Success!

@robalstona your advice was spot-on. For future reference, here's what I did:

• Disassemble the M5StickC:
  • peel the sticker at the bottom from the GPIO pins, do NOT fully remove it
  • use a sharp tool to pry away the black GPIO pin block, forward in the direction that the "5V->" arrow indicates on the sticker that was on it.
  • open the case with a hex tool
  • gently lift and pull out the board
• The battery is attached to 3 different pins: VBATT (red) + GND (black) on the main board and VBATT (red as well) on the extension board/GPIO pins. Cut or desolder the connections on the main board but cut the red wire on the extension board as close as possible to the battery. Then solder the dangling end of this red wire back to VBATT on the main board. Otherwise the new battery cannot be connected via the GPIO pins.
• Now put everything back together in reverse order (first put in the board, then put the GPIO pin cover on, then reattach the sticker)
• Now plug an external 3.7V LiPo battery into the GPIO pins, connected with (-) to GND and (+) to VBATT.
• In my case, the external battery is ~1000 mAh, and I read somewhere that you should charge your lipo batteries with 10% of the mAh rating, so I left the default charging current of 100mA. If using a bigger battery or you want to use higher charge current, then use this command: https://github.com/m5stack/M5StickC/blob/master/src/AXP192.cpp#L470 but keep in mind that you can't use arbitrary currents. You have to pick one of the currents that's compatible with the AXP192 (source: https://usermanual.wiki/Document/axp192xpowers.109566177):

Thanks @robalstona! That's a great pointer. I think I found the code to set the charge current (I use C++/Arduino code): https://github.com/m5stack/M5StickC/blob/master/src/AXP192.cpp#L470
I'll order the new batt, try this out and post my findings here.

Dear M5Stack community,

I would like to add a little lipo battery to the m5stickc and I'm very space-limited (i.e. I can't use the 18650 extension). So I was wondering if I could use a standard 3.7V lipo pack and hook it up to one of the ports. Specifically, there are

• EXT_VDD on the HYP2.0/grove connector
• EXT_VDD on the HAT (5V ->)
• VBUS_VIN on the HAT (5V <-)
• VBAT_Li_Ion on the HAT (BAT)
• VBUS_USB on the USB-C port
  (according to the schematic: https://m5stack.oss-cn-shenzhen.aliyuncs.com/resource/docs/schematic/Core/M5StickC/m5stickC.webp)

Now to which one of these can I connect a lipo battery and have it recognized by the AXP? Or do I have to desolder the internal battery for that to work?

Danke @felmue ,

Your answer pointed me in the right direction.
The voltage between GND and BAT pins was indeed 4.0V+, so I went back to check my code.

I noticed that the official documentation for the AXP on the M5StickC only mentions M5.Axp.GetVbatData() (which is what I used and what gave me weird readings) but the AXP code example uses M5.Axp.GetBatVoltage() and the latter actually gives me the correct number.

What's the difference between these two? Is it just the *1.1/1000?

Cheers,
Flo

Hey folks,

I'm having a bit of an issue here. I have both a M5StickC and a M5StickC Plus and I wrote a simple little Arduino program that displays both the battery voltage and also the percentage charged.
My problem: they don't charge to more than 3.7V. I would like them to be topped off at 4.2V

I tried:

• charging with my PC (5V/0.5A) / charging with my phone charger (5V/3A)
• charging while turned off / charging while turned on
  Nothing seems to do anything.

In all the above cases, I'm charging it via USB-C. If I'm supposed to charge it via the pins, let me know, haven't tried that yet

Any help appreciated,
Flo

EDIT: The problem was that I didn't read the voltage correctly. I was using M5.Axp.GetVbatData() / 1000.0 and that's off by a factor 1.1. The correct way to read the battery voltage is M5.Axp.GetBatVoltage().

How do you guys track the amperage draw of the stick when sleeping? Do you take it apart, cut a power line, hook up a multimeter in between, and later resolder?

Also have you guys experimented with the difference between
M5.Axp.DeepSleep();
and
esp_deep_sleep_start();?

AFAIU Axp.DeepSleep is only supposed to turn off "external devices" (gyro, screen, etc.) but not the ESP itself, right?

And @Morris45 what does the Wire.begin(32, 33); line do in your code above? And why are we commenting out M5.begin()? What's the battery life you're getting from your watch? I'm trying to do a simple infrared remote (that only wakes up when you first pres the main button) and even in full deep sleep, I'm only getting 2 days.