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I have a device that needs a constant power supply and can only use batteries as an energy source. Using multiple batteries, I can swap a drained one for a fully charged one. Since I have to have a constant power supply, I want to install a smaller (lower mah) battery that will supply power during the swap. After I put in the larger (higher mah) fully charged battery, both batteries will contribute power in parallel.

All batteries are Lithium ion 3.7V (only different mah) . The problem arises when I swap batteries. If the difference in voltage between two batteries connected in parallel is greater than 0.2V a huge rush in current is transferred until the batteries equalize. This could be a problem...

The larger battery will need to recharge the smaller one. Would putting in a resistor between the small and large battery do the trick? Or do I need a more complex protection circuit? Thanks in advanced!

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  • \$\begingroup\$ MAX1538 or similar controller. \$\endgroup\$ – Marko Buršič Dec 5 '15 at 9:08
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    \$\begingroup\$ Why do you want a smaller battery rather than two batteries the same size that you can swap directly? \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 5 '15 at 9:18
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A simple diode-or circuit will handle this. Connect a diode from each battery's positive terminal to VCC; when you plug two in, only one will supply load if it's at a higher voltage. If the voltages are close or identical, they will load-share. When you unplug a battery, the other one will take over.

Charging lithium ion batteries is a complex task, best handled by a dedicated charge controller. Instead, why not use two identical size batteries, and simply swap them over by plugging one in before unplugging the other?

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    \$\begingroup\$ Use a power Schottky for the diode. \$\endgroup\$ – Ecnerwal Dec 5 '15 at 12:10

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