2.2 Battery Management System parts
2.2.1 The Power Module (PM)
The basic task of the PM is to charge the battery by converting electrical energy from the mains into electrical energy suitable for use in the battery. An alternative for the mains might be other energy sources, such as a car battery or solar cells. In many cases, the PM can also be used to power the portable device directly, for example when the battery is low. The PM can either be a separate device, such as a travel charger, or be integrated within the portable device, as in for example shavers.Especially in the latter case, the efficiency of the energy conversion process has to be sufficiently high, because the poorer the efficiency, the higher the internal temperature of the portable device and hence that of the battery will be. Long periods at elevated temperatures will decrease the battery capacity [19]. The monitor and control functions for the PM can be divided into two types. First of all, the energy conversion process itself has to be controlled and, secondly,the battery’s charging process has to be controlled. The Energy Conversion Control (ECC) involves measuring the output voltage and/or current of the PM and controlling them to a desired value. This desired value is determined by the CHarging Control (CHC) on the basis of measurement of battery variables such as voltage and temperature. Moreover, the current flowing into the battery can be input for the CHC function. The ECC function is situated within the PM itself, when the PM is a separate device. The CHC function can now be divided between the PM, the portable device and the battery, assuming that this battery can be removed from the portable device.The partitioning of the CHC function will depend on cost, but also on the employed charging algorithm. A removable battery can be charged separately on a so-called DeskTop Charger (DTC). In addition to the standard battery that comes with a product, some users also buy a spare battery. Cellular phone users who make many phone calls during the day sometimes charge the standard battery and the spare battery. It is often possible to simultaneously connect a standard battery in a portable product and a separate spare battery to a DTC. The priority of charging must then be fixed in the system. When the CHC function is incorporated inside the portable device, it has to be present inside the DTC as well. The reason is that the user of the portable product should be able to charge only the spare battery in the DTC. The CHC function inside the portable product can then not be used. Three simple examples of different ways of partitioning the CHC function are given below. A separate PM and a detachable battery are assumed. In the first example, the CHC function is incorporated in the PM. It is incorporated in the portable product in the second example and it is incorporated with the battery in the third example. A dashed box in the examples denotes the DTC indicating that it is optional. For simplicity, the intelligence needed in the DTC to determine charging priority is ignored.