Electronic – Step up converter (3.7V to 5V) that will also charge battery!

batteriesbattery-chargingbattery-operatedpowerpower supply

As a picture often speaks better than words, here is what I'm looking for (the yellow circuit on this picture):

enter image description here

i.e. :

  • a circuit that converts 3.7 V to 5 V when a 18650 battery (3.7V) is connected

  • and that both outputs 5 V and charges the battery if a phone charger is connected

Optionnally, it would be great if the circuit could send a "low battery" trigger to some other chip.

 

Such circuit is probably quite common (in many everyday-use devices that can run in battery or with adapter), do you know where to find one? (either the schematics or even ready-to-use circuits)

Best Answer

Low battery should be easy. You already have a 5V supply and you're measuring a 3.7V battery, so a comparator between the battery and a voltage divider should do the trick.

As for the boost converter, battery charger, and auto-switch, I would be tempted to skip the auto-switch and always run from the battery, even when charging. Sure, you're converting 5V to 4.2V (full-charge) and back to 5V again, but it allows you to design a charger and booster almost independently. Just set the current limit of the charger to cover the application too, but not enough to blow up the battery if the application is off.

While these devices are everywhere, most of them are on the same PCB as the rest of their application, so you may have to roll your own or do some salvaging. Even if there is one you could buy, asking for that route is outside the scope of this site.

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