The key to a green society lies in semiconductor circuits and system-level optimization: Lithium-ion batteries

In the case of lithium-ion rechargeable batteries, semiconductor technology can contribute to accurately grasping the voltage of each battery cell and realizing the required circuit with fewer components. In order to obtain voltage, electric vehicles and hybrid vehicles need to connect several lithium-ion rechargeable battery cells in series. The self-discharge current of each lithium-ion rechargeable battery cell is different. After repeated charging and discharging in series, the capacity difference between the cells will increase (Figure 6). There is also the possibility of overvoltage during the charging process. Therefore, although the voltage of each cell can be monitored, it takes a variety of separate components such as AD conversion IC, reference IC and multiplexer to form a voltage monitoring circuit. Therefore, semiconductor manufacturers are working hard to develop ICs that can integrate these components and realize voltage monitoring of multiple cells on a single chip. A representative product is the "LTC6802" (Figure 7) from Linear Technology of the United States. This product can support 12 battery cells, and the number of components can be reduced to 1/3 to 1/5 of the original. Two ICs can monitor 24 battery cells, and three ICs can monitor 36 cells.

Figure 6: To maximize the use of batteries, battery balance monitoring is necessary This figure shows the charge and discharge status of battery cells when battery balance monitoring is performed and when it is not. When monitoring battery balance and controlling charging by cell, each cell can be charged to a state close to full charge. The driving time of the equipment using the battery will also be extended accordingly. This figure refers to the information of Linear Technology of the United States, and adds a schematic diagram when battery balance monitoring is performed.

Figure 7: Monitoring each cell of a lithium-ion battery The voltage monitoring IC "LTC6802" from Linear Technology is suitable for monitoring the voltage of each cell of a lithium-ion battery. It can support a battery pack consisting of 12 cells. This figure is drawn based on the company's data.

The most significant feature of Linear Technology's LTC6802 in terms of performance is that it can measure voltage with a maximum accuracy of 0.25% Note 1). It is not easy to achieve this accuracy even if a voltage monitoring circuit is composed of separate components. Therefore, "At the time of the product release, we received a large number of inquiries from automobile manufacturers, electrical equipment manufacturers, and battery manufacturers. Most manufacturers are currently evaluating it, and some have decided to adopt it in mass production" (Linear Technology Japan). The company said that the high characteristics of the integrated 12-bit ΔΣ type AD conversion circuit and the voltage reference contributed to achieving this accuracy.

Note 1) It also depends largely on the battery material, but due to the 0.25% voltage measurement detection error, the detection accuracy of the battery charge rate (SOC: state of charge) will decrease by about 3%, and it will decrease by about 5% when LiFePO4 is used.

In order to improve the reliability of the voltage monitoring system, Linear Technology plans to improve the voltage monitoring IC in addition to the auxiliary IC "LTC6801" for backup circuits released in December 2009. It is intended to further improve the accuracy. Higher accuracy would allow for maximum battery usage, help achieve smaller units, and easily detect overvoltage, presumably supporting faster charging.