Definition

The Battery Management System (BMS) is a dedicated technology designed to monitor and control battery packs. These packs consist of multiple battery cells arranged in a series-parallel matrix configuration, enabling them to deliver specific voltage and current ranges over time under expected load conditions. 

The oversight that a BMS provides usually includes:

Monitoring the battery

Providing battery protection

Estimating the battery’s operational state

Continually optimizing battery performance

Reporting operational status to external devices

How Do Battery Management Systems Work? 

A Battery Management System (BMS) is an electronic control system that monitors and manages rechargeable battery packs to ensure safe, efficient, and reliable operation. Its key functions include: 

1. Cell Monitoring & Voltage Balancing

Measures individual cell voltages to prevent overcharging or over-discharging, which can damage cells.

Uses active or passive balancing to equalize charge across cells, improving lifespan and performance.

2. State Estimation (SOC, SOH)

State of Charge (SOC): Estimates remaining battery capacity (e.g., "battery percentage"). 

State of Health (SOH): Tracks long-term degradation (e.g., capacity loss over time).

3. Temperature Management

Monitors cell temperatures using sensors.

Adjusts cooling/heating systems or limits power to prevent overheating.

4. Current & Power Control

Regulates charge/discharge rates to avoid stress on cells.

May disconnect the battery in case of excessive current (short-circuit protection).

5. Communication & Fault Protection

Interfaces with external systems (e.g., vehicle ECUs, chargers) via CAN bus, UART, or other protocols.

Triggers safety shutdowns for faults like overvoltage, overheating, or insulation failure.

6. Data Logging & Diagnostics

Stores historical data (e.g., cycle counts, error logs) for maintenance and optimization. 

Why BMS is Critical？

Without a BMS, batteries risk:

Thermal runaway (overheating leading to fires)
Uneven aging (due to cell imbalances)
Reduced lifespan (from improper charging)

Modern BMS solutions use advanced algorithms (e.g., Kalman filters, machine learning) for more accurate predictions, especially in EVs and grid storage systems. 

Major Components of BMS 

Sensing Components 

Sensing components are a crucial component of BMS. Sensing components are essential for monitoring and managing a battery's numerous properties. For the purpose of maximizing battery life, assuring safe operation, and improving performance, accurate sensing is essential. Voltage sensors, current sensors, and temperature sensors make up the majority of the sensing elements in BMS. 

Voltage Sensors  

Voltage monitoring devices are integral components for overseeing the voltage levels of individual cells within a battery. The maintenance of proper voltage levels stands as a paramount consideration for ensuring both the safety and efficiency of the battery. Typically, these devices gauge the potential difference across the terminals of each cell. This vigilant monitoring of cell voltages empowers the Battery Management System (BMS) to execute cell balancing procedures, guaranteeing uniform charge and discharge across all cells within the battery. Furthermore, it plays a pivotal role in computing the State of Charge (SOC) and serves as a preventive measure against overcharging or deep discharge, circumstances that could potentially inflict harm upon the battery. 

Current Sensors  

Current monitoring instruments find utility in the measurement of the electric current entering or exiting the battery. The monitoring of current assumes critical significance for multiple reasons. Firstly, it contributes to the computation of SOC by integrating the current over time, a technique known as coulomb counting. Secondly, it plays a pivotal role in the identification of abnormal conditions such as over-current or short-circuit incidents, thereby facilitating the implementation of protective measures. An array of current sensors is available, including Hall-effect sensors, shunt resistors, and current transformers. Among these, Hall-effect sensors hold widespread utility within BMS setups due to their versatility in measuring both AC and DC currents and their provision of electrical isolation between the sensor and the current-carrying conductor. 

Temperature Sensors  

Thermal sensors represent essential components tasked with monitoring the temperature conditions prevailing in a battery. The operation of a battery inherently generates heat, and the efficiency of the battery operation is profoundly influenced by its thermal environment. Furthermore, excessive heat can be detrimental, potentially leading to a hazardous state known as thermal runaway, which can result in battery malfunction or even fires. To combat these issues, temperature sensors, encompassing devices like thermocouples or thermistors, are strategically positioned at various locations within the battery pack. Their primary function is to track the temperature of individual cells and the ambient temperature surrounding the pack. The data gleaned from these sensors equips the Battery Management System (BMS) with the information required to make informed decisions. These decisions may involve the activation of cooling systems or the adjustment of charging and discharging rates to uphold safe thermal conditions. 

Protection Circuitry  

A crucial part of a BMS that guarantees the security and dependability of battery systems is the protection circuitry. It continuously checks the battery's condition and adjusts or intervenes in real time to avoid potentially harmful or dangerous situations. Overcharge protection, over-discharge protection, short circuit protection, and heat protection are the main safety features found in a BMS. 

Overcharge Protection 

To stop the battery from being charged above its maximum voltage, overcharge prevention is crucial. The protective circuitry will either stop or redirect the charging current when the voltage hits a predetermined threshold, keeping the battery from receiving any further current. This safeguard is essential for the battery's longevity and safety as overcharging might result in a battery's catastrophic failure. 

Over-Discharge Protection

In a parallel consideration to the risks of overcharging, discharging a battery below a specified voltage threshold can also inflict damage. The mechanism of over-discharge protection diligently monitors the battery's voltage as it undergoes discharge. In the event of a voltage drop nearing or surpassing a predetermined threshold, this safeguard intervenes by either disconnecting the load or implementing measures to prohibit the battery from discharging below the established limit. Such proactive measures hold paramount importance in forestalling capacity depletion, preserving the battery's well-being, and mitigating potential safety hazards. 

Short Circuit Protection  

Short circuit mitigation is an integral safety measure engineered to shield both the battery and the associated circuitry in the occurrence of a short circuit event. A short circuit materializes when the positive and negative terminals of the battery come into direct connection with minimal or negligible resistance in between, thereby triggering an exorbitant surge in current flow. This surge can induce the generation of heat, escalating the risk of fire or even explosion. Typically, short circuit mitigation encompasses the deployment of fast-acting fuses or circuit breakers. These protective elements function by expeditiously disconnecting the battery from the circuit in response to a detected short circuit event, thus averting potential catastrophic consequences. 

Thermal Protection  

Batteries exhibit sensitivity to temperature variations, and straying beyond a prescribed temperature range can prove detrimental to both operational performance and safety. Thermal safeguarding mechanisms are in place to continually monitor the temperature of the battery cells and intervene when the temperature exceeds the defined safe limits. In response to these temperature fluctuations, actions may include the initiation of cooling measures when the cells become excessively hot or, in the case of overly cold conditions, the imposition of restrictions on the charging current, as charging at too low a temperature can lead to damage. 

Balancing Circuit  

A fundamental constituent within the BMS framework is the balancing circuitry. Battery balancing stands as an imperative procedure, especially in battery packs composed of multiple cells, as it guarantees a uniform State of Charge (SOC) across all cells within the pack. This not only guarantees optimal performance but also augments the durability and dependability of the battery pack. Two primary balancing techniques come into play: passive balancing and active balancing. 

Passive Balancing 

Shunt balancing, sometimes referred to as passive balancing, is the process of dispersing extra energy from cells with higher SOCs as heat to lower-charged cells with a higher SOC. Typically, resistors are placed across each cell's terminals. The resistor is engaged when the voltage of a particular cell rises over a predetermined threshold, causing it to redirect some current and release the extra energy. 

Active Balancing 

Active balancing, in contrast to passive balancing, seeks to redistribute the charge across the cells rather than letting it go. Active balancing techniques include employing DC-DC converters, inductors for energy transfer, and capacitors for energy transfer. Active balancing essentially involves the transfer of energy from cells with a higher SOC to those with a lower SOC.