20th May 2020

An Overview of the Essential Functional Blocks of the BMS

(L) FS-XT-BMU;  (R) FS-LT

Introduction

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 each functional block of ION’s BMS!

The Microcontroller Unit (MCU)

The MCU is the brain inside the BMS. An MCU captures all the data from the sensors through its peripherals and processes the data based on the configuration file of the battery pack to make appropriate decisions.

 Click on the image to enlarge.

The MCU has the following functions:

  • Monitor the battery
  • Protect the battery
  • Estimate the battery’s state
  • Maximize the battery’s performance
  • Data logging
  • Report to users and/or external devices through communication channels

For cell safety, the MCU has the following functionality:

  • Prevent any cell from going into an overvoltage situation inside the battery pack. This is achieved by stop charging (giving a turn-off signal to the contactor)
  • Prevent the temperature of any cell from exceeding the upper threshold limit by reducing/stopping the current or by activating the cooling system in the battery pack. This protects the battery from thermal runaway, which is a safety issue.
  • Prevent any cell from going into an under-voltage situation by limiting/stopping the discharge current.
  • Protect the battery pack from short circuit and overload situations by opening the contactors.

Essential functional blocks of the BMS connected to the MCU

The MCU is connected to multiple functional blocks, such as:

  1. Hall Effect Current Sensor
  2. Power Supply
  3. Contactors
  4. SD-Card
  5. Real-Time Clock and Calendar (RTCC)
  6. GPIO Connector
  7. CAN Connector
  8. Bluetooth
  9. High Voltage Interlock Loop
  10. Insulation Monitoring Device (IMD)
  11. Voltage Measurement Port
  12. ISO-SPI Channel

1. Hall Effect Current Sensor

A Hall effect sensor is placed inside the magnetic field produced by a cable that carries the pack current. It produces a voltage that is proportional to that current, that voltage can be measured directly as shown in the figure below.

hall effect current sensor

Hall effect sensors are characterized by the following:

  • The current reported by a Hall effect sensor remains accurate over time and temperature.
  • Hall effect sensors are isolated from the pack current and therefore no isolation is needed.
  • Hall effect sensors suffer from offset at 0 current, which changes with temperature. So, even if they are zeroed at room temperature, they will report a small current when there isn’t one as they get hot or cold. Frequent calibration is required in applications that have periods of 0 current, such as HEVs.

Hall effect current sensors are modules that include their own amplifier, so, unlike the signal of current shuts, their output is at a high level. They can be powered by a supply of 5V bidirectional (can see both charging and discharging current).

Based on that, their output can be referenced to 0A at 2.5V, the output of a bidirectional sensor is bipolar—it will swing above and below ground (2.5V). The voltage output generated by the current sensor will send it back to BMS and the BMS will estimate the actual current based on the sensitivity of the current sensor.

2. Power Supply

For the functionality of the MCU, it requires some power input, which is given by an auxiliary power supply or from the battery pack itself through a DC-DC converter.

3. Contactors

The contactor is a functional block that connects as well as isolates the battery pack between the load and the charger. Generally, a battery pack system has 3 different contactors:

  • Charge
  • Discharge
  • Pre-charge

In some applications like energy storage systems, there will be only one contactor for charge and discharge. The switching of the contactor is controlled by the BMS through a driver circuit. The BMS will give energizing signals to the driver circuit, which energizes the contactor for the operation. The selection of the contactor is done based on the charge/ discharge currents. For low current and low voltage applications, we can go for a MOSFET based solution instead of the contactor.

4. SD-card

It can store the configuration file of the BMS and the data which is generated by the battery. ION’s BMS can store the battery data for up to 15 years. This allows the user to understand the battery pack’s behavior and safeguard the battery from potential damage.

ION’s cloud-connected battery analytics platform “Edison” makes use of this data and has the following features:

  • Advanced analytics and insight into your fleet from a single dashboard
  • Get alerted of any abnormal battery performance and take timely action!
  • Uses Machine Learning Algorithms to suggest corrective measures to prevent battery degradation and improve battery life by 40%.
  • Roll out over-the-air (OTA) updates and feature additions to the BMS over the cloud

5. Real-Time Clock and Calendar (RTCC)

RTCC is used for giving a timestamp for the data stored in the SD card. This helps the user to conduct a root cause analysis if any potential damage happened to the battery pack at any point in time.

6. GPIO Connectors

GPIO connectors give the provision of connecting extra functions to the BMS like cooling system, heating system, Ignition, sensors, etc. Enabling this functionality is done by making changes in the configuration file.

7. CAN Connector

CAN connector is peripherally connected to the MCU, which is meant for internal and external communications. Internal CAN communication is required if there are multiple BMS connected in a system. External CAN communication is generally meant for exchanging messages like Voltage, Current, Errors, etc. External CAN communication is generally used in CAN charger, CAN display, CAN data logger, etc.

8. Bluetooth

It is a wireless mode of communication with the BMS. ION has an application called ION-Lens, which is the companion Android application used to interface remotely with ION Energy BMS via Bluetooth. Through ION Lens’ users can visualize all-important battery information like voltage, current, temperature, SOC, SOH, errors, etc.

9. High Voltage Interlock Loop

Interlock PWM loop mechanism used to detect tampering or opening of the high-voltage equipment or service disconnect switch. High-voltage interlock loop (HVIL) determines if a high-voltage system, such as a power source (e.g., vehicle battery), a load (e.g., vehicle motor), and conductors therebetween, have been properly connected. If not, the battery contactor is not allowed to be closed or, if already closed, it will give a command to open it.

10. Insulation Monitoring Device (IMD)

IMD generally comes in EVs and ESS applications and it will ensure electrical safety and reliability in electric vehicles equipped with a high-voltage battery pack. IMD continuously monitors the insulation resistance in the system between the phase conductor and the ground during charging or driving.

11. Voltage Measurement Port

The terminals are connected across the contactors, which are primarily used for weld check, battery total voltage measurement, and voltage based pre-charge. For high voltage and high current applications, there is a chance that the contractor may get welded.

In that scenario, the contactor is not allowed to be open even if the BMS gives an opening signal. So the BMS identifies that the welding has occurred in the contactor and sends an alert/error message over CAN to external devices for taking an emergency action.

12. ISO-SPI Channel

ISO-SPI channel is meant for internal communication between the master (control circuit) and slave (sensor circuit).

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:

  1. Voltage Sensing Channel
  2. Temperature Sensor
  3. GPIO Connectors
  4. Balancing
  5. ISO-SPI Channel
  6. Analog Filtering
  7. 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.

3. Balancing

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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