> Metis Engineering has unveiled the beta version of its production battery safety sensor, designed to monitor the condition of a lithium-ion battery and detect cell ventilation, an early sign of catastrophic battery failure. Metis Engineering managing director Joe Holdsworth said the team plans to make one or two minor changes before signing off on the final production battery safety sensor specification later this year.

Beta versions of the production battery safety sensor are the first of the new simplified design, optimized for manufacturing in the high volumes required by automotive OEMs. The units are already in trial with a number of electric vehicle manufacturers and battery manufacturers. Initial feedback has been unanimously positive, according to Metis.

The sensor is designed to sense a range of environmental parameters including volatile organic compounds (VOCs), pressure changes, humidity, dew point and also has an optional accelerometer to record shock loads . This data can be used to cross-check with other inputs, such as cell temperatures, to verify cell ventilation.

The sensor relays data through a configurable CAN interface to a control unit, such as the vehicle’s ECU, to alert the driver that cell ventilation has occurred. The sensor can also initiate the circuit breaking process to the battery, giving it the opportunity to cool itself in an effort to prevent thermal runaway.

Limits of current battery management systems. EV battery packs will already be equipped with a battery management system (BMS). As well as managing the charging and discharging of packs, they also attempt to monitor battery health, although this is usually limited to temperature sensors, one for a few cells, and monitoring voltage changes.

This system works well if it is the cell with the temperature sensor that goes bad, but if the cell is some distance from the sensor in the pack, at the time the sensor registers the temperature change, if any, it would most likely be too late. Detecting cell problems through voltage fluctuations can also be difficult in the short term, as other cells in parallel may be sustaining the voltage, thereby masking a cell’s problems.

The Metis Battery Safety Sensor typically detects breakdown within seconds.

There are several stages for a cell to enter thermal runaway. First, there is an increase in temperature, often caused when a battery is overcharged, has a manufacturing defect, or ages. The increase in temperature leads to an increase in pressure inside the battery cell. Eventually, this pressure, if high enough, causes the sealed battery cell to vent. If the load is not removed from the cell, thermal runaway can occur.

Cell ventilation

If a battery fails after being vented, it can catch fire in an auto-oxidation reaction, which will most likely result in a runaway reaction, triggering the cells around it and resulting in the total loss of the vehicle.

Thermal runaway

The Metis Battery Safety Sensor also includes an optional accelerometer, so it can monitor shock loads up to 24G and how long the battery may be impacted.

If the electric vehicle is involved in a collision, the sensor will indicate if the pack has been subjected to loads above safe levels and for how long. This can inform decisions about whether the battery requires service, a second life in a home or industrial energy storage system (ESS), or sent for recycling and any subsequent insurance claims.

Batteries heat up when charged or discharged. To prevent overheating, most packs are cooled in some way. If cooled below room temperature, they may drop below the dew point (the temperature at which air cannot transport moisture and condenses on cooler surfaces), which could result in short circuits and thermal incidents. The Metis Battery Safety Sensor will monitor the dew point in the battery and give a warning before condensation settles on the battery terminals.