Cell Guard's faster thermal runaway detection in EVs confirmed

The tests found the Cell Guard sensor detected a thermal runaway incident seven minutes quicker than other reported evaluations.

Cell Guard is a sensor that provides accurate and detailed information on the health of a lithium-ion battery pack to improve longevity, as well as being capable of detecting cell venting, which is an early sign of catastrophic battery failure vastly reducing the risk of fire in Electric Vehicles (EV) and Energy Storage Systems (ESS).

Unlike current Battery Management Systems (BMS), which typically only measure temperature and voltage change, Cell Guard monitors a range of environmental parameters required to ensure that the battery continues to operate in optimum conditions. The sensor wakes when it detects Volatile Organic Compounds (VOCs) emitted by cell venting and pressure change. It also detects humidity, dew point, and has an optional three axis accelerometer to record shock loads.

Whilst EVs are inherently safer than Internal Combustion Engine (ICE) vehicles and the chances of fires are significantly lower, however when thermal incidents occur, they invariably make the headlines, and this impacts the adoption of this relatively nascent form of transport.

Sandia National Laboratories tested Metis Engineering’s Cell Guard sensor technology along with two competitors, one of which is designed mainly for ESS and retails at many times the price, the other, which is designed for EV applications, but only looks for hydrogen.

In one test the hydrogen sensor took a full seven minutes longer to register that there had been a thermal event, by which time thermal runaway had reached an advanced stage.

Joe Holdsworth, Managing Director at Metis Engineering, said: “We are delighted that our Cell Guard sensor has proven its mettle against two competitors in the market in an entirely independent and third-party test. Not only did our sensor detect a thermal incident in under 60 seconds, the CAN data it provides could be used to inform the driver to stop and evacuate the vehicle”.

Holdsworth continued: “The report states, hydrogen was not detected during the venting phase because hydrogen generation typically occurs during major cathode decomposition at much higher temperatures, around 200^oC. This resulted in a much shorter warning time from the hydrogen-based sensor compared to VOC sensors. Mass Spectrometry detected hydrogen just less than 1 min prior to thermal runaway, compared to the more optimal ~seven-minute warning from the VOC and Combined Gas sensors. A full seven-minute warning from the early detection of degradation markers prior to a thermal event would enable electric buses to stop, pull over and conduct a safe evacuation, as per the Global Technical Regulation on Electric Vehicle Safety #20 (GTR20).”

How does Cell Guard work?

Cell Guard aims to provide the vital extra and often overlooked data required for a more complete picture of battery pack health.

Cell venting and thermal runaway: In both an EV and ESS application, the data Cell Guard provides can be used to crosscheck with other inputs, such as cell temperatures, to check for cell venting. The sensor relays the data over its configurable CAN interface, which can be used to warn the driver there is a problem. The sensor can also trigger the process to cut the circuit to the battery pack, giving it opportunity to cool down with the objective of preventing thermal runaway.

Impact damage: Cell Guard’s optional accelerometer can monitor shock loads up to 24G and impact duration that the battery pack may experience. This information can radicalise the re-sale market of used EVs, with buyers given much needed clarity to be able to make informed decisions regarding battery pack condition, as well as its maintenance. It can also help provide far greater insight into the battery’s condition for repurposing and recycling, as well as any subsequent insurance claims.

Active pack cooling: Cell Guard can monitor the dew point in the battery pack and be used as an input into the batteries thermal management to avoid cooling the batteries to the point where condensation could settle on the battery terminals, which could lead to shorting and thermal incidents.

Water ingress: if humidity levels inside a pack are high this can indicate if water ingress has occurred, Cell Guard has a humidity sensor which allows the system to detect this.

Limitations of current Battery Management Systems

While EV battery packs already have a Battery Management System (BMS) fitted, this system may have limitations when it comes to monitoring the health of the battery pack. For example, temperature sensors may only be present for every few cells, making it difficult to detect issues in a timely manner. Detecting cell issues through voltage fluctuations can also be challenging in the short term because other cells in parallel can prop up the voltage, disguising issues with a cell. Cell Guard can overcome this limitation by supplementing the BMS and providing more detailed information about the health of the battery pack.

Developed using ISO26262 processes and certified to ISO Automotive Standards, Cell Guard is assembled in the UK under strict quality-controlled conditions to satisfy orders from a rapidly growing number of OEMs and Tier 1s where they are being used in ASIL B applications.

With Cell Guard, Metis Engineering is setting a new standard for advanced battery health monitoring in EVs. Increasing safety, reduced scrappage, reducing risk, and providing incomparable insight into battery condition will provide peace of mind for EV drivers and manufacturers alike and help promote the wider adoption of electric vehicles.