For electric vehicle (EV) and hybrid electric vehicle (HEV) battery technology has gained significant progress not only the battery energy density has been steadily improving, but still reliable battery charging and discharging thousands of times. If the design engineer can effectively use these technological advances, then the cost, reliability and longevity, the electric vehicles and hybrid electric vehicles have the potential to compete with conventional cars.

    A battery under the battery capacity is from 100% state of charge to zero charge state can provide power. Charge to 100% state of charge or discharge to zero charge state will quickly reduce the battery life, and should therefore be carefully managed to avoid the battery is fully charged or fully discharged state. And work in 30% to 70% state of charge between (using 40% of capacity) compared to operating at 10% state of charge between 90% state of charge (using the provisions of 80% capacity) battery charging cycle can be The total number down to the original 1 / 3 or less.

    The effective battery capacity and battery life balance between the battery system design engineers a challenge. Consider the use of the previously mentioned 40% of capacity and capacity utilization of 80%. If the battery is limited to only 40% of its capacity, so that the battery life of up to 3 times the original, then the battery size must be increased 1-fold to obtain 80% of capacity and use as many cases the available capacity. But the battery system will increase the weight and volume is doubled, thereby increasing costs and reduce efficiency.

    Car manufacturers generally require more than 10 years of battery life, and on the available battery capacity necessary to do the required. Battery system design engineers face the challenge must do everything we can with the smallest battery to achieve maximum capacity. To achieve this goal, the battery system must use sophisticated electronic circuitry to control and monitor the battery carefully.

    Electric vehicle battery system

    Electric vehicle battery pack composed of multiple cells stacked in series. A typical battery pack about 96 battery, charging to 4.2V lithium-ion battery, this battery can produce more than the total voltage of 400V. Although the automotive power system battery pack as a single high-voltage battery, each battery pack for the entire charge and discharge, but the battery control system must be considered independently of each battery. If the battery in a battery capacity slightly lower than other batteries, so after several charge / discharge cycles, its charge state will gradually deviate from the other battery. If the battery charge status is not cyclical balance with the other battery, then it will eventually enter the deep discharge state, resulting in damage, and eventually the formation of battery failure. To prevent this from happening, the voltage of each battery must be monitored to determine the charge status. In addition, there must be a separate device for charging or discharging the battery to balance the battery charge status.

    Battery monitoring system is an important consideration is the communication interface. Communications on the PC board, the common options include Serial Peripheral Interface (SPI) bus, I2C bus, each bus of the communication overhead is very low, for low interference environment. Another option is the Controller Area Network (CAN) bus, this bus is widely used in automotive applications. CAN bus is very robust, with error detection and fault tolerance features, but its large communication overhead, material costs are high. Although the main battery system to the vehicle CAN bus connection is worthwhile to be, but the battery pack used in SPI or I2C communication is an advantage.

    Linear Technology Corporation (Linear) has introduced a battery system design engineers to meet the demanding requirements of these devices. LTC6802 is a battery monitor IC, capable of measuring up to 12 overlay the battery voltage. LTC6802 as well as internal switch, the battery can be discharged separately, so that they can with the other battery in the battery pack into balance.

    To illustrate the structure of the battery pack, consider a 96 lithium-ion battery system.