High-performance bus bar shunt resistors are helping electric vehicle (EV) designers meet the challenges of the market in new ways. This article reviews the basics of shunt-resistor technology and outlines several recent improvements, including a new metal-injection-molding process and a new form factor. These innovations are leading to breakthroughs in minimizing power loss, reducing physical footprints and managing thermal and conductivity properties.
Overview of shunt resistor technology
A battery in an EV must supply 100 kW of power or greater in response to driver demand. A typical EV battery system consists of hundreds of lithium-Ion battery cells connected in series to generate a voltage of 350 V or more. The voltage, current and temperature of the battery system must be carefully monitored to control charging and discharging, to balance the charge between cells, to keep the cells within their safe operating area (SOA), and to calculate key battery parameters such as state of charge (SOC) and state of health (SOH).
The current sensor must be able to accurately measure currents from milliamps to hundreds of amps over automotive operating temperatures of -40° C to 125° C and an operating life of up to 20 years. A shunt-based current sensing design determines the current (I) by measuring the voltage (V) generated as I flows through a shunt resistor (R) placed in the battery line, as expressed by Ohm’s law:
Figure 1 shows a typical EV arrangement where the shunt is placed in the battery return path. The shunt resistor is typically part of a module that also includes a battery management integrated circuit to measure the voltage across the shunt and communicates with the vehicle network over the industry-standard CAN bus. Note that the current flow can be positive or negative.
Design challenges with existing shunt technology
A practical implementation of a shunt-resistor measurement system poses several challenges for the designer.
The resistance of an “ideal” shunt resistor does not change with time, current or operating temperature; this is not true for a real-world device. For example, any resistor dissipates power according to the equation P = I2R. As I increases, so too does the temperature. In a real component, a change in temperature causes a change in the value of R, characterized in a resistor data sheet as the temperature coefficient of resistance (TCR). Additionally, component aging causes the resistance to change over time, and a real-world device also exhibits parasitic inductance and capacitance. At low currents, error may occur due to thermal electromotive force (EMF) — a voltage in the microvolt (μV) range caused by temperature variations across the shunt resistor.
Isabellenhütte has developed a range of specialized alloys such as Manganin, Isaohm and Zeranin for current measurement applications. Shunt resistors made from these alloys combine low resistances with extremely low inductance, TCR and thermal EMF, and excellent long-term stability.
As shown in Figure 2, the shunt resistor is then combined with two copper (Cu) terminals using Isabellenhütte’s proprietary ISA-WELD® electron-beam welding process. This approach integrates the alloy into a bus bar with a combination of Cu-Manganin®-Cu, or the Manganin can be substituted for another resistive alloy.