BMS

Integrated battery management system for 4-series li-ion packs (based on the BQ76920 from Texas Instruments).

• programmable over/under voltage protection for each cell
• programmable short-circuit detection
• auto cell balancing
• cell voltage monitoring
• coulomb counting for accurate state-of-charge tracking
• status available over BLE

Hardware

Full schematic and PCB layout available in the hardware directory.

• TI BQ76920 analog front-end
• Nordic nRF51822 ARM Cortex-M host controller
• FETs for enabling or disabling charge/discharge
• shunt resistor for current measurement

Firmware

The host controller firmware is based on the Zephyr RTOS. It communicates with the BQ76920 via I2C and handles various error states that may occur in the BQ76920. It also exposes a BLE peripheral for wireless monitoring.

Build

Requirements:

• GNU ARM Embedded Toolchain
• OpenOCD
• Zephyr RTOS (included as submodule)
• elftools

Setup:

• git submodule update --init
• ./setup.sh

Compile and flash:

• make
• make flash

Configuration

There are various configuration options which can be specified via Kconfig. Run make menuconfig to browse the available options and override any of the default values.

• CONFIG_BMS_BOOT_DEVICE (default: GPIO_0)
• CONFIG_BMS_BOOT_PIN (default: 21)
• CONFIG_BMS_ALERT_DEVICE (default: GPIO_0)
• CONFIG_BMS_ALERT_PIN (default: 22)
• CONFIG_BMS_BLINK_DEVICE (default: GPIO_0)
• CONFIG_BMS_BLINK_PIN (default: 18)
• CONFIG_BMS_OVP_ENABLE (default: 3550 mV)
• CONFIG_BMS_OVP_DISABLE (default: 3350 mV)
• CONFIG_BMS_UVP_ENABLE (default: 3000 mV)
• CONFIG_BMS_UVP_DISABLE (default: 3100 mV)
• CONFIG_BMS_SCD_DELAY (default: 10000 ms)
• CONFIG_BMS_OCD_DELAY (default: 10000 ms)

BLE Protocol

The BMS implements a Bluetooth Low Energy peripheral for monitoring cell voltages, current draw and state-of-charge. All status data is part of a single characteristic.

Peripheral UUID: 8D9D7800-5B61-412A-AB71-5C7E0E559086

Characteristic UUID: 8D9D7801-5B61-412A-AB71-5C7E0E559086

Value structure (MSB to LSB):

• status [1 byte]
• cell 1 (mV) [2 bytes]
• cell 2 (mV) [2 bytes]
• cell 3 (mV) [2 bytes]
• cell 4 (mV) [2 bytes]
• current (mA) [2 bytes signed]
• charge used (mAs) [4 bytes signed]
• state of charge (percent) [1 byte]