Fuel-Gauge ( BQ27421 )

Contents

• Introduction

• Fuel-Gauge Functions

• Battery Info Fields

• Example

Introduction

The Fuel-Gauge feature is supported for board shields that have the BQ27421 fuel gauge IC on board. If present, the fuel-gauge firmware interface component will be activated and built with the final firmware image.

This interface component utilizes the open-source driver for this fuel gauge which can be found at this git repository https://github.com/svcguy/lib-BQ27421/tree/master

I2C Bridge Architecture

The fuel gauge implementation uses the MicroPython I2C Bridge architecture for ESP-IDF v5.4+ compatibility:

• Driver: BQ27421 fuel gauge IC communicates via I2C

• Device Address: 0x55 (7-bit I2C address)

• Bridge: mp_i2c_bridge component provides a C interface to MicroPython I2C

• I2C Pins: SCL and SDA are configurable (defaults in fuel_gauge.config)

• Frequency: 100 kHz I2C bus frequency

Fuel-Gauge Functions

• fuel_gauge.init() initializes the module with given battery parameters. It accepts the following optional keyword arguments:

  fuel_gauge.init( designCapacity_mAh=<int>,
                   minSysVoltage_mV=<int>,
                   taperCurrent_mA=<int> )
  

  • designCapacity_mAh (int, default: 1200) - the design capacity of the battery in mAh.

  • minSysVoltage_mV (int, default: 0) - an optional system minimum operating voltage in mV. Pass 0 if not specified.

  • taperCurrent_mA (int, default: 0) - an optional taper current detection threshold of the charger (including charger tolerances). The charger will stop charging below this value. Pass 0 if not specified.

  Raises OSError if the fuel gauge cannot be initialized (e.g. no battery connected).

• fuel_gauge.deinit() deinitializes the module. Before using the module again it has to be re-initialized using fuel_gauge.init().

• fuel_gauge.info() reads current information from the fuel gauge IC and returns a named tuple with all battery parameters. See Battery Info Fields for the complete list of returned fields. Raises OSError if the module is not initialized or the battery is not present.

• fuel_gauge.print() reads and prints all fuel gauge information in a human-readable format to the console output.

Battery Info Fields

The named tuple returned by fuel_gauge.info() contains the following fields:

Field	Type	Unit	Description
voltage_mV	int	mV	Current battery voltage
current_mA	int	mA	Current draw (positive = charging)
temp_degC	float	°C	Battery temperature
charge_percent	int	%	State of Charge (SOC)
health_percent	int	%	State of Health (SOH)
designCapacity_mAh	int	mAh	Battery design capacity
remainingCapacity_mAh	int	mAh	Current usable capacity remaining
fullChargeCapacity_mAh	int	mAh	Current full charge capacity
isCritical	bool	—	Battery voltage critically low
isLow	bool	—	Battery voltage low
isFull	bool	—	Battery fully charged
isCharging	bool	—	Battery is currently charging
isDischarging	bool	—	Battery is currently discharging

Example

import fuel_gauge

# initialize with a 1500 mAh battery
fuel_gauge.init(designCapacity_mAh=1500)

# print formatted battery status to the console
fuel_gauge.print()

Example output of fuel_gauge.print():

Voltage               4200 mV
Current               62 mA
Temprature            30.10 degC
Charge State          100 %
Health State          91 %
Design Capacity       1200 mAh
Remaining Capacity    497 mAh
Full Charge Capacity  497 mAh
is critical           no
is low                no
is full               no
is charging           yes
is discharging        no

Using fuel_gauge.info() for scripting:

import fuel_gauge

fuel_gauge.init()

info = fuel_gauge.info()

print(f"Battery voltage : {info.voltage_mV} mV")
print(f"Charge          : {info.charge_percent} %")
print(f"Health          : {info.health_percent} %")
print(f"Temperature     : {info.temp_degC} degC")
print(f"Remaining       : {info.remainingCapacity_mAh} mAh")

if info.isCharging:
    print("Battery is charging")
elif info.isDischarging:
    print("Battery is discharging")

if info.isCritical:
    print("WARNING: Battery critically low!")

# cleanup
fuel_gauge.deinit()