Advances in HEV Battery Management Systems

A critical element of a hybrid-electric-vehicle (HEV) propulsion system is the battery management system (BMS), which controls the performance of the HEV battery, the costliest and heaviest component of the propulsion system. This paper examines the relevance and criticality of an HEV BMS as a whole; that is, its general functions and “responsibilities”. Of these, its ability to accurately estimate and report the state-ofcharge (SOC) is arguably the most important. This paper will explain why SOC estimation is important, and will examine advances in the state of the art of SOC estimation methods, with a focus on Kalman Filter techniques. HEV BATTERY PACK OVERVIEW Optimizing the cost, weight, size and reliability of major HEV systems is critical in maximizing the value of the HEV to the end customer. Of the components comprising the propulsion system of an HEV, the costliest is the battery pack, which may represent 30– 35% of the total cost of the propulsion system. The battery is among the heaviest components of the propulsion system as well. Therefore, careful design of the battery pack and the BMS can dramatically impact the lifetime affordability of an HEV. Figure 1 provides a block diagram of a typical battery pack. Comprising the pack are the battery cells, junction block(s), BMS, thermal management system, wiring and connectors, and the pack housing. (While it is possible for the BMS to be physically located outside of, or even remote from, the pack housing itself, for this paper it is assumed to be located within the boundaries of the pack.) In most applications, the cells are wired in series to develop the necessary high voltage. The primary functions of the battery pack are to store electrical energy produced by the vehicle (during regenerative braking) and to provide electrical energy for use by the vehicle particularly during acceleration or other peak energy demands. The pack needs to do so in a manner that is safe, reliable, and cost efficient. This includes not only minimizing initial purchase costs, protecting the vehicle from voltage surges or drop-outs, and preventing harmful conditions, but also minimizing operational stresses that can shorten the life of the battery cells. These operational stresses include excessive temperature, excessive discharging, and excessive over-charging. The pack housing provides the physical housing of the pack’s components. The cooling system provides the physical capability for the air (or liquid) thermal media circulation. The junction module provides the highvoltage connection to the vehicle, as well as the necessary relays and safety interlocks for the pack. Remaining is the BMS, which manages the delivery and acceptance of electrical energy to/from the cells, as well as the operation of the cooling system and junction module. The BMS consists of a printed circuit board (PCB) and connectors (and housing, if necessary). It provides the following functions (among others): Pack