BMS Sensors

The Battery Management System relies on a network of sensors to monitor the state of the pack in real time. Without accurate sensor data, the BMS cannot protect the cells, estimate state of charge, regulate charging, or control thermal management. Every algorithm the BMS runs, from SOC estimation to fault detection, is only as good as the sensor inputs feeding it.

Sensors in the BMS context fall into two broad categories: those that measure a physical quantity directly (hard sensors) and those that estimate a quantity from a model or data fusion (soft sensors, or virtual sensors). Many modern systems use both in combination.

Sensor Summary

The table below summarises the sensor types covered in this page, the typical measurement technology used, which application domains they appear in, whether the function is safety-critical, and whether a virtual (model-estimated) alternative exists.

Cell Voltage

Cell voltage is the most fundamental measurement in any battery pack. The BMS reads the terminal voltage of each cell or group of cells continuously, using this to monitor the state of charge, detect cell imbalance, enforce over-voltage and under-voltage protection limits, and support impedance-based diagnostics.

Voltage measurement is typically performed by a dedicated cell monitoring IC (CMIC) or battery monitoring IC (BMIC), which samples each cell in the stack and reports to the main BMS controller. Accuracy requirements are tight: errors of a few millivolts can lead to significant SOC estimation error near the top or bottom of charge where the OCV-SOC curve is steep, particularly for LFP chemistries with a flat voltage profile.

Key considerations for voltage sensing:

• Measurement accuracy: typically 1-5 mV across the full temperature range
• Sampling rate: fast enough to detect transient events and support impedance measurement
• Isolation: the measurement circuit must be isolated from low-voltage control electronics in high-voltage packs
• Redundancy: some functional safety architectures require independent voltage measurement channels

Current

Pack current is measured at the main current path, typically in the negative path of the high-voltage circuit, and feeds directly into coulomb counting for SOC estimation as well as overcurrent protection and power limit calculations.

Two principal technologies are used in production battery systems:

Shunt resistors measure current by reading the voltage drop across a precision low-resistance element. They are accurate, stable over temperature, and well suited to high-accuracy SOC estimation. The four-wire (Kelvin) measurement technique is standard practice, using separate sense and current-carrying connections to eliminate contact resistance errors.

Hall-effect sensors measure the magnetic field generated by the current and are galvanically isolated from the high-voltage circuit, which simplifies integration. They are generally less accurate than shunts at low currents but avoid the power dissipation of a shunt at high continuous currents.

Some systems use both: a shunt for accuracy at low currents and a Hall-effect sensor for high-current events. Although, a single-sensor designs with a high-quality shunt are most common in automotive applications.

See Current Sensor for further detail on technology types and manufacturers.

Cell Temperature

Temperature is the second most critical measurement after voltage. The BMS uses cell temperature to enforce charge and discharge limits, control thermal management, support lifetime modelling, and detect the early stages of thermal runaway.

Temperature sensing for cells is covered in depth at Cell Temperature Sensing, but the key points from a BMS sensor integration perspective are:

Sensor type: NTC thermistors dominate production battery packs due to low cost, small size, and adequate accuracy. Thermocouples are preferred in test environments where wide temperature range and robustness are more important than cost. RTDs offer better linearity but are less common in pack applications.

Placement strategy: It is not practical to instrument every cell individually. Sensors are typically placed at representative locations, hotspots identified through thermal simulation and physical testing. The BMS using the readings to infer the thermal state of the wider group. The number and location of sensors needs to be validated against the actual thermal distribution of the pack under worst-case conditions. → How Many Temperature Sensors are Required?

Redundancy: A single thermistor is a low-cost component but can be expensive to replace once the pack is assembled. Using two sensors per zone, with the BMS cross-checking readings, is common practice. A sensor that reads outside plausible bounds, or deviates significantly from its neighbour, can be flagged as failed and handled gracefully.

Virtual sensors: Where physical sensors cannot reach, the core of a large prismatic cell, for example, model-based virtual sensors can estimate the local temperature from surface measurements combined with a thermal model of the cell. See Virtual Temperature Sensors for more on this approach.

Pack Voltage

Pack voltage is the sum of all series cell voltages and can be measured independently of the individual cell voltages. It serves as a cross-check on the cell voltage measurements, feeds into isolation resistance monitoring, and is required for certain power electronics interfaces.

In practice, pack voltage is often measured by the same hardware that monitors the high-voltage bus rather than by a dedicated BMS sensor, but the BMS must have access to this value for both protection and diagnostics.

Isolation Resistance

A high-voltage battery pack must be electrically isolated from the vehicle chassis or system ground. Isolation resistance monitoring, often called insulation resistance monitoring, checks that this isolation remains intact during operation. A degraded isolation resistance can indicate damaged cabling, a failing component, or ingress of coolant or moisture.

Isolation resistance monitoring is a safety-critical function. Regulatory requirements such as ISO 6469 and FMVSS 305 set minimum thresholds for isolation resistance in electric vehicles (typically 100 Ohm/V for the complete system). The BMS must be capable of detecting isolation faults and responding appropriately, including alerting the driver and in some cases disconnecting the pack.

See Isolation Resistance of a Pack for the measurement principles and typical threshold values.

Earth Bonding Impedance

Distinct from isolation resistance, earth bonding impedance is the resistance of the protective earth connection, the low-impedance path that ensures exposed conductive parts of the pack and vehicle are bonded to a common reference. This is a passive safety measure: in the event of a fault, a low-impedance earth bond ensures that fault current flows through the protection device rather than through a person.

Earth bonding checks are typically performed as part of end-of-line testing but may also be monitored during operation in higher-integrity systems.

Humidity

Moisture inside a battery pack is a significant risk. It can degrade electrical insulation, promote corrosion of busbars and connectors, and in extreme cases create conductive paths that compromise isolation. A humidity sensor inside the pack enclosure allows the BMS to detect ingress events. Whether from a seal failure, condensation during thermal cycling or coolant leakage and raise a fault before damage occurs.

Humidity sensors for battery applications need to be compatible with the chemical environment inside the pack and function reliably across the full operating temperature range.

Pressure and Force

Gas pressure monitoring is relevant in sealed or semi-sealed cell formats, particularly pouch cells and certain prismatic designs. Cells that are generating gas internally due to: electrolyte decomposition, lithium plating or the early stages of thermal runaway, will cause internal pressure to rise. A pressure sensor or force sensor on the cell stack can detect this swelling before it is visible externally.

In liquid-cooled systems, coolant circuit pressure is also monitored separately as part of the thermal management system rather than the BMS proper.

Gas Sensing

As a lithium-ion cell approaches thermal runaway, it first vents gases before any thermal event becomes visible. Detecting these gases early provides the maximum possible warning time for system shutdown and user alert. The primary gases of interest are electrolyte vapours (EC, DEC, EMC, DMC) in the early venting stage, followed by CO and H₂ as the event escalates.

Electrolyte Detection

During a thermal runaway event, various gases are released. In the early stage, the cell vents a high concentration of electrolyte vapor. This includes gases such as Ethylene carbonate (EC), Diethyl carbonate (DEC), Ethyl Methyl carbonate (EMC) and Dimethyl carbonate (DMC) etc.

As the event progresses to an explosion, there is an increase in H2 and CO.

Honeywell

Gas sensing is a relatively recent addition to production battery systems, driven by tightening safety regulations and the push for early warning in large-format and stationary applications. Sensor placement must account for gas flow paths within the pack enclosure to ensure gases from any cell reach the sensor before accumulating to hazardous concentrations.

See Gas Sensing for the sensor technologies and example implementations.

Coolant Temperature and Flow

In liquid-cooled packs, the BMS or its thermal management subsystem monitors coolant temperature at the inlet and outlet of the cooling circuit. The temperature difference across the pack gives a direct measure of how much heat is being removed, and together with the flow rate allows the actual heat rejection to be calculated.

Coolant flow rate sensing, typically using a turbine or ultrasonic flow meter, confirms that the pump is operating correctly and that the circuit is not obstructed or partially blocked. A loss of coolant flow is a safety-critical event that requires the BMS to reduce power limits immediately to avoid thermal damage.

In some architectures the coolant temperature and flow sensors report to a separate thermal management controller rather than directly to the BMS, but the BMS must receive this data to make correct protection decisions.

Force and Deformation

Lithium-ion cells expand and contract with state of charge and temperature, and swell progressively as they age. In module and pack designs that constrain the cells mechanically, which most do, to maintain contact pressure and structural integrity then this expansion generates measurable force. Load cells or strain gauges on the module endplates or pack structure allow the BMS to track cell swelling over life, detecting abnormal expansion that may indicate lithium plating or internal damage and confirm that the mechanical assembly remains within design limits.

Force sensing is more common in development and testing than in volume production, but its use in production systems is increasing as pack-level SOH estimation becomes more sophisticated.

Dielectric Strength of Coolant

Where a conductive coolant path passes close to high-voltage components, the dielectric strength of the coolant fluid is relevant to safety. This is particularly the case in immersion cooling architectures and in cooling plates that run close to cell terminals. Dielectric fluid monitoring, checking conductivity rather than making a direct dielectric strength measurement, can be a measure of contamination of a nominally non-conductive coolant over time. → Dielectric Immersion Cooling

Measurement, Estimation and Sensor Fusion

Not everything the BMS needs to know can be measured directly. State of charge, state of health, internal temperature, and remaining useful life are all estimated rather than measured. They are outputs of algorithms that combine multiple sensor readings with cell models and historical data. These estimated outputs are sometimes called virtual sensors or soft sensors.

The accuracy of any virtual sensor is ultimately bounded by the accuracy of the physical sensors feeding it. Errors in current measurement accumulate over time in coulomb counting; errors in temperature measurement propagate into lifetime models. Sensor calibration, both at end of line and tracked over the life of the pack, is therefore as important as the sensor specification itself.

See Virtual Temperature Sensors and Battery Management System Algorithms for more on how measured inputs are converted into estimated states.

See also: BMS Testing, Humidity Sensors, EV Current Sensors: The Basics, Thermocouple vs Thermistor, Seebeck Error: When a Sense Wire Adds Its Own Voltage