What should be paid attention to in the design of battery pack and management state of charge in HEV/EV battery management system

What needs to be paid attention to in the design of HEV/EV battery management system for battery packs and managing state of charge? As shown in the figure below, the basic transmission system of an electric vehicle (EV) consists of three system modules.

A battery pack is an array composed of multiple batteries (usually lithium-ion batteries in pure electric vehicles), which can generate up to hundreds of volts. The voltage of the battery pack depends on the system requirements of the electric vehicle.

The second component of the system is the inverter. The AC traction motor used in electric vehicles can provide acceleration when the vehicle is completely stopped and is very reliable. The voltage of the battery pack is direct current (DC), which is converted into alternating current (AC) (usually three-phase) by an inverter. Like voltage, the number of phases depends on the system requirements and the type of motor used, but it is usually three-phase.

The motor used is usually an induction motor, which requires AC voltage. Such motors are often used in electric vehicles because they are easy to drive, reliable, and cost-effective. The outer component of the motor is the stator, on which three coils are wound. The inner layer is usually a rotor made of copper or aluminum bars.

Figure 1: The simple flow of the electric vehicle transmission chain-battery management system (BMS) to the inverter, then to the three-phase AC motor

This article will introduce the precautions related to battery packs and managing the state of charge. Since the battery pack is made up of multiple batteries in series, its effective performance is based on the weakest single battery. The difference in battery capacity is due to chemical imbalances in the manufacturing process, location in the battery pack (heat changes), and changes related to usage or life.

The difference between battery voltages indicates battery imbalance at the system level. The reason for this difference is still under study. It is very important to fully understand this, because it affects the duration of the battery pack in terms of power output, as well as the usable life of each single cell and the service life of the battery pack.

One of the most important parameters to consider is the state of charge. Since the power of each single battery is different, we use a percentage to reflect the power imbalance between the batteries. If the state of charge of one battery is 94%, and the state of charge of the other battery is 88%, there is a 6% imbalance in the power of the two. In addition, each battery also has a different voltage, called open circuit voltage (OCV), which is the chemical state of charge.

The challenge for battery packs is that when drawing current, not every battery loses power at the same rate. Therefore, even if the batteries are connected in series, the discharge rate will occur at a different rate. Since some batteries have lower absorption capacity than others, their ability to recover and absorb electricity will change over time. Other conditions (including temperature) will accelerate the cycle. As mentioned earlier, some battery cells may become hotter due to their positioning or location close to the heat dissipation element.

The main cause of battery failure is the complete breakdown of the battery, which will affect the battery voltage, because the battery is basically just a resistor that reduces the voltage. One way to avoid this situation is through battery balancing, which is the process of managing how to fully charge each single battery. There are several techniques to achieve cell balancing; the simplest method is to connect a resistor and a metal oxide semiconductor field effect transistor (MOSFET) in parallel to each cell, and monitor the voltage of each cell through a voltage monitoring comparator , And use a simple algorithm to turn on the MOSFET to shunt the battery. The disadvantage of this method is the waste of bypass energy.

Another technique is called charge transfer, which does not use resistors and only connects a capacitor between the cells. This technique does not cause waste of bypass energy, but it is more complicated because you need to connect the battery over a wider distance instead of bypassing each single battery.

The technology used in electric vehicles is usually inductive charging, where a transformer is connected to an unbalanced single battery because it is a higher power system. Circuit design tends to be large, which requires the design to include a larger area to accommodate the number of circuits required to implement the solution.

All these balances are based on extensive research on the characteristics and chemistry of single cells, expressed by spreadsheets and mathematical formulas that run them using tools such as MATLAB. The microprocessor plays an important role in ensuring the correct execution of all balances in the system. In order to supply power to the microprocessor, the DC/DC converter is directly connected to the battery pack and provides 48V or 12V output according to the system design to power the system. TI has two devices that can power microprocessors; both can withstand the transient characteristics under harsh conditions and a wide voltage range.

LM5165-Q1 is a 3V to 65V, ultra-low output synchronous step-down converter that can provide high efficiency over a wide range of input voltage and load current. The device has integrated high-end and low-end power MOSFETs, which can provide up to 150mA of output current with a fixed output voltage of 3.3V or 5V or an adjustable output voltage. The converter is designed to simplify the solution while optimizing the performance of applications such as battery management systems. When the operating temperature is as high as 150°C junction temperature (Tj), the device can withstand the high operating temperature range of electric vehicles.

The LM46000-Q1 SIMPLE SWITCHER® regulator is a synchronous step-down DC/DC converter that can drive up to 500mA of load current within the input voltage range of 3.5V to 60V. When you need high input voltage or larger current of the system, LM46000-Q1 provides excellent efficiency, output accuracy and voltage drop voltage with a very small solution size.

There are many ways to manage the balance of lithium-ion batteries in a battery pack, but the design appearance depends on many factors, such as cost, size, thermal characteristics, and accuracy requirements. Before implementation, all the above factors need to be taken into consideration in the design strategy. Learn more about TI products that meet stringent automotive and system requirements, and view the system block diagram of the HEV high-cell-count battery pack.