Big bus puts new demands on the battery management system

4 min read

Today's vehicles have an electrical system operating from a 12V battery with an associated power management system. Two requirements, however, are stressing the 12V power system to the limit of its capability.

Firstly the European Union's 2020 standard for cars and vans sets a CO2 emission target of 95g/km across a manufacturer's car fleet, which is only achievable through the introduction of more electrified drivetrains. Secondly consumer demand is driving the wider adoption of desirable features such as climate control, advanced safety systems, navigation systems, on-board entertainment and in-seat heating. Both of these factors are driving the load on the electrical system beyond 3kW, which is roughly the limit of the classic 12V system's capability. The result: batteries that suffer from early failure, and users' dissatisfaction with the short life of their vehicle's battery. Now two dramatic changes in automotive power systems have been proposed to address the problem. First, some vehicle manufacturers intend to introduce a 48V power bus. This higher-voltage network enables higher loads (up to 10kW) to be supported with the same or even narrower cable diameters. Second, the venerable lead-acid battery type is to be replaced by lithium-based batteries (preferably the LiFePO4 or LiTi2O3 types), which will support more charge/discharge cycles, resulting in longer battery life. This new system, however, requires dramatic changes to the electrical topology of a car. The new 48V system will operate side-by-side with the conventional 12V one; the 48V bus will only supply those functions which need the high power input and output it provides – the remaining functions will continue to run from the 12V bus. A DC-DC converter will allow battery power to be distributed between both voltage domains. The new 48V system might also require modification of communication systems such as the popular CAN (Controller Area Network) bus. Last but not least, the use of new lithium batteries, which provide sufficient capacity to support the rise in electrical power usage, calls for a much more sophisticated battery management and diagnosis system than is required for a lead-acid battery. The challenge for automotive electronics designers is to implement a battery management system (BMS) that provides for a combination of safe operation, long battery life and the separation of the low- and high-voltage domains without requiring the use of numerous components in a complex circuit design. Since the development of 48V automotive power systems is in its infancy, there is as yet no single, preferred architecture or approach to achieving these goals. The circuit shown in Figure 1 shows a highly integrated approach to the task and implements the four main functions of a 48V BMS: 1. Measurement of the battery pack voltage, individual cell voltages and current. These data are used to maintain the battery within its safe operating area. 2. Cell balancing 3. Separation between the 12V and 48V domains 4. Fail-safe disconnection of the 48V domain The first function is a normal requirement of a BMS system. Today's 12V cars equipped with an automatic start/stop feature require the battery's state of charge (SOC) to be maintained at no lower than 50%. This ensures that there is permanently sufficient capacity to drive the engine-start mechanism. This is surprisingly difficult to achieve. A lithium-based battery – most of all the LiFePO4 type – has a very flat discharge voltage curve (see Figure 2). That is, its output voltage declines very gradually as it is discharged, and only falls at a quicker (more easily measurable) rate when its SOC is close to 0%. This makes voltage measurement alone an ineffective method for determining the SOC. In fact, the best way to monitor a lithium battery's SOC is to measure the open-circuit voltage to establish a measurement start point, and then to perform 'coulomb counting' – measuring the total current as it exits the battery. This calls for very high accuracy on the voltage channel, and an offset-free current measurement path: in the circuit shown in Figure 1, the ams AS8510 battery sensor interface offers both. For measurement of the pack voltage, the AS8510, which integrates signal conditioning functions and two 16-bit ADCs, is combined with AS880x high-precision attenuators: this circuit's accuracy exceeds that of traditional discrete implementations, reaching an accuracy level of 0.2%. Prolonged battery life The system design shown in Figure 1 also provides for cell balancing, a necessity in lithium-based batteries. Like lithium battery packs, lead-acid batteries are composed of multiple individual cells (energy-storage units). In every battery, there are random variations between one cell and another which mean that some cells become fully charged before other cells do. The goal of battery management is to ensure that all cells maintain an equal state of charge. The closer the BMS gets to achieving this goal, the greater the battery's capacity and the longer its life. Lithium batteries are extremely sensitive to overcharging, and so this technique cannot be used for balancing lithium cells. Instead, the charge must continually be re-distributed between cells throughout the charge/discharge cycle. The AS8506 IC from ams uses internal synchronised offset-free comparators to make the decision locally about the cells that need to be balanced (see Figure 3). Unlike conventional cell-balancing designs, this system operates without the involvement of a microcontroller, and so is easier to implement. Separating 48V and 12V Separation between the voltage domains is achieved through the use of high-resistance voltage dividers. In the event of a failure, a series resistor in conjunction with the 3.9V Zener diode dissipates the excess voltage before it can damage any other components, or cross to the CAN bus. This design allows for safe separation of the voltage domains without the need for isolation components such as optical or magnetic couplers. Should the vehicle be involved in an accident, or should the battery reach the limit of its operating range, it is advisable to disconnect the battery pack completely from the 48V path. This should also be done when the vehicle is not used for a long time. This is achieved by placing a high-power relay on the battery's positive voltage rail. Finally, the AS8601 System Basis Chip (SBC) provides power to the entire BMS, and integrates a CAN transceiver. The 48V power system is one of the last remaining ways to achieve large reductions in fuel consumption in ICE-powered vehicles. The cost of deployment and the need to completely redesign the topology of the electrical network have held back deployment before now. But with lithium battery prices falling, and car manufacturers under intense pressure to reduce CO2 emissions, the 48V power bus is on the verge of a breakthrough. Gernot Hehn is Application Engineer for ams AG.