Battery management systems explained: protection, monitoring and cell balancing

A rechargeable battery pack is a system of cells, interconnects, sensors, switches, electronics, firmware, enclosure and thermal paths.

The battery management system, or BMS, observes and controls important parts of that system. Depending on the product, it may measure individual cell voltages, pack current and temperatures; estimate state of charge; balance cells; control charge and discharge switches; log faults; communicate with a charger or host; and place the pack into a protected state.

The required architecture changes enormously between a small portable pack, an industrial battery, an energy-storage rack and an electric vehicle. The principles are shared, but component selection, redundancy, isolation and compliance cannot be generalised into one reference design.

What the BMS is protecting

Rechargeable cells operate within defined voltage, current and temperature limits. Operation outside those limits can reduce life, damage the cell or create a safety hazard.

The BMS must therefore detect relevant conditions and take an appropriate action. Typical protections include:

• cell overvoltage during charge;
• cell undervoltage during discharge;
• overcurrent in charge or discharge;
• short circuit;
• overtemperature and undertemperature;
• open sensing wires;
• internal communication or measurement faults.

Thresholds, delay times and recovery behaviour must come from the cell data, pack design and applicable standards. A generic BMS board with adjustable values is not automatically suitable for a different cell chemistry or series count.

Cell-voltage monitoring

In a series pack, the total voltage does not reveal the condition of every cell group. One group can approach overvoltage while the pack total remains inside its broad limit.

A battery monitor measures each series-connected cell or parallel cell group. Accuracy, input range, common-mode capability and synchronisation matter. Measurement errors affect protection and state estimation.

The sensing connections should be routed and protected according to the monitor manufacturer's guidance. Open-wire detection can identify a broken sense lead that might otherwise present a misleading voltage.

Filtering must control noise without slowing a protection response beyond the permitted time.

Current measurement

Pack current is commonly measured using a shunt resistor and amplifier, or a magnetic sensor in higher-current or isolated systems.

The BMS may integrate current over time for coulomb counting, detect overcurrent and short-circuit events and estimate power or remaining capacity.

Shunt selection involves resistance, power dissipation, tolerance, temperature coefficient and pulse capability. Kelvin connections are important because high-current copper can create a voltage drop comparable to the measured signal.

Fast short-circuit protection may require a dedicated comparator or analogue path rather than waiting for firmware to process a normal measurement.

Temperature monitoring

Cell temperature strongly influences charging limits, available power, ageing and safety.

Sensors should represent the hottest or most vulnerable parts of the pack across real operating conditions. One sensor mounted beside the BMS electronics may not detect a remote cell heating inside an enclosure.

The BMS can reduce charge or discharge current, stop operation or signal the host as temperature approaches a limit. The charger and system controller must respect those commands.

Thermal design remains a mechanical responsibility too. Electronics cannot remove heat that has no adequate path out of the pack.

Protection switches and contactors

Small packs often use MOSFETs to disconnect charging or discharging. Larger systems may use contactors, pre-charge circuits, fuses and pyrotechnic disconnects.

The switching elements must interrupt the credible fault current at the pack voltage. MOSFET safe operating area, contactor breaking capability, arc control and fuse coordination all matter.

High-side and low-side switching affect ground reference, charging paths and communications differently. Some packs need separate control of charge and discharge currents.

The BMS should know whether a commanded disconnect actually occurred. Welded contactors, shorted MOSFETs or an unintended current path can defeat the assumed safe state.

What cell balancing does

Series-connected cells do not remain perfectly matched. Small differences in capacity, leakage, temperature and state of charge accumulate over cycles.

The usable pack range is limited by the first cell to reach its upper or lower limit. Balancing reduces state-of-charge mismatch so more of the pack capacity can be used without driving an individual cell outside its range.

Passive balancing

Passive balancing discharges higher-voltage cells through resistors. It is simple and common but converts energy into heat. Balance current, resistor power and thermal conditions determine how quickly it can correct a mismatch.

Active balancing

Active methods move energy between cells or modules using capacitive, inductive or transformer-based circuits. They can improve energy utilisation in large packs but add cost, control complexity and components.

Balancing cannot repair a damaged or substantially degraded cell. A growing imbalance can be a diagnostic signal rather than a condition to hide indefinitely.

State of charge and state of health

State of charge estimates the remaining usable charge. Voltage alone is insufficient for many chemistries because the voltage curve can be flat and depends on load, temperature and recent history.

Coulomb counting integrates current into and out of the pack but accumulates sensor and timing error. Practical systems combine current, voltage, temperature and cell models and periodically correct the estimate at known conditions.

State of health describes changes in capacity, resistance or power capability over life. It is an estimate based on defined criteria, not a directly measured universal percentage.

The user interface should not present more precision than the model and sensors can support.

Communication and system control

The BMS may communicate over SMBus, I²C, CAN, isolated daisy chains or another interface. It can report measurements, limits, faults and permitted charge or discharge power.

Define what happens if communication is lost. Safety should not depend entirely on a host processor continuing to send the correct command.

Firmware update, diagnostics, logging and cybersecurity matter in connected systems. A BMS that cannot be securely maintained can become the lifecycle limit of an otherwise serviceable battery.

Redundancy and independence

Higher-consequence systems may require independent monitoring or shutdown paths so that one measurement, processor or software fault cannot remove protection.

The appropriate architecture comes from hazard analysis and sector requirements. Redundancy is not achieved simply by reading one sensor twice through the same circuit.

Review common-cause failures including shared power, connectors, references, clocks, communication paths and software.

Mechanical design is part of battery safety

The pack must manage cell spacing, compression where required, insulation, venting, fire propagation, ingress, impact and service access.

Sense wiring should be protected from abrasion and correctly fused or current limited. High-current paths need controlled resistance and robust joints. Temperature sensors must remain attached through vibration and ageing.

Cell selection, matching and assembly quality can determine whether the BMS ever sees a manageable pack.

Validation must include faults

Test normal charge and discharge across temperature, then verify the detection and response to credible faults.

Depending on the system, this may include:

• overvoltage and undervoltage thresholds;
• current-limit and short-circuit response;
• sensor open and short faults;
• sense-wire disconnects;
• communication loss;
• stuck switches or welded contactors;
• unbalanced cells;
• failed updates and reset recovery;
• charger and load interactions.

High-energy battery testing requires suitable facilities, containment and competent personnel. Do not create unsafe faults on an ordinary electronics bench.

Questions to answer before selecting a BMS

1. What cell chemistry and exact cell model will be used?
2. How many cells are in series and parallel?
3. What are the permitted voltage, current and temperature limits?
4. What fault current can the pack deliver?
5. What isolation and communications are required?
6. Is passive balancing adequate, and at what current?
7. Which functions must remain independent of firmware?
8. What standards and transport requirements apply?
9. How will the pack be programmed, calibrated and traced?
10. How will field faults and firmware updates be handled?

A BMS is not a universal safety board attached to a set of cells. It is one part of a battery architecture whose electrical, thermal, mechanical and software assumptions must all agree.

Technical sources

• Texas Instruments, battery-monitor and cell-balancer guidance.
• Texas Instruments, HEV and EV battery-management design resources.
• Texas Instruments, multi-cell monitoring, protection and balancing reference designs.
• The selected cell manufacturer's specification and applicable product, transport and safety standards.