A complete low- or medium-voltage BMS typically consists of three ICs: an analog front-end (AFE), a microcontroller (MCU), and a fuel gauge. The fuel gauge can be a standalone IC or embedded within the MCU. The MCU is the core component of the BMS; it obtains information from the AFE and the fuel gauge, while also interacting with the rest of the system.
The AFE provides the MCU and fuel gauge with voltage, temperature, and current readings for the battery. Because the AFE is physically closest to the battery, it is recommended that the AFE also control a circuit breaker, which would disconnect the battery from the rest of the system if any fault is triggered.
The fuel gauge IC obtains readings from the AFE (Automatic Feedback Controller) and then uses sophisticated cell modeling and advanced algorithms to estimate key parameters such as SoC (System-on-Chips) and SoH (Solar Horizon). Similar to the AFE, some tasks of the fuel gauge can be included in the MCU code; however, using a dedicated fuel gauge IC offers several advantages:
• High-efficiency design: Using dedicated ICs to run complex power calculations allows designers to use lower-specification MCUs, thereby reducing overall cost and current consumption.
• Enhanced Insight and Safety: Dedicated fuel gauges can measure the individual SoC and SoH of each series-connected cell combination in a battery pack, enabling more accurate measurement precision and aging detection throughout the battery's lifespan. This is important because battery impedance and capacity change over time, affecting runtime and safety.
• Fast time-to-market: The fuel gauge IC has undergone comprehensive testing across various scenarios and test cases. This reduces the time and cost of testing complex algorithms, while accelerating time-to-market.
Improve SoC and SoH accuracy
The primary goal of designing a precise BMS is to provide accurate calculations for the battery pack's SoC (Cycles On-State) and SoH (Lifetime and State of Charge). BMS designers might think the only way to achieve this is by using a very expensive AFE (Automatic Factor Analyzer) with precise battery voltage measurement tolerances, but this is only one factor in overall calculation accuracy. The most important factors are the fuel gauge battery model and the fuel calculation methodology, followed by the AFE's ability to provide simultaneous voltage-current readings for battery resistance calculations.
The fuel gauge uses its internal algorithms to perform complex calculations, converting voltage, current, and temperature measurements into System-on-Chips (SoC) and SoH (Solar Hourly Rate) outputs by analyzing the relationship between these values and a specific battery model stored in its memory. The battery model is generated by characterizing the battery under different temperature, capacity, and load conditions, mathematically defining its open-circuit voltage and resistive and capacitive elements. This model enables the fuel gauge's algorithm to calculate the optimal SoC based on variations in these parameters under different operating conditions. Therefore, if the fuel gauge's battery model or algorithm is inaccurate, the final calculation will be inaccurate no matter how precise the AFE (Aspect-to-Fuel Parameter) measurement is. In other words, employing a high-precision fuel gauge has the greatest impact on the SoC accuracy of the BMS (Body Management System).
Voltage and current synchronous reading
While almost all AFEs offer different ADCs for voltage and current, not all AFEs provide actual synchronous current and voltage measurements for every battery. This feature, called synchronous voltage-current reading, enables the fuel gauge to accurately estimate the battery's equivalent series resistance (ESR). Because ESR varies under different operating conditions and over time, real-time ESR estimation allows for a more accurate estimation of the system-on-chip (SoC).
The SoC error during synchronous readout is significantly lower than that during asynchronous readout, especially after several discharge cycles. These results were obtained using the MPF42791, which integrates ESR detection and thermal modeling.
AFE Direct Fault Control
As mentioned earlier, the most important role of the AFE in a BMS is protection management. The AFE can directly control the protection circuitry, protecting the system and 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, thus increasing firmware complexity.
Advanced AFEs use their ADC readings and user configuration to detect any fault conditions. The AFE reacts to faults by turning on protection MOSFETs, ensuring true hardware protection. The AFEs are also thoroughly tested, making it easy to guarantee system safety and reliability. This allows the MCU to be used as a secondary protection mechanism for an even higher level of safety and robustness.
The MP279x series integrates two protection control methods. This allows designers to choose whether to control fault response and/or protection via an AFE or an MCU.
High-end and low-end battery protection
When designing a BMS, it is important to consider the placement of battery protection circuit breakers. These circuits are typically 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 (positive terminal of the battery) or the low-voltage side (negative terminal of the battery).
The advanced architecture ensures that ground (GND) always has a good reference, thus avoiding potential safety and communication problems in the event of a short circuit. Furthermore, a clean, stable GND connection helps reduce reference signal fluctuations, which is crucial for the accurate operation of the MCU.
However, when an N-channel MOSFET is located at the positive terminal of the battery, driving its gate requires a voltage higher than the battery pack voltage, which makes the design process more challenging. Therefore, dedicated charge pumps integrated into the AFE are typically used in high-end architectures, which increases overall cost and IC current consumption.
For low-end configurations, since the protection MOSFET is located at the negative terminal of the battery, a charge pump is not required. However, achieving effective communication is more difficult in low-end configurations because there is no GND reference when the protection is on.
The MP279x series employs a high-side architecture, providing robust protection while minimizing BOM (Bill of Materials). Furthermore, high-precision charge pump control enables soft-turn-on of the N-channel MOSFETs without requiring any additional pre-charge circuitry, further reducing BOM size and cost. Soft-turn-on is achieved by slowly increasing the gate voltage of the protection FET, allowing a small current to flow through the protection device to pre-charge the load. Several parameters can be configured to ensure safe switching, such as the maximum allowable current or the time it takes for the protection FET to turn off without triggering a fault.
Extend battery life through battery balancing
Battery packs powering large systems, such as electric bicycles or energy storage systems, consist of many cells connected in series and parallel. Each cell is theoretically identical, but due to manufacturing tolerances and chemical variations, each cell typically behaves slightly differently. Over time, these differences become more pronounced due to varying operating conditions and aging, severely impacting battery performance by limiting its usable capacity or potentially damaging it. To avoid these dangerous situations, it is important to periodically equalize the voltage of the cells connected in series through a process called battery balancing.
Passive balancing is the most common method for equalizing battery voltages. It requires discharging the most charged batteries until all batteries have equal charge levels. Passive battery balancing in an AFE can be achieved externally or internally. External balancing allows for a larger balancing current but also increases the Bill of Materials (BOM).
On the other hand, internal balancing does not increase the BOM, but it typically limits the balancing current to a lower value due to heat dissipation requirements. When choosing between internal and external balancing, consider the cost of the external hardware and the target balancing current.
Another important aspect of battery balancing is the physical connection. For example, the MP279x AFE series uses the same pins for voltage sensing and balancing. This significantly reduces the IC size, but means that consecutive batteries cannot be balanced simultaneously, thus increasing the time required to perform battery balancing. Using dedicated balancing pins can reduce balancing time, but this significantly increases IC size and overall cost.
AFE safety features
As discussed in this article, the AFE (Active Front-End) for control system protection and fault response is extremely important in BMS design. The AFE must be able to detect adverse conditions before turning on or off the protection FETs.
Battery and battery pack level faults, such as overvoltage (OV), undervoltage (UV), overcurrent (OC), short circuit (SC), overtemperature (OT), and undertemperature (UT) faults, should all be monitored. However, AFEs can also provide other beneficial protections and functions for certain applications. For example, self-tests allow the IC to detect whether its internal ADC has failed, thus preventing system measurement errors. Enhanced watchdog timer functionality also ensures robustness and safety when the main MCU is unresponsive.
The MP279x series offers the aforementioned fault protection with high configurability, allowing users to define different thresholds, despiking times, and hysteresis for each fault. These devices also rely on two separate comparators to handle SC and OC fault conditions to minimize response time. They also offer an automatic fault recovery configuration, meaning they can automatically recover from most faults without any action from the MCU.
in conclusion
A Battery Management System (BMS) monitors the battery pack to protect the battery and other parts of the system. A substandard BMS not only reduces system safety but also provides inaccurate battery SoC management. These inaccuracies have a significant impact on the final product quality, as they can lead to potentially dangerous failures or malfunctions that negatively affect the user experience. To mitigate these issues, this article explains what designers should expect and look for when designing a BMS. Learn more about how battery management systems work and how to design them using MPS's BMS evaluation toolkit.
