Battery Module BMS Requirements: The Integration Boundary
Short answer: A battery management system must monitor cell voltage, temperature and current; estimate SOC and SOH; control balancing; drive contactors; and handle faults. In a module-based pack, how much of the cell-level sensing layer comes with the module varies per module and per project: integrated temperature sensing is common, a cell supervision circuit is not a given. The integrator builds the master controller, communication stack and safety logic on top.
A module-based pack divides BMS responsibility at a well-defined boundary. Understanding that boundary before selecting a module — and before writing a BMS specification — avoids late-stage integration surprises. This article works through what a BMS actually does, where the module supplier's scope ends, and which interfaces to verify before purchase.
What a BMS measures and controls
The BMS is the control and protection layer of a battery pack. Its core functions are measurement, estimation, protection and actuation.
Cell voltage monitoring
Each prismatic cell in a module must be monitored individually. Cell voltage monitoring is the foundation: it detects cell imbalance, triggers low- and high-voltage protection, and feeds the SOC estimator. Measurement is typically performed by a cell supervision circuit (CSC) — sometimes called a slave board — mounted on or near the module. Accuracy requirements depend on chemistry and application; realistic figures for a well-designed analogue front-end are ±2–5 mV across the operating temperature range, though this varies with circuit design, calibration and temperature compensation. Do not accept a supplier's headline accuracy figure without asking for the conditions under which it was measured.
Temperature measurement
Thermistors (NTC type in most automotive modules) are embedded in the module at representative points — typically two to four per module, positioned near cell terminals or mid-stack. The BMS reads these to enforce charge and discharge temperature limits and to derate current at extremes. For LFP chemistry, charging below 0 °C causes lithium plating and must be inhibited; for NCM, the upper limit on charge is typically 45 °C. The module datasheet should state how many thermistors are present and their physical locations.
Current measurement
Current is measured at pack level, not cell level, using a current sensor — typically a Hall-effect or shunt resistor type — installed by the integrator on the main bus. Some module formats include a current sense resistor on the slave board for balancing current monitoring only; this does not replace a pack-level sensor.
SOC and SOH estimation
State of charge (SOC) is the integrator's calculation, performed in the master BMS. Common methods are coulomb counting (integrating current over time) corrected by voltage-based resets at known reference points, or model-based observers such as extended Kalman filters. State of health (SOH) is estimated from capacity fade and impedance rise over cycle history. Neither function lives in the module; they require a master controller with sufficient compute and memory.
Balancing
Cell-to-cell voltage differences accumulate from manufacturing spread and differential ageing. Balancing corrects these differences during charging. The two primary architectures differ substantially in complexity, heat and cost.
| Parameter | Passive balancing | Active balancing |
|---|---|---|
| Mechanism | Bleeds excess energy from high cells through a resistor | Transfers charge between cells via inductor, capacitor or transformer |
| Balancing current (typical) | 50–200 mA | 1–5 A |
| Heat generated | Dissipated in module as resistive heat | Low; energy is reused |
| Cooling impact | Must be accounted for in thermal design | Minimal additional heat load |
| Component cost | Low | Significantly higher |
| When it matters | Adequate for packs with good initial cell matching and moderate cycle count | Justified for high-cycle or high-capacity packs where imbalance recovery time or heat budget is constrained |
| Typical location | On the CSC/slave board inside the module | On the slave board (integrated) or external to the module (discrete) |
Where a module ships with a CSC, the balancing resistors usually sit on that board. When selecting battery modules from Avantis Energy, confirm whether the slave board supports only passive balancing or whether active balancing circuitry is present or can be populated.
Contactor control and fault handling
The master BMS drives the main contactors (positive, negative, pre-charge) that connect and disconnect the pack from the load. Fault handling — overvoltage, undervoltage, overcurrent, overtemperature, communication loss, insulation fault — is implemented in the master software. In a module purchase these functions are integrator scope.
The module supplier's scope
A module supplier covers the cell layer. How far the sensing interface comes with it differs per module and per project, so treat the list below as the scope to settle per product rather than as a standard delivery. When evaluating battery modules or prismatic LFP and NMC cells, establish which of these are included:
- Cells assembled into a module with busbars, compression structure and housing.
- CSC/slave board (if included): analogue front-end ICs measuring cell voltages and thermistor inputs, with a digital output to the master.
- Thermistors wired and positioned per the module design.
- Voltage tap connectors: a standardised multi-pin connector giving the master access to individual cell voltages and temperatures.
- Mechanical and thermal interfaces: cooling plate connections, mounting points, busbar geometry.
The master BMS controller, current sensors, contactors, pre-charge circuit, insulation monitoring, system-level fault logic and the CAN/SMBus communication stack all sit above the module. In a module purchase they are integrator deliverables. In a system project they can be part of the supplier's scope: Avantis Energy covers pack design, thermal management and BMS/EMS integration at that level. Which of the two you are buying is worth settling before the BMS specification is written.
Interfaces to verify before committing to a module
Before purchase, request the following from any module supplier:
- CSC included or not? Some module formats are supplied without a slave board; the integrator connects directly to voltage tap pins.
- Connector pinout and mating connector part number. Non-standard connectors create sourcing risk.
- Communication protocol from CSC to master. Common options are isoSPI, SPI, I²C or CAN; verify compatibility with your master hardware.
- Balancing type and maximum balancing current. Confirm the thermal impact for your enclosure.
- Number and location of thermistors. Verify coverage is adequate for your thermal model.
- Voltage tap accuracy under temperature. Ask for the specification conditions, not just the headline figure.
- Isolation voltage between voltage tap connector and module housing. Relevant for high-voltage packs.
For complete module systems intended for stationary or vehicle applications, these interfaces are documented in the module datasheet and technical reference manual. Ask for both before finalising your BMS architecture.
Master–slave architecture
A master–slave BMS architecture assigns one CSC (slave) per module and one master controller per pack or string. The master polls each slave, aggregates measurements, runs SOC/SOH estimation, enforces protection thresholds and controls contactors. Communication between master and slaves is typically a daisy-chain bus to avoid the isolation challenges of parallel connections across high-voltage strings.
The number of slaves scales with the number of modules; the master is usually a single embedded controller. This architecture is well-suited to the standardised module formats described by the VDA (see VDA for format documentation) because each module presents an identical interface to its slave board, and the slave boards present an identical interface to the master.
IEC 62619, published by the IEC, sets safety requirements for stationary lithium battery systems and is a relevant reference when specifying the protection functions that the master must implement.
What this means for module selection
If you are building a pack for commercial vehicles or stationary storage, module format standardisation (VDA 355, MEB 590) reduces mechanical integration risk, but the BMS integration boundary still requires careful definition. A module that ships with a CSC shortens your development path; a module without one gives you more flexibility at the cost of more analogue design work.
For context on how module format standardisation affects pack architecture in vehicle applications, see why the electrification of commercial fleets in Europe is accelerating. For stationary applications, the same principles apply; the business case for commercial and industrial energy storage increasingly depends on integrators being able to source modules with well-documented BMS interfaces.
Frequently asked questions
What does a battery module BMS slave board (CSC) typically include?
A cell supervision circuit on a module-level slave board typically includes an analogue front-end IC measuring individual cell voltages and NTC thermistor inputs, passive balancing resistors, and a digital communication interface (isoSPI, SPI or CAN) to the master controller. It does not include current measurement, contactor drivers or protection logic.
What is the difference between passive and active cell balancing in a battery module?
Passive balancing dissipates excess charge from high cells through a resistor, generating heat in the module at typical currents of 50–200 mA. Active balancing transfers charge between cells at 1–5 A with low losses. Active balancing is justified when heat budget or imbalance recovery time is constrained; passive balancing is adequate for well-matched cells at moderate cycle counts.
Which BMS functions are the integrator's responsibility in a module-based pack?
In a module purchase the integrator is responsible for the master controller, SOC and SOH estimation algorithms, current measurement, contactor and pre-charge circuitry, insulation monitoring, pack-level fault logic, and the communication stack to the vehicle or system controller. The module supplier covers the cell layer and whatever sensing is integrated in the module, which differs per module — confirm it per product rather than assuming a slave board is included.
What interfaces should I request from a module supplier before designing my BMS?
Request: whether a CSC/slave board is included; the voltage tap connector pinout and mating part number; the CSC-to-master communication protocol; balancing type and maximum balancing current; number and location of thermistors; voltage measurement accuracy under temperature; and isolation voltage between the tap connector and module housing.
Does cell voltage monitoring accuracy vary between modules?
Yes. Voltage measurement accuracy depends on the analogue front-end design, calibration, temperature compensation and reference voltage quality. Realistic figures for a production CSC are ±2–5 mV across the operating temperature range. Always ask suppliers to state the conditions — temperature range, voltage range, after calibration — under which their accuracy figure applies.
Discuss your integration requirements
If you are specifying a module-based pack and need to confirm which BMS interfaces are covered at the module level, discuss your pack requirements with Avantis Energy. Module datasheets and technical support on pack architecture are available before you commit to a design.