With the rapid development of new energy vehicles, the application of BMS has become commonplace. The BMS is responsible for monitoring and protecting the battery against conditions that could harm the battery, the vehicle, the user, or the surrounding environment. BMS is also responsible for providing accurate SoC and SoH estimates to ensure that battery performance and capacity degradation are minimized throughout the battery's life cycle, ensuring the user's driving experience.

The main structure of a BMS typically consists of three ics: an analog front end (AFE), a microcontroller (MCU), and a coulometer (Figure 1). The coulometer can be a standalone IC or embedded in the MCU. The MCU is the core component of the BMS, and while connected to the rest of the system, it also obtains information from the AFE and coulometer.

Figure 1. BMS architecture block diagram

AFE provides voltage, temperature, and current information for cells and modules for MCUS and coulometers. Because the AFE is physically closest to the battery, the AFE can also control the circuit breaker, which will disconnect the battery from the rest of the system if any failure is triggered.
The coulometer IC takes cell information from the AFE and then uses sophisticated cell modeling and advanced algorithms to estimate key parameters, such as SoC and SoH. Coulometer functions can be implemented through an MCU, but there are several advantages to using a dedicated coulometer IC:

· Efficient design: Using dedicated ics to run complex algorithms allows designers to use lower specification MCUS, reducing overall cost and current consumption.
· Improved safety: A dedicated coulometer measures individual SoC and SoH for each tandem cell combination in a battery pack, enabling more precise measurement accuracy and aging detection throughout the battery's life cycle. This is important because battery impedance and capacity diverge over time, affecting uptime and safety.

Improve SoC and SoH accuracy
The main goal of designing a high-precision BMS is to provide accurate calculations for the SoC and SoH of the battery pack. BMS designers may think that the only way to achieve this is to use higher-precision AFEs, but this is only one factor in overall computational accuracy. The most important factors are the coulometer battery model and the coulometer calculation algorithm, by the AFE's ability to provide synchronous voltage-current readings for the battery resistance calculation.
The coulometer converts voltage, current, and temperature measurements into SoC and SoH outputs by analyzing the information calculated in real time by the algorithm in relation to a specific battery model stored in its memory. The cell model is generated by characterizing the cell under different temperature, capacity, and load conditions, mathematically defining its open-circuit voltage as well as its resistance and capacitance components. This model enables the algorithm of the coulometer to calculate the optimal SoC based on the variation of these parameters under different operating conditions. Therefore, if the battery model or algorithm of the coulometer is inaccurate, the calculation is inaccurate regardless of the accuracy with which the measurement is made by AFE.

Voltage and current synchronous reading
Although almost all AFEs offer different ADCs for voltage and current, not all AFEs offer actual synchronous current and voltage measurements for each cell. This feature, called voltage-current synchronous reading, enables the coulometer to accurately estimate the equivalent series resistance (ESR) of the battery. Because ESR varies with different operating conditions and time, estimating ESR in real time allows for more accurate SoC estimates.

Figure 2 shows the error of a synchronized read versus an unsynchronized read.

Figure 2 Comparison of SoC errors with and without synchronous reading

AFE direct fault control
As mentioned earlier, the most important role AFE plays in a BMS is protection management. AFE can directly control the protection circuit, protecting the system and the battery when a fault is detected. Some systems implement fault control in the MCU, but this results in longer response times and requires more resources from the MCU, increasing the complexity of the firmware.
Advanced AFE uses its ADC reading and user configuration to detect any failure conditions. AFE responds to failures by turning on a protective MOSFET to ensure true hardware protection. In this way, the MCU can act as a secondary protection mechanism for higher security and robustness.

Battery protection for high and low voltage measurements
When designing a BMS, it is important to consider where the battery-protected circuit breaker is placed. Typically, these circuits are implemented using n-channel MOSFETs because they have lower internal resistance compared to p-channel MOSFETs. These circuit breakers can be placed on the high voltage side (the positive terminal of the battery) or the low voltage side (the negative terminal of the battery).
High side architecture ensures good grounding (GND) to avoid potential security and communication issues in the event of a short circuit. In addition, a clean, stable GND connection helps reduce reference signal fluctuations, which are key to accurate MCU operation.
However, when the n-channel MOSFETs are placed at the positive end of the cell, driving their gate requires a voltage higher than that of the battery pack, making the design process more challenging. Therefore, specialized charge pumps integrated into AFE are often used in high-end architectures, which increases the overall cost and IC current consumption.
For low-end configurations, charge pumps are not required, but it is more difficult to achieve effective communication in low-voltage side configurations because there is no GND reference when protection is turned on.

Battery balance to extend battery life
A power battery pack usually consists of a number of cells in series and parallel. Each cell is theoretically identical, but each cell usually behaves slightly differently due to manufacturing tolerances and chemical differences. Over time, these differences become more significant, so battery balancing is essential.
Passive equalization is the most common method, which requires discharging the most charged batteries until they all have equal charges. Passive unit balancing in AFE can be done externally or internally. External balancing allows for a greater balance current, but also increases the BOM (as shown in Figure 3).

Figure 3 External battery balance diagram

Internal balance, on the other hand, does not increase the BOM, but it usually limits the balance current to a lower value due to heat dissipation (Figure 4). When determining internal and external balance, the cost of the external hardware and the target balance current need to be considered.

Figure 4 Internal unit balance block diagram