Battery management design for lithium battery 18650

　　Lithium-based battery chemistries are rapidly replacing lead-acid and nickel metal hydride (NiMH) materials in high-power industrial and transportation systems. However, the energy and power density advantages of lithium chemistry are offset by the greater complexity of battery management circuits. In these "next generation" battery management systems (BMS), developers need to deal with very challenging design constraints. They must measure the voltage of each cell with very high accuracy in a harsh noisy environment, over a wide temperature range and in the presence of hundreds of volts of common-mode voltage.

　　Lithium battery packs are made from a set of single-cell lithium-ion cells with typical voltage/current values of 2.5 to 3.9V and 4 to 40Ahr. In many systems, the battery pack consists of 36 to 200 cells connected in series. Battery packs are used in hybrid electric vehicles, which must provide rapid rechargeability, and consumers also require battery packs to last at least 10 years and have a driving range of 100 miles on a single charge. Peak charge and discharge currents It is about 200A. The most important thing is that the possibility of ROE (rapid oxidation event, that is, fire) must be smaller than that of fuel-powered vehicles. Of course, all this performance must be delivered in a way that has minimal impact on the cost of the car. In summary, the design of electric vehicles (EVs) using lithium battery packs requires a balance between performance, economy, and safety. Two key elements are battery design and battery management circuitry.

　　Charging a lithium-ion battery to 100% or discharging it to 0% will reduce its long-term capacity. Therefore, the working state of charge of lithium-ion batteries is usually limited to a smaller range, such as 30% to 70%. In order to fully utilize the range of available battery capacities, the battery system must very accurately monitor the voltage of each cell (which directly corresponds to the state of charge). This is because lithium-ion batteries have a relatively flat charging curve (see Figure 1). For example, a change in battery voltage of just a few millivolts represents a 1% change in state of charge.

　　The use of multiple lithium-ion cells in a high-voltage battery pack adds significant complexity to the goal of maintaining a specific state-of-charge range. A lithium-ion battery pack cannot charge and discharge like a single power source. Due to manufacturing differences, individual cells have slightly different capacities, and this capacity difference increases over time as poorer cells age faster than others. In the case of batteries with smaller capacities, their state of charge will gradually deviate over many charge and discharge cycles. If the state of charge of each cell is not periodically equalized, some cells will eventually overcharge or discharge, causing damage and ultimately battery pack failure.

　　In addition, high-power battery applications generally must deal with significant noise generated by the negative output of the power supply, switching regulators, relays, starting devices, and other sources. Figure 2 shows the output of a 100-cell battery pack with spikes from the 10kHz negative output powering an electric motor. In this example, each cell has an average DC value of 3.7V, and the 100V transient voltage is spread equally among the 100 cells. The top cell has a common-mode voltage of 370V, a common-mode switching transient voltage of 100V, and a differential transient voltage of 1V.

　　Figure 2 High-voltage battery pack switching noise

　　Obviously, battery management circuits face a very big challenge. These circuits must quickly and accurately measure the voltage of each cell. This requires the ability to extract a small differential voltage from a common-mode voltage of 0 to 1000V. Today's battery management systems (BMS) mostly use a combination of off-the-shelf components arranged in a modular fashion. As shown in Figure 3, a battery pack composed of 36 batteries is monitored in 3 groups of 12 batteries. Doing so reduces the common-mode voltage on each set of analog circuits. A 2-cell module provides local power and ground to the analog circuitry, which uses the LTC6802-1 battery pack monitor.

　　Figure 3 Typical battery pack configuration

　　The LTC6802 handles data acquisition tasks for large battery packs, and is especially suitable for lithium-ion batteries. It connects directly to each cell in the battery string. The voltage of each cell is digitized into a 12-bit word with a resolution of 1.5mV. Using a unique level-shifted serial interface, multiple LTC6802s can be stacked in series without the need for optocouplers or isolators. The device accurately measures voltage directly from each cell as highly noise-immune serial data is sent along the battery pack to a system controller. Additionally, when multiple LTC6802 devices are connected in series, they can operate simultaneously, enabling fast and accurate voltage measurements of all cells in the stack.

　　Voltage measurement accuracy is better than 99.75% across the entire temperature and voltage range, and the voltage of all cells in the battery pack can be measured within 13ms. Each cell is monitored for undervoltage and overvoltage conditions, and the input to each cell has a MOSFET switch that can be used to discharge any overcharged cells. These switches are used in so-called passive charge balancing to address the battery balancing challenges described above. Each LTC6802 communicates through a 1MHz serial interface that supports broadcast and addressed instructions. The device also includes two thermistor inputs to measure ambient or battery pack temperature, two GpIO lines, and a 5V regulator. Linear Technology has made special considerations for the challenging automotive environment; the LTC6802 is designed to operate over the industrial temperature range, has high ESD, EMI and noise immunity, and has built-in diagnostics and self-test capabilities.

　　After years of hard work and continuous progress, high-energy battery systems will soon be feasible for daily use, especially as components of electric and hybrid electric vehicles. Until this happens, technical issues at all levels must be addressed for practical, economical, and reliable battery systems. The LTC6802 provides a good solution. This IC integrates data acquisition functions into a single device and can support long battery strings composed of many cells. Advances like this will ensure the commercial success of electric and hybrid electric vehicles and create opportunities for many other applications.

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