Is a LiFePO4 or Lithium-ion battery better for eCall systems?

The European Parliament’s eCall regulation law passed in 2015 and goes into effect in April 2018, mandating that all cars that are released in the European market are required to be equipped with eCall. Due to the characteristics of the eCall system, it is important to make the system self-contained, reliable and maintenance-free, with its own battery independent of the car battery. The battery must have enough energy to make a 10-minute phone call, remain on the cellular network for 60 minutes after the initial call and operate at any time. The most common battery chemistries used in this application are lithium-ion (Li-ion) and lithium-ion phosphate (LiFePO4). LiFePO4 batteries are safer due to their higher thermal runaway temperature but have higher self-discharge, which can cause balancing issues with aging. Li-ion batteries have higher energy density but require a protection circuit to operate safely. Table 1 shows the comparison between LiFePO4 and Li-ion batteries. Li-ion has higher density and has been used for space limited applications to meet the run time requirements.

Table 1: Comparison between LiFePO4 and Li-ion Batteries

Figure 1 is a power-tree diagram of a typical eCall system using a single-cell LiFePO4 or Li-ion battery. As you can see, a DC/DC buck converter is required to step down the voltage to 5V from a 12V car battery. Then a linear charger from a 5V power rail can charge the battery, while power rails stepping from the 5V voltage rail can supply the GSM (Global System for Mobile communication) module, MCU, GPS module and audio amplifier.
e 1: Comparison between LiFePO4 and Li-ion Batteries

Figure 1: eCall system power-tree diagram (image courtesy of Chris Glaser, BMS Deep Dive 2015)

As an example of a battery-management solution, the bq25071-Q1 can charge a LiFePO4 battery, while the bq24081-Q1 and bq24075-Q1 can handle Li-ion battery charging. All three devices are automotive Q100-graded with integrated safety features such as undervoltage, overvoltage, short-circuit and thermal protection, which helps to ensure safe charging operation. The bq27441-G1 uses TI’s patented Impedance Track™ algorithm for battery fuel gauging and helps maximize battery lifetime. For additional safety protection, the bq29700 provides an accurate monitor and trigger threshold for overcurrent protection during high discharge/charge current operation or battery overcharge conditions.

As eCall becomes mandatory in automotive vehicles, TI is ready to provide a complete turnkey solution to support a self-contained and reliable eCall system.

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