4th February 2022
An Overview of a Microcontroller Unit (MCU) 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 Microcontroller Unit (MCU) of a 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 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 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:
- Hall Effect Current Sensor
- Power Supply
- Real-Time Clock and Calendar (RTCC)
- GPIO Connector
- CAN Connector
- High Voltage Interlock Loop
- Insulation Monitoring Device (IMD)
- Voltage Measurement Port
- 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 proportional to that current. That voltage can be measured directly, as shown in the figure below.
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 their output is at a high level, unlike the signal of current shuts. A supply of 5V bidirectional can power them (can see both charging and discharging current).
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. Based on that, their output can be referenced to 0A at 2.5V.
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.
The contactor is a functional block that connects and isolates the battery pack between the load and the charger. Generally, a battery pack system has three different contactors:
There will be only one contactor for charge and discharge in some applications like energy storage systems. The BMS controls the switching of the contactor through a driver circuit. The BMS will give energizing signals to the driver circuit, which energizes the contactor for the operation. The contactor selection is made based on the charge/ discharge currents. We can go for a MOSFET-based solution for low current and low voltage applications instead of the contactor.
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 behaviour and safeguard the battery from potential damage.
ION’s cloud-connected battery analytics platform “Altergo” 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.
Bluetooth enables a wireless mode of communication with the BMS. ION-Lens, ION Energy’s application, 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. A 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 correctly 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, 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 emergency action.
12. ISO-SPI Channel
ISO-SPI channel is meant for internal communication between the master (control circuit) and slave (sensor circuit).
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About ION Energy
ION Energy is an advanced battery management and intelligence platform. We’re focused on building technologies that improve the life and performance of lithium-ion batteries that power electric vehicles and energy storage systems.
Our customers typically choose ION Energy because of its reliability, transparency, commitment towards customer success, and innovative business models. OEMs and battery pack manufacturers across the globe choose ION’s integrated battery management solutions to continuously improve the life and performance of the battery.