11th February 2022
An Overview of the Analog Front-End of the BMS
(L) FS-XT-BMU; (R) FS-LT
Functional blocks of the BMS (Battery Management System) comprises a control block which is the Microcontroller (MCU) and a sensing block, which is the Analog front end (AFE).
Let’s delve deeper into the Analog front end (AFE) block of ION’s BMS!
The Analog Front-end (AFE)
The AFE is an Integrated Circuit (IC) in the BMS, which is designed to wrap all the analog circuitry required for the design and operation of a BMS system up into a small package. It contains inputs to measure the cell voltages of each cell position in the cell stack. The AFE can measure 3 to 15 series cells in a battery pack in a 0-5V range.
AFE also plays a significant role in triggering the balancing circuitry. The AFE IC contains an in-built temperature sensor which is meant for measuring the BMS board temperature. The AFE has an internal small digital state machine that manages the sequential measurement of voltages present at the input to IC, along with providing the I2C interface.
Click on the image to enlarge.
The features of an AFE include:
- Measuring each cell voltage and setting it to MCU
- Measuring temperature (usually via NTC thermistor)
- Balancing circuitry for each cell
Essential functional blocks of the BMS connected to the AFE
AFE is connected to multiple functional blocks, such as:
- Voltage Sensing Channel
- Temperature Sensor
- GPIO Connectors
- ISO-SPI Channel
- Analog Filtering
- PSU AFE
1. Voltage Sensing Channel
The voltage sensing channel takes the readings from the individual cells in the pack and sends it to the AFE. It also acts as a channel for balancing the cell.
2. Temperature Sensor
NTC thermistor temperature sensors are a key component in Li-Ion battery safety. They provide critical temperature data required to keep the Li-Ion battery in the optimum condition during the charging/discharging cycle. The temperature sensor will be placed in the hotspot of the battery pack, which measures temperature in the form of voltage and the data will be sent to the AFE.
When there is an imbalance between the cell in a pack that occurs over a prescribed limit, the AFE will give a triggering signal which is controlled by MCU to the balancing circuit in the BMS.
4. GPIO Connectors
GPIO connectors give the provision of connecting extra functions to the BMS like cooling system, contactor control, heating system, Ignition, sensors, etc. Enabling this functionality is done by making changes in the firmware.
5. ISO-SPI Channel
ISO-SPI channel is meant for internal communication between slave(sensor circuit) and the master(control circuit). This is also known as the internal CAN communication channel.
6. Analog Filtering
It filters electrical noises in voltage and temperature measurements.
7. PSU AFE
It represents the power supply unit required for the functioning of AFE. Usually, it is tapped from the cell stack.
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