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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)

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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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  • \$\begingroup\$ Thanks! Just a question: you say ow battery should be easy. You already have a 5V supply and you're measuring a 3.7V battery : when running on battery alone, we don't have a 5V supply... \$\endgroup\$ – Basj Jun 2 '15 at 21:58
  • \$\begingroup\$ @Basj: Sure you do. It's your output. If the output isn't on, then the application isn't powered, and so the indicator doesn't mean much anyway. \$\endgroup\$ – AaronD Jun 2 '15 at 22:01
  • \$\begingroup\$ Oh, that's right! Something else: do you have a schematics / part in mind that could do the whole thing? \$\endgroup\$ – Basj Jun 2 '15 at 22:14
  • \$\begingroup\$ @Basj: Search google images for "boost converter schematic" without quotes, and you should get a decent idea for that part. Change "boost" to "buck" for the charger. Both types have transistors or switches controlled by poorly defined sources because the source is automatically adjusted according how the output is drifting away from spec. You can get all the parts to prototype both kinds on a breadboard, and I would recommend a small microcontroller to monitor the circuit via some ADC channels and control the transistors via PWM. Or you could use an arduino for that job too. \$\endgroup\$ – AaronD Jun 2 '15 at 22:40
  • \$\begingroup\$ Make sure you get the level translations right, because there's going to be a lot of them. You're controlling and monitoring circuits that are likely to be above the voltage rating of the chip itself. \$\endgroup\$ – AaronD Jun 2 '15 at 22:44
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Take a look at TI's battery management ICs.

Some include USB OTG outputs which provide 5V at varying current outputs (you didn't specify power requirements). They will take care of charging your single Li-Ion cell while maintaining the 5V output if an external supply is connected. When the external supply is not connected. the output is supplied from the battery through an internal boost circuit.

It's off-topic to request a single product recommendation, but the general device you're looking for is a battery management IC.

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  • \$\begingroup\$ Thanks! Oh I see 199 matching parts, I'm really a noob about such electronics question and don't want to make a wrong choice, could you give an example of part you'd use? \$\endgroup\$ – Basj Jun 2 '15 at 21:59
  • \$\begingroup\$ @Basj You're going to need to learn at some point if you're going to be building this circuit. I'd suggest you look at the BQ24188 and see if it meets your requirements. \$\endgroup\$ – Samuel Jun 2 '15 at 22:03
  • \$\begingroup\$ Thanks @Samuel, so it will work straighforward with just one such chip? No other part required (resistors, capacitors?). What does such a part look like? I saw some schematics, datasheets, but no real photo: will it be SMD or can I try it / prototype it on a breadboard / veroboard? If you were in my situation, in which direction would you go first? \$\endgroup\$ – Basj Jun 2 '15 at 22:13
  • \$\begingroup\$ Sidenote: I want to add this thing to my project samplerbox.org \$\endgroup\$ – Basj Jun 2 '15 at 22:14
  • \$\begingroup\$ @Basj I'm beginning to suspect this task is beyond your skill level. The IC does not work on its own, external passive components like capacitors, resistors, and inductors are required. The example schematic can be found in the product datasheet. This is a surface mount component, it looks like a little black box smaller than your fingertip. You need to design a PCB to put it on or get a breakout board. If this is all too much you can just buy a USB battery , kludge it inside your case, and be done with it. \$\endgroup\$ – Samuel Jun 2 '15 at 22:24

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