Battery health-related tweaks

[pastavibescameo]

Motivation:

It's commonly considered bad for a lithium-ion battery's long term health to be charged up to, or left at, a high level of charge for an extended amount of time (say, if the hiby is commonly used plugging in as a USB DAC.) The consensus seems to be to keep the battery within a charge level from 20 to 80% for the least degradation.

Conveniently, the R1's PMU, the X-Powers AXP2101, offers an I2C interface to adjust different charging parameters. Doubly-conveniently, the R1 also bundled in the tools needed to read and write to the I2C bus directly.

Disclaimer: I didn't go to battery school; so I had Claude, the fancy text completion engine, doing the preliminary research for this. I have since done my due diligent to double check its claims to the datasheet and tested on my own R1. As far as I can tell, these mods (probably) won't turn yours into a pipe bomb.

Posted here since I feel this need more investigating & testing.

References:

Datasheet: https://files.waveshare.com/wiki/common/X-power-AXP2101_SWcharge_V1.0.pdf
(see section 6.13.2 for detailed descriptions on what each registers do)

Common syntax

At least on my device, the PMU is on bus 0 at address 0x34. We have to force the read/write with -f, else it would fail:

# Over adb shell

# Read
i2cget -f -y 0 0x34 0x<RegisterIndex>

# Write
i2cset -f -y 0 0x34 0x<RegisterIndex> <HexValue>

Tweaks

I have tried the following:

Lower voltage cutoff threshold:

REG 64 holds the battery voltage threshold where charging stops. Default value upon boot is 0x05 or 101 - which correspond to 4.4 Volts as per the datasheet above.

Changing it to 0x03 (011 or 4.2 Volts) seems to prevent the battery from going above 83%.
And setting the limit to 4.1V (0x02) stops charging at at roughly 72%.

Example:

adb shell i2cset -f -y 0 0x34 0x64 0x03

Have not tested with any other values.

Slower charging speed:

NOTE: I have not verify this imperially. As in: I didn't bother busting out the timer and A/B-test it through.

REG 62 controls the constant current charge current limit. Default value reads 0x0d (1101/700mA). Changing the value to 0x06 or 0x04 should limit the current to a max of 150mA and 100mA, respectively.

Example:

adb shell i2cset -f -y 0 0x34 0x62 0x06

Making the changes permanent:

The values gets reset once the R1 fully shutdowns. So I added these at the top of /usr/bin/hiby_player.sh:

# ...

# Stop charging when battery is at 4.2 volts instead of 4.4
i2cset -f -y 0 0x34 0x64 0x03
# Limit charge current to 150mA
i2cset -f -y 0 0x34 0x62 0x06

# ...

There are probably better places to inject these.