Battery Fuel Gauge Fix

Overview

Battery Fuel Gauge Fix (authored by DuduSki) is a low-level battery calibration module designed to eliminate discrepancies between Android’s status bar battery percentage and the physical battery management system (BMS).

On devices powered by Qualcomm Snapdragon platforms, the Power Management Integrated Circuit (PMIC) maintains a dedicated Battery Monitoring System (BMS). The hardware BMS measures charge entering and exiting the cell via a precision sense resistor (coulomb counting). However, Android’s framework daemon (BatteryService) applies complex smoothing algorithms that can become desynchronized after custom ROM installations, kernel swaps, or aging battery health.

This desynchronization leads to frustrating issues: phones suddenly turning off while displaying 15% charge, or charging indicators remaining stuck at 100% for hours before plummeting. Battery Fuel Gauge Fix bypasses framework smoothing by binding the UI directly to kernel BMS telemetry.

Technical Architecture & How It Works

The module establishes a supervisory daemon during the late_start service stage:

1. Hardware Register Discovery

The module inspects the kernel power supply class hierarchy in /sys:

# Primary Qualcomm BMS raw capacity node (measured in basis points: e.g., 8500 = 85.00%)
/sys/class/power_supply/bms/capacity_raw

# Secondary fallback hardware capacity node
/sys/class/power_supply/bms/capacity

# Real-time charging state node
/sys/class/power_supply/battery/status

2. Synchronization Loop

1. Telemetry Sampling: The background daemon divides the raw integer from capacity_raw by 100 to extract the true single-percentage integer value.
2. Framework Alignment: When a change in true capacity is detected, the script updates Android’s battery state machine via standard shell IPC:

   dumpsys battery set level <calibrated_level>

3. Adaptive Power Scheduling:
   • Discharging: The daemon sleeps for 20-second intervals to minimize CPU wakeups and prevent idle drain.
   • Charging: The polling interval shortens to 5 seconds to provide smooth, responsive progression as the battery charges.

3. Native Binary Acceleration

In addition to the POSIX shell monitor in service.sh, the module packages an optimized native binary bat_capacity_fix. On supported kernels, this executable hooks directly into kernel sysfs events, eliminating shell invocation overhead entirely.

Verification & Manual Testing

To verify that your kernel supports the BMS sysfs nodes utilized by the module, run:

su -c "cat /sys/class/power_supply/bms/capacity_raw"

If your device returns an integer (e.g. 7842 for ~78.4%), the Qualcomm hardware fuel gauge is accessible and active.