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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

- See more at: http://www.newelectronics.co.uk/electronics-technology/big-bus-puts-new-demands-on-the-battery-management-system/56462/#sthash.Uy68rfZW.dpuf