XIAO with Lipo Battery Charging Circuit

Products & Technology XIAO
1

I am designing a very small wearable product that uses the XIAO. The XIAO is stacked with a custom PCB with an IMU, a Bluetooth module, and a battery. The board has a charging circuit and regulator on it. My question involves how to connect these to the XIAO.

I should be able to connect the regulator output (+5) to the 5V pin of the XIAO. The schematic shows a diode from the VUSB line. Should I add the same diode to the regulator output before connection to the XIAO 5V pin? (If so, what diode do you recommend?)

The second part of the question is where to connect the input to the charging circuit (MCP73833)? Normally, this would connect to VUSB. If no USB connection was present, the charging circuit would be inactive. When USB power is present, the charger would monitor the battery and charge if necessary. The output of the charger circuit is connected to the battery and the regulator. VUSB is not available on the XIAO.

Is this the right way to add a battery with charging circuit to the XIAO? Do you have any other recommendations?

Thank you.

2

After doing some more research, I believe that I can add battery support to the XIAO for my project. My thought is that if the VUSB voltage is available, that can be connected to the input to the charging circuit. The output of the charging circuit would go to the lipo battery, through a switch, and into a 5V regulator. The output of the 5V regulator would then go through a low forward voltage diode and then connect to the VIN/5V pin. With this circuit, the on-board regulator would be driven by either by USB power or the lipo battery through the regulator. If USB power and lipo power were present at the same time, the diodes would protect the USB and external regulator.

Is the diode shown on the schematic present on the top of the XIAO? If so, the anode side of that diode would be VUSB. Soldering to it may be a challenge but this might be a way to add battery support.

3

More research…

I have looked at the schematic for the XAIO Groove Shield that has battery management. If I read it and the XAIO schematic correctly, when USB power is present, +5 from the XAIO is an input to the battery management IC (U5) on the shield. The Vsys output of U5 is connected to the battery for charging. It is also connected to the input to the LDO 3V3 regulator on the shield. The 3.3v output of regulator would power everything on the shield. The output of that regulator is also connected to the 3V3 pin on the XIAO. That is how the XIAO receives its power.

When there is no USB connection present, there is no +5 input to U5 so U5 passes the battery voltage to the LDO regulator. That powers the shield and also powers the XAIO through the 3V3 pin.

My only problem with this approach is that in both cases, power is being applied to the output of another regulator. When USB power is present, the XAIO LDO regulator is producing 3.3 volts but since U5 is active and the battery is present, the shield LDO is also producing 3.3 volts. Both these regulators are on and producing 3.3 volts and are connected to each other. Is this OK?

When USB power is not present, there is no voltage to the input to the XIAO regulator so it is off. The battery voltage is at the input of the shield regulator and so it is on and producing 3.3 volts. That 3.3 volts is applied back to the XAIO and therefore also being applied to the output of the off XIAO regulator. Is this OK?

I may be completely missing something so please tell me where I am wrong. If I am right, please let me know that it is OK for the output of these two regulators to be connected to each other. If that is OK, I will do something similar for my project.

Thanks.

4

Hi Chevelle,

I understand the schematic in the following way:
You can connect your battery to the two connector pads (Vin and Grnd) on the bottom side of the XIAO and connect the same XIAO via USB at the same time. (Of course, this will not charge the battery).

I am planning to do exactly the same thing.
I already bought a 4.2V lipo battery and charger , but I cannot entirely test yet, since I have a different issue:
My C# program runs perfectly as long as I connect the XIAO with the PC. But when I connect the Xiao with the same USB cable to an external power bank, the XIAO will indicate its green light, but my C# program won’t start. My i2C devices connected to the XIAO will blink once, but that’s it.

I tried the same with the 4.2 battery connected to the bottom connectors, same behaviour.

As soon as I disconnect the USB cable from the external power bank and connect to the USB port of my PC instead, the XIAO will work again.

Maybe you could give me a hint on how to start the XIAO when only connected to a USB power bank ?
Do I have to modify the bootloader of the XIAO ? I cannot find anything in the internet.

As soon as this is solved, I will try to operate the USB and battery in parallel and I will inform you on my observations.

Regards,

Volker.

5

Hi Volker,

Because Seeed Studio hasn’t responded to my question, I have had to move on to another microcontroller for this application. I am now using am ESP32. It is slightly larger but with BLE built in, I think it is a better fit.

As far as your issue goes, you mentioned an external power bank. What voltage? The Vin pin goes into a LDO voltage regulator. You cannot put your battery there. A 1S battery has a nominal voltage of 3.7V and can go as low as 2.8-3 before needing to be charged. The maximum voltage after being fully charged is 4.2V. If Vin is too low, that be be causing your issue.

Per the schematic, the +5V from the USB connector is connected directly to the Vin pin. To test your design, make sure it is all working when being powered off of the USB connection to your computer. Then disconnect the USB connector, connect +5V from your power pack to Vin. Everything should run OK.

Good luck.

6

I’m necroposting considerably, but now in late-2024 several XIAO boards have on-board battery management circuits so that Lipo batteries may be connected directly to the board. In my own parts bin, the XIAO ESP32-C3 and new XIAO RP2350 boards do, but the XIAO RP2040 do not have on-board Lipo battery management.

I presume the OP was posting about the XIAO SAMD21, which does not have on-board Lipo battery management.

If you look at the bottom of your XIAO board(s) and see BAT+ and BAT- solder pads, then it’s likely your board supports battery charging and discharging. Check the schematic PDF associated with your specific model to be sure.

Note that overvoltage/undervoltage protection is likely not present. Lipo batteries will be damaged if they’re discharged below roughly 2.8VDC. If you have an analog input pin free, using a 2:1 voltage divider and monitoring the battery voltage periodically may be wise when designing your code.

Such an example is here, written for the XIAO ESP32-C3, but largely applicable to other boards (except perhaps for the voltage calibration supported by eFuse on the ESP32-C):

Hope this info helps for folks new to this thread!

1 Like
7

Two 220kΩ resistors connected in series will allow a current of about 10 microamps to flow, depending on the battery voltage.
It is important to note that this leakage current will not protect the battery from over-discharging.
Therefore, a connector can be used to disconnect the battery. Soldering this is also safe.

Also, the measurement value is unstable, so I wonder if the current is insufficient. The XIAO-MG23 is measured with two 10kΩ resistors.

Lipo_connector
Lipo_connector1024Ă—1143 381 KB

8

Hi there,

So Before you even think about using a battery, Don’t be pound wise and Penny foolish. Start with a GOOD Battery. Use a PMIC or Voltage divider if that is all you have.
Good-quality LiPo batteries, like what I use is the 3.85V 450mAh cell, usually include built-in protection circuits. These Protection Circuit Modules (PCMs) or Protection ICs prevent:

  1. Over-discharge (deep discharge) – Cuts off power before voltage drops too low.
  2. Overcharge protection – Prevents charging past the safe limit (typically ~4.35V for 3.85V cells).
  3. Overcurrent & Short Circuit Protection – Prevents excessive current draw or shorts.

:small_blue_diamond: How to Check If Your Battery Has Protection

:one: Look for a Small PCB Attached to the Battery

  • Most protected LiPo cells have a small circuit board near the terminals.
  • If it’s a bare cell without a PCB, it may NOT have protection.

:two: Check the Datasheet or Manufacturer Specs

  • If the spec sheet mentions “PCM,” “BMS,” or “Protection Circuit”, then it includes built-in safety.

:three: Test Voltage Cutoff Behavior

  • If you discharge the battery below ~3.0V and it cuts off, it has protection.
  • If it drops below 2.8V and still works, it might be unprotected.

:battery: What If the Battery is Unprotected?

  • If no protection is built-in, your Xiao must handle over-discharge cutoff.
  • The Xiao nRF52840 includes a fuel gauge and voltage monitoring, so you can program a low-voltage cutoff (e.g., shut down at 3.0V).
  • If you charge externally, use a LiPo charger with built-in protection.

The tiny PCB with two chips on top of the 3.85V 450mAh LiPo battery is the Protection Circuit Module (PCM). That confirms the battery has built-in overcharge, over-discharge, and short-circuit protection! :battery::fire:

:small_blue_diamond: What the Tiny Chips Do:

  1. Battery Protection IC – Manages voltage levels and prevents overcharge/discharge.
  2. MOSFETs – Act as switches to disconnect power if unsafe conditions occur.

:microscope: How the Protection Works

:heavy_check_mark: Overcharge Protection → Cuts off charging if voltage goes above 4.35V.
:heavy_check_mark: Over-discharge Protection → Cuts off load if voltage drops below ~3.0V (typically 2.8V-3.0V).
:heavy_check_mark: Short Circuit/Overcurrent Protection → Shuts off if too much current is drawn.

:rocket: What This Means for Your Xiao Setup

:heavy_check_mark: You don’t need to manually monitor voltage in software.
:heavy_check_mark: The battery will shut off before deep discharge damage occurs.
:heavy_check_mark: Safe to charge directly using a proper LiPo charger.

Even with protection, you can:

  • Monitor battery voltage via ADC (for display or logging).
  • Set a software cutoff at ~3.2V to extend battery lifespan.
  • Use a fuel gauge IC (like MAX17048) for more accurate charge monitoring.

I’m waiting for the Xiao to get a fuel gauge ic… :wink: :crossed_fingers:

:battery: How the Xiao’s Battery Charger Chip Works with Your Protected LiPo

The Seeed Studio Xiao nRF52840 (and similar Xiao boards) include an onboard LiPo charging circuit, which works together with your battery’s built-in protection to safely manage charging.

:rocket: How It Works:

:one: Xiao Provides a Safe Charging Voltage

  • The Xiao’s charger limits the charge voltage to 4.2V or 4.35V, depending on the model.
  • Your 3.85V LiPo reaches 4.35V at full charge, so check if the charger matches.

:two: Battery Protection Circuit Prevents Overcharge

  • Your LiPo battery’s built-in PCM will disconnect the battery if the charger tries to overcharge.
  • This prevents damage even if the Xiao’s charger malfunctions.

:three: Battery Disconnects If Voltage is Too Low

  • If the battery voltage drops too low (~2.8V-3.0V), the PCM cuts off output.
  • The Xiao won’t see the battery until it’s charged back above the protection threshold.

:four: Charger Handles Safe Current Flow

  • The Xiao’s charging chip limits current (typically 100mA to 500mA).
  • This prevents overheating and ensures the battery charges at a safe rate.

:mag: What You Need to Check:

:heavy_check_mark: Check Your Xiao’s Charge Voltage

  • Xiao nRF52840: 4.2V charger
  • Xiao ESP32C3/C6/S3: 4.2V charger
  • Some models (not Xiao) can go up to 4.35V
  • If the charger is 4.2V, your 3.85V battery won’t fully charge (stops at ~85% capacity).

:heavy_check_mark: Charging Current (mA)

  • If the charger provides 500mA, your 450mAh battery takes ~1 hour to charge.

:heavy_check_mark: USB Power vs. Battery Use

  • When USB is plugged in, the Xiao runs on USB and charges the battery.
  • When USB is unplugged, it automatically switches to battery power.

:dart: Final Thoughts

:white_check_mark: Your battery’s built-in protection ensures safe operation.
:white_check_mark: The Xiao’s charging chip properly manages charge voltage & current.
:white_check_mark: If the Xiao charges to 4.2V max, your 3.85V LiPo won’t reach 100% (but still works fine).
:white_check_mark: No external charge controller is needed—Xiao and the battery PCM handle it!

HTH
GL :slight_smile: PJ :v:

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