Plastic optical fibre for battery management systems in 48 V powertrain architectures

October 11, 2017 // By Carlos Pardo

Electromagnetic noise is a major topic in any electrical power train, either full electrical or hybrid architectures. The primary source of electrical noise generation is the continual switching of the IGBTs and MOSFETs embedded within the power electronic subsystems.

The DC-AC inverter produces an AC waveform from the battery current that drives the electric motor or, alternatively, rectifies the current from the motor to store it in the battery. On the DC-AC inverter board, AC waveforms in the order of 8-10 KHz are typically generated.  The DC-DC bidirectional converter steps up the voltage of the battery pack to that employed on the high voltage bus and generates harmonics in the 50 KHz band. Another source of high frequency AC ripple on the DC link comes from the electrical motor itself. The electrical machine produces odd harmonics arising from the non-sinusoidal back-EMF waveform.

The control of all the subsystems involved in the electrical powertrain requires a communication bus that transports the control, actuation and sensor signals among the different components. The communication bus has to be immune to the above-mentioned electromagnetic noise and, at the same time, comply with the mechanical, temperature, and weight constraints of the overall vehicle.

Figure 1: A hybrid powertrain architecture

An optical-based communication technology like 1000BASE-RH complies with these requirements. Moreover, it can operate at 100 Mbit/s for most current needs while also supporting future needs at 1 Gbit/s.

Isolation risks in battery management systems

The batteries used in electrical powertrains consist of physical clusters of cells, assembled in enclosures called packs. Packs typically contain between six and twenty-four cells in series. Altogether, a commercial battery involves one hundred or more cells providing hundreds of volts. A typical li-Ion battery consists of approximately 96 stacks of cells, developing a total voltage in excess of 400 V.

It is generally accepted that more than 60 volts may be lethal to human beings, so safety is a key concern for more than the surrounding electronic equipment. Although inherently dangerous, stacks need to be monitored and managed. For this purpose, a safe and reliable communication system between the cell monitoring devices in the packs and the central battery management system (BMS) is needed. For lithium-ion chemistry, for example, it is necessary to monitor the voltage of each cell. Moreover, although individual cell temperature monitoring is not mandatory, the facility to do so should be available. These measurements are taken by specific standard product ICs (ASSPs), which typically handle from six to twelve cells.

Figure 2: Battery packs grouped into a series of electrically separate clusters

A common approach to organizing cells in a battery stack is to group the battery packs into a series of electrically separate clusters, as shown in figure 2.

Each cell pack communicates with a control module that in turn sends and receives control information from the BMS. The chain of cells is connected in series with a switch that is normally closed when the vehicle is in normal operation. In emergency situations, the switch is opened so the stack voltage disappears at the terminals. In order not

Electromagnetic noise is a major topic in any electrical power train, either full electrical or hybrid architectures. Using plastic optical fibre for battery management systems provides a new way to implement 48 V powertrain systems

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48V hybrid powertrain

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