The BMS Definitions & Glossary is an A to Z extension to our website that just gives you an alternative way of finding information.
Active Balancing – the idea here is to redistribute the energy across the cells. Give energy from the cells with the highest SoC to the cells with the lowest SoC. This is the ideal cell balancing approach.
Active Mode – the BMS is on, communicating and monitoring all sensors.
Adaptive Observer – a state observer whose internal parameters update online as the system’s true parameters (e.g. cell resistance or capacity) drift, rather than assuming they’re fixed at design time.
ADC – Analogue to Digital Converter
AFE – Analogue Front End (the IC that measures cell voltages)
Ah – Ampere-hour is the unit of cell capacity.
ASIL – Automotive Safety Integrity Level (ISO 26262; critical for BMS functional safety)
Balancing – all about the dissipation or movement of energy between cells. The aim being to align them all with respect to state of charge. Aligning the state of charge of all of the cells in a pack will allow the pack to deliver the most energy and power. This becomes more crucial as the pack ages and differences between cells become more significant.
Battery Charge Control Module (BCCM) – the module that sits between the battery and the charger, managing the interface and control signals during a charge event.
Battery Plant Controller – the aggregation layer between the rack- or string-level BMS units and the plant’s PPC/EMS, rolling up cell-level data from every DC block into a single plant-wide state-of-charge, state-of-health and fault picture.
BMU = Battery Management Unit – an alternative name some manufacturers (LG, among others) use for the BMS in cell-level datasheets, specifically when describing the protection functions the pack-level electronics must implement around that cell rather than the vehicle-level system as a whole.
CAN — Controller Area Network (the dominant BMS communication protocol)
Capacity – cell nominal capacity is defined as the quantity of charge, in ampere hours (Ah) that a cell is rated to hold.
Cell-Level Telemetry at Scale – the data-architecture problem unique to grid-scale BESS: aggregating and storing voltage and temperature readings from many thousands to millions of cells across a plant, several orders of magnitude beyond what a single EV pack’s BMS has to handle.
Cell Matching – what level of cell matching do you do prior to assembling a battery pack? Assuming the battery pack will be balanced the first time it is charged and in use.
Cell Sensor Circuit (CSC) – the hardware that combines voltage sensing, temperature sensing and balancing for a sub-group of cells and reports back to the master in a master-and-slave BMS architecture.
Cell Voltage Delta – difference in resting voltage for cells at different points in a series string. A threshold maximum voltage difference will trigger cell balancing.
Centralised BMS – long leads are required to connect the central control unit to every cell in the pack.
Charge Curve – the profile of charge power or current against state of charge during a charging session, typically holding peak power through a low-to-mid SoC band before tapering as the cell nears full charge.
Charging Protocols – the family of control strategies (CC-CV, MCC-CV, SCC-CV, pulse, boost, variable current profile) a BMS can use to manage a charge event.
Chipsets – the semiconductor building blocks an AFE-based BMS is architected around; Texas Instruments, NXP, Analog Devices and Renesas are the four dominant suppliers.
Closed-Loop Control – control action where the output is measured and fed back to correct the input, as opposed to running from a fixed schedule with no feedback.
Closed-Loop Observer – a state observer that feeds its output error back against the real system’s measured signals, typically terminal voltage, to continuously correct its own state estimate.
Communication Architectures – the internal (isoSPI, SPI, I²C, daisy-chain UART) and external (CAN, CAN-FD, LIN) bus configurations connecting cell sensor circuits, slave boards and the master controller.
Contactor – the switch, effectively a relay, that the BMS operates to connect and disconnect the high-voltage circuit.
Controllability – a system property describing whether every internal state can be driven to a desired value by manipulating the available inputs within finite time.
Coulomb Counting – SOC Estimation by Coulomb Counting is based on the measurement of the current and integration of that current over time.
C-rate – a measure of the rate at which a battery is charged or discharged relative to its capacity. It is the charge or discharge current in Amps divided by the cell capacity in Ampere-hours. An nC-rate is the constant-current charge or discharge rate that a cell can sustain for 1/n hours. This current (in A) equates to the cell nominal capacity (i.e. C Ah) multiplied by n h−1 i.e. i=nC. For example, a fully charged 20Ah cell should be able to deliver 10A for 1/n = C/i = 20/10 = 2hrs, or the c-rate is n=i/C = 0.5h-1
CTI – Comparative Tracking Index (used in creepage/clearance material selection)
Current Derate – a reduced current charge or discharge capability.
Cutoff Voltage – the specified voltage at which a charge or discharge is terminated to protect the cell.
DCIR – Direct Current Internal Resistance is the internal resistance of the cell. This is the resistance in charge and discharge to a direct current demand applied across the terminals.
Deep Cycling – an application in which a cell or battery is successively and repeatedly charged, then fully discharged. This needs careful consideration of the upper and lower voltage limits.
De-Rate – when the charge or discharge current (power) limits communicated to the vehicle are reduced.
Diagnostics – the general BMS function that monitors sensor and system data for developing faults and reports them to the wider vehicle or system controller.
Digital Twin – a software model that mirrors a physical cell, module or pack in real time, using live sensor data and an electrochemical or equivalent-circuit model to estimate internal states that can’t be measured directly.
Discharge Rate – the rate at which electrical current is removed from a cell or battery, expressed in Amps or as a C-rate.
DoD – Depth of Discharge is equal to 1 – SoC
Dual/Joint Estimation – an estimation scheme that estimates a cell’s states (e.g. SoC) and its parameters (e.g. resistance, capacity) simultaneously, rather than assuming the parameters are already known and fixed.
DVA – Differential Voltage Analysis (used alongside ICA for SoH)
ECM – Equivalent Circuit Model is an electrical circuit designed to represent the dynamic behaviour of a battery cell using electronic components (capacitors, resistors and inductors).
EMC – Electromagnetic Compatibility
EMS – Energy Management System
Extended Kalman Filter (EKF) – a Kalman Filter variant that linearises a cell’s nonlinear voltage-SoC relationship at each estimation step, extending Kalman filtering to systems that aren’t already linear.
Fallback Modes – the reduced-capability operating states a BMS switches to when a fault or sensor loss prevents normal operation.
Fault Injection & Diagnostic Coverage – BMS testing that deliberately introduces faults at every stage of the test ladder against a requirements traceability matrix, so each fault class is provably caught at the level it needs to be.
Feedback Control – control action driven by comparing a measured output against its target and correcting for the difference.
Feedforward Control – control action calculated directly from a known or predicted disturbance and applied ahead of any measured error, rather than in response to one.
FET / MOSFET – used in balancing circuits and BMS switching
Fire Suppression Interface – the communication link between a rack-level BMS and a DC block’s fire detection and suppression system, so thermal-runaway precursor signals the BMS already sees electrically can trigger or pre-arm suppression alongside the dedicated gas sensors.
Flexible Printed Circuits (FPCs) – the hidden nervous system of high-voltage batteries, yet their reliability is often overlooked.
FMEA – Failure Modes and Effects Analysis (standard safety engineering tool)
Fuel Gauge – the colloquial industry term for the SoC-reporting function of a BMS, particularly in consumer electronics and smart-battery contexts.
Fuzzy Logic – a state-of-charge estimation approach that derives a result from multiple imprecise or conflicting inputs (voltage, current history, temperature) using weighted rules rather than a single deterministic model.
Gain Scheduling – switching between pre-tuned observer or controller gains depending on operating conditions such as SoC, temperature or C-rate, rather than using one fixed gain everywhere.
HARA – Hazard Analysis and Risk Assessment (ISO 26262 process step)
Hardware-in-the-Loop (HIL) – testing real BMS hardware (AFE, master, harness, sensors) closed-loop against a real-time plant model, with programmable power supplies and cell simulators standing in for the cells.
H-infinity Observer – a state observer designed to guarantee bounded estimation error under worst-case model uncertainty and noise, rather than assuming the noise statistics are known the way a Kalman Filter does.
Hybrid Pulse Power Characterisation (HPPC) – a pulse test protocol used to characterise a cell’s internal resistance and power capability across SoC, feeding directly into State of Power estimation.
Insulation – use of a poor conducting material to restrict flow of current to almost zero.
Insulation Monitoring Device (IMD) at DC Block Scale – isolation monitoring extended from the single-pack case an automotive BMS handles to an entire DC block with many parallel strings, where a fault has to be traced to a specific string rather than just flagged at the pack level.
Insulation Resistance – value of resistance between any two points.
IMD – Isolation Monitoring Device
Inrush Current – the brief, high-magnitude current surge at the moment of contactor closure or charge initiation, driven by the charger’s capacitive input stage or unmanaged pre-charge.
Isolation – disconnection and separation of electrical equipment from every source in such a way that the disconnection and separation is secure.
Isolation Resistance – the value of resistance, measured at a specified voltage, between a HV bus and ground.
isoSPI – isolated SPI (dominant protocol for multi-cell AFE communication chains)
Kalman Filter – an estimator of information of a system from noisy (or uncertain) directly or indirectly related measurements.
Kelvin (4-Wire) Voltage Sensing – placing a cell’s voltage-sense tap points symmetrically relative to the current path so the resistive voltage drop along the interconnect cancels out between adjacent cells, rather than showing up as a false imbalance reading.
LIN – Local Interconnect Network (lower-speed vehicle bus used in some BMS)
Logging – the BMS function that records operational data, faults and state history over the pack’s life for diagnostics and warranty purposes.
Lossless Balancing – this approach switches cells in and out of the circuit during charging. This means we have a lot of switches and that these switches have to be designed to carry the full current.
Luenberger Observer – a deterministic state observer that estimates a cell’s internal states from its measured terminal voltage and current using a fixed correction gain; the non-statistical counterpart to a Kalman Filter.
Master and Slave BMS – a slave will monitor and control a sub-set/module of cells and communicate back to the master.
MAVD – Moving Average Voltage Deviation (safety diagnostic in BMS)
Model-in-the-Loop (MIL) – testing a BMS control algorithm, still as a model, against a plant model of the cell, pack and thermal system, before any production code exists.
Model Predictive Control (MPC) – a control strategy that repeatedly solves an optimisation over a short future horizon using a plant model, then applies only the first step of the resulting plan before re-solving.
Multi-Stage Overvoltage / Overdischarge Protection – a protection scheme with two or more distinct voltage thresholds rather than one: an initial trip that simply halts charge or discharge, and a second, more severe threshold that locks the BMS out entirely until a technician resets it.
Observability – a system property describing whether a system’s internal states can be reconstructed from its measured outputs.
Open-Loop Observer – a state observer that runs a model of the system forward using only the system’s inputs, with no correction from measured outputs, so estimation error can grow unchecked over time.
Ohm Ampere-Hour – a figure of merit used when comparing cells for the lowest internal resistance.
Open Circuit Voltage (OCV) – is the potential difference between the positive and negative terminals when no current flows and the cell is at rest.
OCV Hysteresis – the fact that a cell reads a different open-circuit voltage at the same SoC depending on whether it was last charged or discharged, with the gap between the two varying across SoC and temperature.
Operational Limits Protection – the BMS function that derates or cuts off charge and discharge current as the pack approaches its voltage, temperature or SoC limits.
OTA – Over The Air (firmware updates to BMS)
Overcharge – charging a cell beyond its maximum safe capacity, a critical safety condition the BMS must prevent.
Particle Filter – a state estimation method that represents the probability of each possible state using a large population of weighted samples, useful when a cell’s error distribution isn’t well approximated as Gaussian.
Passive Balancing – simple form of balancing that switches a resistor across the cells. In the example shown with the 3 cells the balancing resistor would be switched on for the centre cell. Discharging this cell and losing the energy to heat in the balance resistor (typically 30Ω to 40Ω).
PID Controller – a feedback controller that combines proportional, integral and derivative terms on the measured error to compute its output.
Plant Model – the mathematical model of the system being controlled or estimated. Here, the cell, pack or thermal system against which an observer, controller or test rig is run.
Precharge – when closing battery contactors onto a capacitor load there would be a very high current that could cause damage to the contactors or cells, or result in the fuse blowing. Thus a precharge resistor and contactor allows that maximum current to be controlled.
Preconditioning – active heating (occasionally cooling) of the pack ahead of a planned DC fast-charge event, bringing cell temperature into the window where lithium plating risk is low and charge acceptance is high.
Processor-in-the-Loop (PIL) – testing the compiled BMS binary running on the actual target microcontroller, or a cycle-accurate equivalent, still closed-loop against the plant model.
PTC – Positive Temperature Coefficient (heater and protection device)
PWM – Pulse Width Modulation
Rack-Level / String-Level BMS – the master/slave hierarchy in a BESS runs one level deeper than in an EV pack: each rack or string runs its own local BMS, and a plant may run tens to hundreds of these in parallel, all reporting up to a Battery Plant Controller rather than a single vehicle-level master.
Reactive Power Support – the capability of a BESS’s inverters to export or absorb reactive power independent of charging or discharging real power, used to help regulate local grid voltage.
Recursive Least Squares (RLS) – an online parameter-estimation method that updates its parameter estimate incrementally as each new measurement arrives, without needing to store or reprocess the full measurement history.
Redundant BMS Architecture – an N+1 or similar redundancy design for the master BMS controllers in a large BESS installation, so a single controller fault doesn’t take an entire DC block or string offline; a reliability requirement that matters far more over a BESS’s 15–25 year design life than in a vehicle’s.
Remaining Useful Life (RUL) – a key function declared by the battery management system. A prediction of how many cycles the pack has before hitting the minimum requirements for operation.
Runtime Balancing – each cell is connected to an individual low DC-DC power converter, then each converter is connected in series. This then allows the power delivered and received by each cell to be completely controlled based on their capability.
Safety Diagnostics – BMS functions that continuously monitor for overvoltage, undervoltage, overcurrent, overtemperature, isolation faults and internal short-circuit indicators, triggering protective action when thresholds are exceeded.
Seebeck Error – A measurement error source where a voltage is generated at the junction of two dissimilar metals if there’s a temperature difference across it, which can corrupt precision cell voltage or resistance readings if not accounted for in sense-wire and connector material selection.
Sense Lead / Balancing Lead Separation – routing a cell’s active balancing current through a separate wire from its voltage-sensing wire, so the balancing current itself doesn’t create a resistive-drop error in the very voltage reading the BMS uses to decide how much to balance.
Sleep Mode – the battery pack is isolated, there is no balancing, the BMS will be in a low power mode where it occasionally looks at sensor inputs and listens out for a communications request to wake.
Sliding Mode Observer – a state observer that drives its error to zero using a discontinuous switching correction term, giving fast convergence and robustness to model uncertainty at the cost of estimation chatter.
SMBus (System Management Bus) – a two-wire communication protocol, built on I²C, specifically used by “smart batteries” in laptops and consumer electronics to report SoC, cycle count, manufacturer data and fault codes to a host device.
Software-in-the-Loop (SIL) – testing the production BMS code, compiled for a host PC, against the same plant model used in MIL and HIL, off-target.
State of Charge (SoC) – abbreviated as SoC and defined as the amount of charge in the cell as a percentage compared to the nominal capacity of the cell in Ah.
SoCE – State of Certified Energy (EU Battery Regulation mandatory metric from 2026)
SoE – State of Energy
SoF – State of Function
SoQ – State of Capacity/Quantity
SoR – State of Resistance
SOX – shorthand for the full family of “state of” functions (SoC, SoH, SoP, SoE, SoQ, SoR, SoF) a BMS publishes to the wider system.
State of Health (SOH) – this is the total available charged capacity of the cell as a percentage compared to the nominal capacity in Ah when the cell was new.
State of Power (SoP) – the maximum power the pack can deliver or accept over a short window, typically 10–30 seconds, accounting for SoC, temperature and SoH.
State-Space Model – a system representation in terms of a set of internal state variables and the equations governing how they evolve and relate to measured outputs; the mathematical basis for observer-based estimation.
Temperature – a critical parameter that you need to know before charging or discharging a cell. A cell is a 3 dimensional structure that is also inhomogeneous and hence you will observe temperature gradients within the cell. The temperature limits, gradients and heat rejection rate will define the overall power capability of the battery.
Temperature Gradient – the maximum temperature differential in a cell is normally specified as ~2°C to minimise the degradation in capacity of the cell. This requirement will drive the cell selection versus application along with the cooling system design.
- High temperature and the SEI layer on the anode grows faster. If the SEI layer grows fast it tends to be more porous and unstable.
- At low temperatures we see slower diffusion and intercalation with the possibility of lithium plating. Lithium plating removes lithium from the active cell, reducing cell capacity. Also, lithium plating can subsequently into lithium dendrites that can cause electrical shorts.
Temperature Limits – these temperatures will change with chemistry and by cell manufacturer, therefore, it is really important to use the limits as advised by the manufacturer. In addition you will need to test the cell to gain the detailed understanding of how the cell behaves in your application versus temperature.
The limits will also be blurred by the design of the battery and control system. One example is the maximum operating temperature for the cell.
Throughput-Based Dispatch Limiting – the function, split between BMS and EMS, that caps how hard a stationary pack is cycled against its contracted degradation curve, trading near-term revenue-stacking opportunity against the warranty and augmentation schedule the asset owner is holding the OEM to.
Unscented Kalman Filter (UKF) – a Kalman Filter variant that propagates a small set of deterministically chosen sample points through the true nonlinear cell model, avoiding the linearisation step an EKF relies on.
Usable SoC Window – If we want a battery cell to last a lot of cycles, extend the life in a power application or to ensure the available power is consistent then we need to set a usable SoC window that is smaller than 100%.
Increasing the Usable SOC Window – If you are in a battery design role there is always pressure on increasing usable SoC window. Nobody wants to pay for and carry around unused battery capacity. However, there are some really good reasons to restrict that window.
Usage Histogram (Service Histogram) – time-in-range data (e.g. seconds spent in each 10°C temperature band, each SoC band, each voltage band) logged by the BMS over a pack’s life and stored in non-volatile memory, used for warranty analysis and lifetime/degradation modelling rather than real-time control.
Virtual Temperature Sensor – a software algorithm that estimates cell temperature at an unmeasured location from a thermal model driven by current, voltage and a limited number of physical sensor readings, reducing harness cost and complexity.
wBMS – Wireless BMS
Weakest Cell – the principle that a series string’s performance is capped by its worst cell: the first cell to hit a voltage or temperature limit stops charge or discharge for the whole string. (Has its own dedicated page; missing from the glossary.)
Wireless BMS – one option for reducing the wiring complexity and weight in a battery pack is to use a wireless connection. Here the modules or cells report voltage and temperature signals back to the central control system in the battery wirelessly.
See also: Abbreviations and Jargon