Seeking empirical data and configuration paths for charge termination behavior and charge limiting on Redmi Note 9 Power (Snapdragon 662, ₆₀₀₀ mAh)

Objective

Characterize how MIUI (Android 10/11/12 builds) terminates charging on this model across different power protocols (5V/2A, QC 3.0 18W, USB-PD 18W).
Determine whether this platform exposes reliable, non-hacky mechanisms to cap charge at 80-90% (to improve cycle life) via kernel/HAL/sysfs/vendor properties, and how these interact with MIUI’s power/thermal stack.
Compare stock MIUI vs custom ROMs/kernels in terms of: termination current, float voltage, taper behavior, and re-charge hysteresis near 100%.

Background and hypotheses

Many Qualcomm SMB5-based chargers (PM6150 family + SMB13xx) expose sysfs for float voltage, taper/termination current, and step/JEITA charging. Xiaomi sometimes gates or overlays these via power HAL or thermal-engine policies.
On several Xiaomi mid-range devices, the battery driver may present nodes such as:
- /sys/class/power_supply/battery/voltage_now, voltage_max, constant_charge_current_max
- /sys/class/power_supply/battery/charge_full, charge_counter, capacity, health
- /sys/class/power_supply/battery/charge_stop_level or charge_control_limit[_max] (when implemented)
- /sys/class/power_supply/usb/input_current_limit or /sys/class/power_supply/main/…
Charge termination typically occurs when current tapers below a threshold (e.g., C/20-C/50) near float voltage. Observed behavior can vary by charger type and thermal headroom.

Questions for owners/developers
1) Termination characteristics

What final float voltage (voltage_now near 100%) do you observe on stock MIUI? Does it settle around 4.35 V nominal (common on high-voltage LiPo) or lower (e.g., 4.2-4.32 V)? Any variance by firmware build?
What termination current do you see at the moment capacity flips from 99% to 100% (derived from input_current_limit or battery current nodes such as current_now, if exposed)?
Does MIUI re-trigger charge cycles between 98-100% when idle on charger (i.e., hysteresis width and frequency)?

2) Protocol/charger dependency

Compare behavior on:
- 5V/2A USB-A (dumb)
- QC 3.0 9V profiles (Xiaomi 18W)
- USB-PD 9V 2A PPS-off (if any)
Does float voltage or termination current differ by protocol? Any thermal throttling artifacts (e.g., stepping down to 5V under sustained heating)?

3) Charge limiting interfaces

Do you have charge_stop_level or charge_control_limit exposed under /sys/class/power_supply/battery on this device with:
- Stock MIUI kernel
- Custom kernels (CAF-based) or ROMs (LineageOS, ArrowOS, etc.)
Any effective vendor properties for limiting SOC (examples seen on other Xiaomi devices: persist.vendor.smart_charge_limit, persist.vendor.charge_stop_level), and do they persist across reboots/OTAs?

4) Thermal/Power HAL interplay

Does thermal-engine.conf (or vendor thermal XML) impose aggressive tapering or early termination under modest surface temps? Any differences between “Balanced” vs “Performance” power modes in MIUI affecting charge current caps?
On custom ROMs using AOSP power HAL, do you observe higher stability in taper and less oscillation near 100%?

5) Battery health telemetry

Is charge_full and (if available) charge_full_design readable? What degradation percentages are seen on multi-year units?
Any accessible cycle count nodes or vendor BMS stats (often under /sys/class/power_supply/bms or /sys/devices/platform/soc/…/power_supply/)?

Suggested measurement methodology (non-invasive)

From 70% to 100%, screen off, airplane mode:
- Log every 30-60 seconds:
- capacity, voltage_now, current_now (signed), temp
- usb/main input_current_limit and voltage
- status, health
- Note charger type and ambient temperature.
Sample shell snippet (adjust paths as present on your build):
while true; do
date +“%F %T”
for n in capacity voltage_now current_now temp status health; do cat /sys/class/power_supply/battery/$n 2>/dev/null; done
for n in input_current_limit voltage_now online; do cat /sys/class/power_supply/usb/$n 2>/dev/null; done
sleep 60
done
Optional: correlate with logcat tags from healthd/batterysvc and check for thermal events.

What to share

ROM/firmware build (MIUI version and Android base), region variant, bootloader status (locked/unlocked), kernel (stock/custom).
Charger model and protocol negotiated (as reported by sysfs or a PD/QC analyzer if available).
Final float voltage observed and inferred termination current.
Presence and effectiveness of any charge limit controls (sysfs paths, app/kernel hooks).
Any kernel source references confirming the charger IC and BMS driver used on this device.

Goal

Build a reproducible profile of the Redmi Note 9 Power’s charging behavior and identify reliable, user-accessible ways to cap SoC for longevity-ideally without breaking OTA or requiring persistent root-plus document any ROMs/kernels that already implement proper charge limiting on this platform.