Battery management architectures for HEVs

Squeezing the maximum capacity out of the smallest battery pack in HEVs requires careful control and monitoring of the SOC

BY JIM DOUGLASS
Linear Technology, Milpitas, CA
http://www.linear.com

Vehicle manufacturers typically demand battery lifetimes exceeding 10 years, and they also specify the required usable battery capacity. The challenge to the battery system designer is to squeeze the maximum capacity out of the smallest battery pack. To accomplish this, the battery system must carefully control and monitor the batteries using precision electronics.

Electric vehicle battery pack systems

An electric vehicle battery pack consists of dozens of batteries stacked in series. A typical pack might have a stack of 96 or so batteries, developing a total voltage in excess of 400 V for Li-ion batteries charged to 4.2 V.

While the vehicle power system sees the battery pack as a single, high-voltage battery—charging and discharging the entire battery pack at once—the battery control system must consider each battery’s condition independently. If one battery in a stack has slightly less capacity than the other batteries, then its state of charge will gradually deviate from the rest of the batteries over multiple charge/discharge cycles.

If that cell’s state of charge is not periodically balanced with the rest of the batteries, then it will eventually be driven into deep discharge, leading to damage, and eventually complete battery stack failure. To prevent that from happening, each cell’s voltage must be monitored to determine state of charge. In addition, there must be a provision for cells to be individually charged or discharged to balance their SOCs.

An important consideration for the battery-pack-monitoring system is the communications interface. For communication within a PCB, common options include the serial peripheral interface (SPI) bus and inter-integrated circuit (I2 C) bus. Each has low communications overhead, suitable for low-interference environments.

Another option is the controller area network (CAN) bus, which has widespread use in vehicle applications. The CAN bus is very robust, with error detection and fault tolerance, but it carries significant communications overhead and high materials cost. While an interface from the battery system to the main vehicle CAN bus may be desirable, SPI or I2 C communications can be advantageous within the battery pack.

Devices such as the LTC6802 battery stack monitor IC enables battery system designers to meet these difficult requirements and measure the cell voltages of up to 12 stacked cells. The device also has internal switches that provide for the discharge of individual cells to bring them into balance with the rest of the stack.

To illustrate the battery stack architecture, consider a system with 96 Li-ion cells. Eight battery stack ICs would be required to monitor the entire stack, with each device operating at different voltage levels.

Using 4.2-V Li-ion batteries, the bottom monitoring device would straddle 12 batteries with potentials scaling from 0 to 50.4 V. The next group of batteries would have voltages ranging from 50.4 to 100.8 V, and so forth, up the stack.

Communicating between these devices, at different potentials, presents a difficult challenge. A variety of approaches have been considered, and each has advantages and disadvantages in light of the priorities of the vehicle manufacturers.

Battery-monitoring requirements

At least five major requirements need to be balanced when deciding between battery-monitoring-system architectures. Their relative importance depends on the needs and expectations of the end customer.

1. Accuracy. To take advantage of the maximum possible battery capacity, the battery monitor needs to be accurate. A vehicle, however, is a noisy system, with electromagnetic interference over a wide range of frequencies. Any loss of accuracy will adversely affect battery pack longevity and performance.

2. Reliability . Automobile manufacturers must meet extremely high reliability standards, irrespective of the power source. Furthermore, the high energy capacity and potentially volatile nature of some battery technologies is a major safety concern. A fail safe system that shuts down under conservative conditions is preferable to catastrophic battery failure, although it has the unfortunate potential of stranding passengers. As a result, battery systems must be carefully monitored and controlled to ensure complete control over their entire life in the system. To minimize both false and real failures, a well-designed battery pack system must have robust communications, minimized failure modes, and fault detection.

3. Manufacturability . Modern vehicles already contain a vast array of electronics with complicated wiring harnesses. Adding sophisticated electronics and wiring to support an EV/HEV battery system is an additional complication for automobile manufacturing. The total number of components and connections must be minimized to meet stringent size and weight constraints and ensure that high-volume production is practical.

4. Cost . Complicated electronic control systems can be expensive. Minimizing the number of relatively costly components, like microcontrollers, interface controllers, galvanic isolators, and crystals, can significantly reduce total system cost.

5. Power . The battery monitor itself is a load on the batteries. Lower active current improves system efficiency and lower standby current prevents excessive battery discharge when the vehicle is off.

Battery monitoring

Four architectures for battery-monitoring systems are described below. Table 1 summarizes the pros and cons of each architecture, assuming a 96-battery system organized into 8 groups of 12 batteries.

Table 1. Battery-monitoring Architecture Comparison

Each architecture is designed to be an autonomous battery-monitoring system. Each provides a controller-area network (CAN) bus interface to the vehicle’s main CAN bus and is galvanically isolated from the rest of the vehicle.

Parallel independent CAN modules

Each 12-battery module contains a PC board with an LTC6802, a microcontroller, a CAN interface, and a galvanic isolation transformer. The large amount of battery-monitoring data required for the system would overwhelm the vehicle’s main CAN bus, so the CAN modules need to be on local CAN subnets. The CAN sub-nets are coordinated by a master controller that also provides the gateway to the vehicle’s main CAN bus.

Fig. 1. Parallel independent CAN module.

Parallel modules with CAN gateway

Each 12-battery module contains a PC board with an LTC6802 and a digital isolator. The modules have independent interface connections to a controller board containing a microcontroller, a CAN interface, and a galvanic isolation transformer. The microcontroller coordinates the modules and provides the gateway to the vehicle’s main CAN bus.

Fig. 2. PDiagram of parallel modules with CAN gateway

Single monitoring module with CAN gateway

In this configuration, there is no monitoring and control circuitry within the 12-battery modules. Instead, a single PC board has 8 LTC6802 monitor ICs, each of which is connected to its battery module. The LTC6802 devices communicate through non-isolated SPI-compatible serial interfaces. A single microcontroller controls the entire stack of battery monitors via the SPI-compatible serial interface, and it also is the gateway to the vehicle’s main CAN bus. A CAN transceiver and a galvanic isolation transformer complete the battery-monitoring system.

Fig. 3. SDiagram of single monitoring module with CAN gateway.

Series modules with CAN gateway

This architecture is similar to the single monitoring module, except each LTC6802 is on a PC board within its 12-battery module. The eight modules communicate through the LTC6802 non-isolated SPI-compatible serial interface, which requires a three- or four-conductor cable to be connected between pairs of battery modules.

A single microcontroller controls the entire stack of battery monitors via the bottom monitor IC, and also acts as the gateway to the vehicle’s main CAN bus. Once again, a CAN transceiver and a galvanic isolation transformer complete the battery-monitoring system.

Fig. 4. SDiagram of series modules with CAN gateway.

Battery-monitoring architecture selection

The first and second architectures are generally problematic due to the significant number of connections and the external isolation required for the parallel interface. For this added complexity, the designer has independent communication to each monitor device. The third (single monitoring module with CAN gateway) and fourth (series modules with CAN gateway) architectures are simplified approaches with minimal limitations.

The LTC6802 can address all four configurations, leaving the choice to the system designer. Two variants of the device have been created, one for series configurations and one for parallel configurations. The LTC6802-1 is designed for use in a stacked SPI interface configuration. Multiple devices can be connected in series through an interface that sends data up and down the battery stack without external level shifters or isolators. The LTC6802-2 allows for individual device addressing in parallel architectures. Both variants have the same battery-monitoring specifications and capabilities.

Electric vehicles place huge demands on battery packs. Vehicle manufacturers expect cost-effective battery systems that meet their stringent reliability requirements. The latest battery-monitoring ICs give system designers the flexibility to choose the best battery pack architecture without compromising performance. ■

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