I'm curious about how long the capacitor can run the clock when fully charged and I have disabled charging again. But my device spends most of it's time on USB power so the duration is hard to measure; so lets try to estimate it :-)

If the frequency out and I2C is disabled (inien = True) the device should typically draw 0.3 µA when on backup battery (table 5 in the datasheet).
The capacitor is possibly a ML414H; 0.07 F.
Table 3 in the datasheet says that the clock oscillator minimum voltage is 1.1v, and the start voltage is VStandby; 3.3v. So we just need to work out the discharge time.

Start with working out the equivalent resistance at 3.3v; R=Voltage/Current = 3.3/0.0000003 = 11000000 = 11 MΩ

Now use an online discharge calculator to do the 'heavy lifting'...
https://3roam.com/capacitor-discharge-time-calculator/
(I like that site; it shows the maths used)

This gives an estimated! time of 845932 seconds; which is ~235 hours, almost ten days. Which seems reasonable for a capacitor backup.

FYI: I ran a test and after charging the RTC battery / supercap for a couple of minutes, setting a start time, then removing all other power sources and checking after about 7 hours later, the time in the RTC was still correct.

Thanks
Felix

RTC supercapacitor

Once again, thankyou, :-) I am getting cynical, it never occurred to me to just check the specs. doh!

It's a supercapacitor, which explains what I'm seeing. the chip loses time if power is off (fully off, no main battery or usb power) for more than a few minutes. I was hoping that is because it is a supercapacitor and needs a charge, but reluctant to turn charge on if it was just a dud silverNickel cell.

For context: I'm writing a micropython driver for this chip and testing / debugging on my Tab5. I need to go to the datasheet and work out the correct bits to set in my driver. That link to the M5Unified library is really appreciated too!

@sam_20 :
The rx8130ce has built-in fault detection for the backup battery. So it makes sense for M5 to enable it in the M5Unified init() routines. I will be working out (or just copying..) the correct settings myself and setting up my boot.py to enable 'charge' for this as well as the main battery at boot. My driver is generic, it will support all of the 8130's power modes and flags. It will default to charge off because that is safest.

hmm, I don't think I understand you statement. Doesn't every electronic part that's faulty need to be swapped? What am I missing here?

Thanks
Felix

it is listed in the specifications here.

RTC Chip: RX8130CE (supports timed interrupt wake-up), RTC supercapacitor specification: 70000μF/3.3V, size Φ4.8×1.4mm

Edit: If I am not mistaken RTC battery charging is enabled with this line in M5Unified library.

Thanks
Felix