PowerHab - Lithium Ion Battery Management System PrototypeFollow article
This article highlights the design and manufacturing of the PowerHab student teams Lithium-Ion battery management system prototype as well as an explanation of its operation. Battery management systems are used to monitor battery cells and provide safety in order to prevent thermal runaway events from occurring. Therefore, PowerHab presented the BMS prototype at the IGLUNA 2020 Virtual Field Campaign to various experts from the space and engineering community.
RS Components provided the PowerHab team with a home electronics laboratory throughout the COVID-19 pandemic to allow for prototype manufacturing throughout lockdown. Several IGLUNA teams struggled to produce all their planned prototypes for the field campaign and had it not been for PowerHabs sponsorship with RS Components, the likelihood would have been the same with no physical prototypes produced for the field campaign in 2020. However, as RS provided PowerHab with a home laboratory consisting of all the required equipment and components, this allowed prototype manufacturing to continue. An image of the home laboratory provided can be seen below.
Being able to use a laboratory at home provided many advantages compared to university provided lab spaces. These included advantages such as the quality of equipment being significantly better, the quick ease of access, being able to organise the lab to the specific needs of the project and the ability to work out of university hours. The ability for the team to still bring a manufactured prototype played a large part in the success of the PowerHab team and so the partnership with RS Components proved invaluable.
Prototype Design and Manufacture
The BMS prototype brought to the IGLUNA field campaign was designed to provide a small-scale demonstration of what is required to protect and manage cells correctly. As such, the prototype consisted of a battery pack capable of combining cells in series, an emergency stop button which allows for the battery pack to be disconnected from the BMS in the event of a fault or a short circuit and battery management circuitry capable of monitoring cell voltages and providing cell balancing.
The battery pack contains 3 lithium Ion Panasonic NCR18650BF cells all arranged in series and provides output wires at the positive terminal of each cell to allow for their individual cell voltages to be measured. This is shown in the following figure.
As can be seen, the battery pack provides outputs for cell 1 (brown), cells 1 and 2 (yellow) and cell 1, 2 and 3 (green) which should provide measurements of approximately 3.7 V, 7.4V and 11.1V respectively. These outputs allow for the BMS to determine individual cell voltages by using the measured cell 1 voltage to calculate cell 2 and then cell 3 voltages. This is a key fundamental of battery system management and so all measurements were verified both on the BMS and using a multimeter and oscilloscope. The actual manufactured battery pack can be seen in the figure below.
The battery pack was originally made using nickel strips which were soldered directly onto the cell terminals. This was found to be extremely difficult to achieve and resulted in loose connections on the output wires. So, a second battery pack was manufactured using a standard 3 series 18650 cell pack with output wires soldered to the wire connecting the cell terminals which was found to be much easier and safer to manufacture. The second battery pack functioned correctly and provided the expected output voltages
Emergency Stop Button
Emergency stop functionality was implemented on the advice of experts at SSC and ESA to allow for the battery pack to be connected and disconnected quickly as well as ensuring a user is not required to touch live wires when connecting/disconnecting the pack. The emergency stop button was connected across the 11.1V and Gnd output and can be seen in the following diagram and image.
In an ideal BMS design, emergency disconnect functionality would be controlled by the BMS itself however for safety of the user, a manual option was determined to be best.
The BMS circuit design consists of several elements which allow it to accurately monitor and balance cells, shut off charge/discharge currents and prevent a thermal runaway event from occurring. The BMS was manufactured firstly on a breadboard to allow for easy fixing and debugging and then secondly on Veroboard to make the design safer and more portable. The two manufactured prototypes can be seen below.
In order to understand the operation of the circuitry elements involved, a highlighted diagram of prototype 1 as well as element descriptions are provided as follows.
The chosen microcontroller was an Arduino Nano which allows for a monolithic BMS design and can provide monitoring of up to 4 cells in series. It also provides analog, digital and PWM signal pins as well as being capable of being powered solely by the battery pack to make it a stand-alone design.
An LCD screen was implemented in order to display the individual cell voltages as well as a mode status update as to whether the BMS is implementing cell balancing.
Voltage Sensing Circuitry
This circuitry sends the battery cell output voltages through a resistor voltage divider and into the Arduino to allow for the cell voltages to be read. The voltage division ensures that no potentially damaging high voltages are input to the Arduino.
Cell balancing circuitry
This circuitry comes into effect if any of the measured cell voltages are found to be at unsafe levels. The voltage limit can be set when programming the Arduino and if any cells are found to have a voltage above said limit, the cell will begin discharging through a logic level MOSFET. The excess charge of the cell will either be redistributed amongst the remaining cells or dissipated into the circuit ground. LEDs are used to indicate whether a cell is discharging.
Input Current regulator
This element was unfortunately not tested for the field campaign due to a lack of time in order to source a specific lithium-Ion cell charge. This circuitry is responsible for allowing current to flow into the battery pack if it needs recharging and to prevent current flowing if the battery is at full charge or in an unsafe state. This element will hopefully be demonstrated at future campaigns.
The following tests were carried out to verify the functionality of the BMS.
Multimeter readings taken for each cell were recorded, compared to the displayed BMS readings and documented in the following table.
Multimeter Reading (V)
BMS Reading (V)
As shown, the results were within +/- 0.1V of the actual values demonstrating the accuracy of the BMS and that it is monitoring the cell voltages correctly. The figures provided below highlight the recorded cell voltages both on the LCD screen and digital multimeter proving the readings are correct.
BMS Cell Calibration Test – LCD Display highlights recorded cell voltages
BMS Cell Calibration Test – Multimeter recorded cell voltages for verification
Cell Balancing Test
Another test carried out was cell balancing in which one cell voltages was offset within the programming to read 4.31V. The corresponding cell discharge circuitry came into effect and the corresponding LED light up. This indicates power flowing through the circuit and so it can be verified that the “overcharged cell” is now discharging down to 4.2 V demonstrating cell balancing. Therefore, the test was passed.
BMS Cell Balancing Test – Cell 3 discharge LED is illuminated, and LCD displays it is in discharge mode
Emergency Disconnect Test
The emergency stop functionality was tested in order to ensure the battery disconnects from the circuit properly and verifies that the safety elements are in place. When the emergency stop was activated, the BMS would power down and no voltage would be recorded in the circuitry demonstrating a cell pass.
VFC Presentation Video
The BMS prototype was successfully demonstrated at the IGLUNA virtual field campaign via video and viewed by members of the Swiss Space Center, European Space Agency and interested members of the public. The PowerHabs team presentation, as well as the BMS demonstration, is provided below.
It is important to note for anyone wishing to work with lithium-ion batteries that they should take extra care when handling or working with the cells. If you wish to attempt battery management yourself then there are a few key dos and don’ts based on the experience of this project that is recommended to follow:
- Keep your cells at below 50% charge when testing circuitry to avoid the risk of overcharging
- Ensure to test circuit elements individually before assembling the full system
- Monitor your cells with a multimeter to ensure redundancy and to verify the BMS results are correct
- Have a lithium-ion fireproof bag and/or blanket in case of emergencies
- Solder directly to the battery cells as this can be extremely difficult and excessive heating can cause the cells to go into thermal runaway
- Wire your cells directly to the BMS as you will need to be able to remove the cells in the event of a circuit malfunction
- Leave use an unregulated power supply to charge the cells, specific lithium-ion approved chargers are recommended
What Comes Next
It is hoped that the PowerHab team will be able to bring similar prototypes to future field campaigns further expanding on the technology involved in battery management and battery systems. It has been the aim of PowerHab to optimise the design of our subsystems as much as possible and to provide lightweight and space efficient options. Therefore, we envision that the mass of future proposed battery energy storage systems can be reduced through designing of our own BMS circuits that may even become commercially available.