Introduction and Design Considerations for HEV/EV Battery Management System (I)

A battery pack is an array of multiple batteries (usually lithium-ion batteries in a pure electric vehicle) that can generate voltages of up to several hundred 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 provides acceleration when the car is completely stopped and is very reliable. The voltage of the battery pack is direct current (DC), which is converted to alternating current (AC) (usually three-phase) by the inverter. As with voltage, the number of phases depends on the system requirements and the type of motor used, but is usually three-phase.

The motor used is usually an induction motor and requires an alternating voltage. These motors are commonly used in electric vehicles because they are easy to drive, reliable and cost effective. The outer component of the motor is the stator with three coils wound around it. The inner layer is usually a rotor composed of copper strips or aluminum strips.

This article describes the precautions associated with battery packs and managing charge states. Since the battery pack is made up of a plurality of batteries in series, its effective use performance is based on the weakest single battery. The difference in battery power is due to chemical imbalances in the manufacturing process, location in the battery pack (heat changes), and changes in usage or life.

The difference between battery voltages indicates an imbalance in the system level battery. The cause of this difference is still under study. It is important to fully understand this because it affects the duration of the battery pack's power output, as well as the usable life of each individual battery and the life of the battery pack.

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

The challenge for battery packs is that not every battery loses its power at the same rate when drawing current. Therefore, even if the batteries are connected in series, the discharge rate will occur at different speeds. Since some batteries absorb less than others, their ability to recover and absorb electricity will change over time. Other conditions, including temperature, will speed up the cycle. As mentioned earlier, some battery cells may become hotter due to their positioning or location near the heat dissipating components.