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This question already has an answer here:

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.

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

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    \$\begingroup\$ You need an OR gate, with two diodes. Figure 1.71, Horowitz The Art of Electronics, 3rd Edition. \$\endgroup\$ – user2798 Aug 24 '16 at 12:17
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    \$\begingroup\$ 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? \$\endgroup\$ – Dampmaskin Aug 24 '16 at 12:41
  • \$\begingroup\$ 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. \$\endgroup\$ – user86234 Aug 24 '16 at 14:39
  • \$\begingroup\$ And for pointing me in the right direction. The duplicate answer has what I'm looking for. \$\endgroup\$ – TomL Aug 26 '16 at 6:43
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There is a major question here, how much current does the load pull from the batteries?

To charge a set of batteries properly the charger has to provide enough current to satisfy the load and to charge the batteries at the same time. If your load pulls 100mA and your batteries want 200mA of charge current then it's pretty easy. Microchip makes charge controllers MCP73812 for example. Two batteries in parallel balance each other and don't require additional balancing. This setup can also be supported on a regular USB port that can supply up to 500mA with enumeration. An FT230X FTDI chip could be used to enumerate and juice the bus to full current.

If you load current plus charge current are greater than 500mA then a wall charger is needed. This means that you won't be able to charge from a computer and only specific high current charges would work. Microchip makes charge controllers for this too MCP73861 for example. (double check the chip voltage is compatible with your battery voltage)

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  • \$\begingroup\$ yes, this solution is what I'm looking for... \$\endgroup\$ – TomL Aug 26 '16 at 6:44

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