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Measuring charge/discharge current of EV batteries and other current sense apps

Texas Instruments tells us that Battery-monitoring systems are used to estimate state of health (SOH) and state of charge (SOC), especially in Electric Vehicles (EVs). To get detailed information about SOH and SOC, integrating accurate sensors into the battery-monitoring system is critical to protect the battery from damage. For a typical battery, current, voltage and temperature, sensors must measure the following parameters:

  • The current flowing into (when charging) or out of (when discharging) the battery.
  • The pack voltage.
  • The individual cell voltages.
  • The temperature of the cells.

Let’s look at the location of current sensors in a block diagram of a battery-control unit. See Figure 1.

Figure 1

The location of the current sensor in a battery-control system (Image courtesy of Texas Instruments)

The location of the current sensor in a battery-control system (Image courtesy of Texas Instruments)

Using an isolated shunt current-sense measurement system provides excellent isolation between the hot and cold sides in an automotive application; this system helps with battery sensing reliability. TI has the following devices like the AMC1301 which provides current signal isolation and the ISOW7821 that provides power isolation. The PGA400-Q1 eliminates the offset and gain errors. See Figure 2.

Figure 2

An isolated shunt current-sense measurement system (Image courtesy of Texas Instruments)

An isolated shunt current-sense measurement system (Image courtesy of Texas Instruments)

See also Signal Chain Basics #113: Precision current measurement optimizes motor control

Formula E racing car battery current measurement

Isabellenhütte is using the FIA Formula E racing series as an innovation driver for its measurement technology products. This event exposes their shunt-based IVT measurement technology to the harshest requirements. Future generations of IVT series products will benefit from the knowledge gained.

With ISA-WELD and ISA-PLAN technology Isabellenhütte has developed an advanced shunt technology, which is applied in precision measurement. With those shunts and the simple formula, I = U/R, these devices will measure current the most precise way possible with state-of-the-art technology. Thus, in collaboration with a semiconductor producer, the company developed an ASIC, which transfers the measured values into a data stream, that can be used for further processing.

The IVT-F is used in Formula E race cars and measures the charge and discharge volumes of the battery units. These systems achieve high energy densities when high voltages are applied. This is why the isolation electric strength of the measurement sensors must be correspondingly high.

Figure 3

The IVT-F used in Formula E race cars (Image courtesy of Isabellenhütte)

The IVT-F used in Formula E race cars (Image courtesy of Isabellenhütte)

This shunt-based measurement system is a custom-made product for the FIA. The device must meet elementary physical and technological framework conditions due to the FIA racing regulations. In the racing trim, the sensor system used must be extremely precise with high level isolation capabilities. With a plastic potting that surrounds the IVT-F, the standard insulation used from the standard product IVT-S, which is between 600 and 800 volts, is therefore additionally amplified. In this way, the IVT-F achieves an isolation electric strength of 1,000 volts. The very good linearity, the custom-made electronics, the quick sampling and the excellent calibration give a precise measurement.

The heat generated while racing is countered by the IVT-F with a high temperature resistance. The system, which is designed from specially-resistant materials, has a very low temperature coefficient that protects the IVT-F from malfunctioning. The system ensures precise measurement results up to an operating temperature of +105o C. A microcontroller monitors the sensor status to meet the safety requirements.

As with the standard product IVT-S, an A/D converter in the IVT-F also converts a precise transformation of the voltage drop into digital signals. A CAN bus interface was installed for the data transfer. It ensures the communication between IVT-F and the battery control unit.

Allegro Microsystems uses a Hall Effect current sensing technique in Hybrid Electric Vehicles (HEVs).

The A1360 Hall-effect sensor is positioned in the gap of a ferromagnetic toroid that surrounds the inverter phase conductor in the motor.

Figure 4

A Hall-effect sensor is running through the gap of a ferromagnetic toroid that surrounds the inverter phase conductor in the motor (Image courtesy of Allegro Microsystems)

A Hall-effect sensor is running through the gap of a ferromagnetic toroid that surrounds the inverter phase conductor in the motor (Image courtesy of Allegro Microsystems)

In an HEV power design, the main battery voltage is inverted as shown in Figure 4, and the resulting AC voltage is applied to the motor that drives the wheels (This is where AC current sensing is done). In the process of regenerative braking, the AC motor acts as a generator. When the regeneration system is active, the output of the motor-generator is rectified and converted to a DC voltage to charge the HEV battery cells (Here is where the DC current is sensed while going to the battery), completing the power cycle.

Figure 5

An HEV power system architecture (Image courtesy of Allegro Microsystems)

An HEV power system architecture (Image courtesy of Allegro Microsystems)

The three-phase, full-bridge driver in a typical inverter will convert DC battery voltage to a 3-phase AC voltage required for efficient operation of the system motor. The inverter phase currents are measured (See Figure 4) and the resulting information is typically used to control the pulse-width modulated (PWM) inverter switches (typically IGBTs). The inverter control loop requires high bandwidth current sensors to improve accuracy, and to maximize motor torque and overall motor efficiency. High-side current sensors with fast response times also enable overcurrent protection during a short circuit condition from a motor phase to the system ground node.

Figure 6

A DC/DC converter is on the left side of this diagram and the Three-phase DC/AC Converter is on the right side. (Image courtesy of Allegro Microsystems)

A DC/DC converter is on the left side of this diagram and the Three-phase DC/AC Converter is on the right side. (Image courtesy of Allegro Microsystems)

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