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| 25 | Table 1 Introduction T he need to monitor the state of health of lithium- ion cells in battery packs during charging and discharging is a key requirement for Battery Management Systems (BMS) in high energy applications such as hybrid electric vehicles (HEVs) and battery electric vehicles (BEVs). Maintaining performance and safe operation is of paramount importance, as lithium-ion chemistries are prone to degradation and aging issues. A dangerous thermal runaway condition can occur if the ambient temperatures are out of specification or if the batteries are undercharged or overcharged, possibly causing cracks in the carbon and lithium plating. The three parameters measured by the BMS to determine the state of a cell's health are cell voltage, temperature, and current. The traditional solution employed to measure these parameters has been the use of shunt resistors. These passive components feature a relatively high absolute tolerance of 5 percent. However, the overall accuracy, when combined with a current sense module, can be reduced to as low as 0.01 percent. This application note provides helpful BMS design information on the accuracy level that can be obtained using Bourns ® Model CSM2F Series shunt resistors combined with a current sense module designed for this purpose. Bourns® Shunt Resistors for Battery Current Measurement Bourns offers three shunt resistor models qualified by Bourns for harsh environment energy storage applications. Table 1 outlines the key characteristics of these products. The resistive element in all three models consists of large copper terminals, as shown in the examples of the CSM Series on the left. Given that the resistivity of copper is 1.72 x 10-8Ωm and that the resistance will increase by 0.393 percent for every extra degree Celsius in temperature, the CSM Series overall coefficient of resistance between the two points will be higher than the resistivity of the Current Sense Resistor's resistor alloy (maximum 50ppm/ °C or 0.05 percent/ ºC). If the distance between the two measurement points in copper is 3 mm in total, as is the case with CSM2F-8515, the temperature coefficient of resistance, or TCR, will increase from 50ppm/ °C (TCR of element) to 150ppm/ °C (TCR of the combined element plus copper terminal). The maximum current these shunts can carry is quite high. For example, the Bourns ® Model CSM2F-7036 uses Ohm's Law to carry 1000A, DC/DC at a maximum power 50W. A typical battery pack for an HEV is 24kWH. This is equivalent to 500 ampere hours in a 48V vehicle. Therefore, the current can be very high, especially during periods of high power, such as acceleration when moving or fast charging when stopped.