16 at 14:39
• And for pointing me in the right direction. The duplicate answer has what I'm looking for. – TomL Aug 26 '16 at 6:43