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I am looking to design a USB rechargeable device that feature a small microcontroller set up to run at low clock frequency driving a few LEDs. I'd like to power it off a 2032 coin cell battery or similar in order to achieve a streamlined form factor.

I'm doing research on recharge circuitry for coin cells and I'm wondering what I need to do to allow the 5V from USB to charge my coin cell batteries and power the rest of the circuitry at the same time (most of the recharge schematics I'm seeing are in isolation.)

For example, I'm looking at this Analog Devices part, the LTC4054, which is an integrated charging circuit. For this schematic could I just power the main load circuitry in parallel with the battery from the charge output, or do I need to do some kind of switching so they're not both connected at the same time?

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    \$\begingroup\$ Coin cells are primary and thus have low efficiency for charging and all you use is a 3.0V LDO \$\endgroup\$ Commented May 21, 2022 at 3:20
  • \$\begingroup\$ What do you mean by primary? \$\endgroup\$ Commented May 21, 2022 at 5:00
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    \$\begingroup\$ 'Primary' cells are not typically rechargeable to any useful degree. 'Secondary' cells are intended to be recharged. \$\endgroup\$
    – brhans
    Commented May 21, 2022 at 11:25
  • \$\begingroup\$ exceptions exist: Alkaline cells (primary) can be recharged (with knowledge and special care) up to 10 times with adjusted (pulsed current) chargers for rechargeable and disposable alkaline cells (rechargeable alkaline batteries) and there are rechargeable lithium ion 2032 cells also (some ~65-230mAh, maybe only ~3.6V) \$\endgroup\$
    – beyondtime
    Commented May 21, 2022 at 12:30
  • \$\begingroup\$ Primary means if its dead you can restore maybe 5 to 10% capacity charging it. This is the 2nd layer of charge in most batteries that only is about 10% of the total capacity from the double layer effect. Some have more. "dead" means 2.5V . almost useless means less than that. But an LED doesn't last very long on any coin cell.. < Half a day bright then dim for another day. Bad choice. \$\endgroup\$ Commented May 22, 2022 at 0:18

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To your question:

What I need to do to allow the 5V from usb to charge my coin cell batteries and power the rest of the circuitry at the same time?

The example shown with LTC4054-4.2 uses a Lithium rechargeable battery and your mention to “coin cell” would mean a small Li-ion battery, then:
It is possible to specify proper parameters for charging the battery and monitoring its state of charge, while using it to power your device circuit at the same time, having some points of attention for proper sizing:

  • Li-Ion battery will probably be 100% of the (charger-connected) time maintained at 4.2V as this is the “fully-charged” or termination voltage.
    Some can argue about Li-Ion being maintained at 4.2V could shorten the long-term life, but cycling number, readiness and convenience of use may be the most important features, so for me 100% always charged = “ok”.
  • Charging current should be higher than average consumption of device, but could be lower than maximum consumption; in this later case, battery would just supplement the worst case drain. As your are considering “coin” sized batteries, the charging current will ok for both USB and LTC sides.
  • There might be cases where LTC4054 stops charging the Li-ion cell if the consumption of device happens to be less than 10% of the programmed current - PROG’s resistor sizing in the datasheet.

These features could be designed using the LTC4054 controller, but other ones may not have all these features. How important each will be in your device, would depend on the expected way of using the device, its expected lifetime, how much effort you expect to put to optimize the project and the parts to find, etc.

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