Battery State-of-Charge indication

One of the tasks of a Battery Management System is to keep track of a battery’s State-of-Charge (SoC), as discussed in chapter 1 of this book. Information on the SoC can be used to control charging and discharging and it can be signalled to the user. An accurate SoC determination method and an understandable and reliable SoC display to the user are important features for a portable product. However, many examples of poor accuracy and reliability can be found in practice. This can be pretty annoying, especially when a portable device suddenly stops functioning whereas sufficient battery capacity is indicated. A poor reliability of the SoC indication system may induce the use of only part of the available battery capacity. For example, the user may be inclined to recharge the battery every day, even when enough battery capacity is indicated on the portable device. This will lead to more frequent recharging than strictly necessary, which in turn leads to an earlier wear-out of the battery. The effect of inaccuracy of SoC indication can be even worse when the SoC value is also used to control charging. The battery is either not fully charged or it is overcharged. In the former case, the battery will be recharged more often than needed, which will lead to an earlier wear-out. In the latter case, frequent overcharging will lead to a lower cycle life. Some methods with which an SoC indication system can be realized will be described in this chapter.