Protecting Lithium-Ion Batteries for Electric Vehicle Applications

Apr 1, 2010 12:00 PM
SAM DAVIS, Editor-in-Chief

With Li-ion batteries the choice for electric, hybrid electric, and plug-in hybrid vehicles, battery monitoring systems to protect hundreds of individual cells are critical

Electric vehicles of all kinds must have battery stacks that provide the power required to drive the associated traction motor(s). Depending on whether it is an electric vehicle (EV), hybrid electric vehicle (HEV), or plug-in hybrid (PHEV), the required voltage may be in the range of about 200V to 400V. To supply the required voltage and current, the overwhelming choice is now lithium-ion (Li-ion) rechargeable batteries that will require multiple cells to drive the traction motor(s).

Li-ion batteries are now the popular choice because they have a better energy-to-weight ratio than the previously-used NiMH batteries. Also, Li-ion batteries offer more efficient storage capacity over multiple charge-discharge cycles, and suffer less charge leakage when not in use. And unlike NiMH batteries that have been used in some high-voltage applications, battery stacks using Li-ion technology can use fewer individual cells to produce hundreds of volts.

Using Li-ion batteries has its challenges, however. Each cell must be properly monitored and balanced to ensure user safety, improve battery performance and extend battery life.

Protection and monitoring are a necessity because fires have occurred in lower voltage notebook computers that experienced over-voltage peaks. At the higher operating voltages experienced in electric vehicles, this type of fault can be catastrophic. Although the quality of battery fabrication has improved, guarding against higher temperature and fault conditions in any automotive application remains crucial for reliable operation.

Figure 1 shows one version of an HEV power train. Each cell in the Li-ion battery stack produces 3.0V to 3.9V, depending on its state of charge/discharge. It is not unusual to have 100 cells connected in series to bring the total stack voltage up to the hundreds of volts. Here, batteries in the Battery Management System power dc-ac inverters that drive its ac induction motors. Use of a high battery voltage reduces their average current, which cuts I²R power losses, allows smaller diameter cables, and reduces overall vehicle weight.

Analog Devices, Inc. (ADI) has addressed the requirements of Li-ion battery manufacturers and power system designers by developing a Li-ion battery monitoring and protection system, that integrates all necessary components, including voltage and current measurement, signal isolation and safety monitoring. ADI's system performs these functions while also allowing power system designers to replace costly discrete components, decrease power consumption and reduce system space.

ADI's Li-ion battery monitoring and protection system (Figure 2) performs five main functions, including:

• AD8280 SAFETY MONITOR CURRENT-MODE CONTROL

  Safety monitors that guard against hazardous or battery-damaging conditions.

• Voltage measurement that aids the monitoring and balancing of battery cells.

• Current measurement that monitors the battery stack's current.

• Isolators that bring the measurement signals safely from the high-voltage batteries to the low-voltage Battery Management System.

• AD7280 VOLTAGE MEASUREMENT DEVICE

  Battery Management (processor) System that controls and manages battery functions to optimize vehicle operation.

Overcharging or overheating the battery can cause thermal runaway, creating the potential for fire or venting of toxic gases. A safety monitor protects against discharging the battery too much, which can damage the batteries to the point where a replacement cost can be over $10,000. In addition, the safety monitor:

• Ensures safety of operators/maintenance personnel.
• Validates primary monitor measurements, which is a relatively low cost form of redundancy.
• Aids the OEM/system producer in meeting mandated safety requirements.

The AD8280 safety monitor is an integrated solution that monitors six cell voltages and two temperature inputs. The AD8280 safety monitor is housed in a 48-lead LQFP (low-profile quad package). It is powered completely from the battery stack, providing either a shared or a separate alarm for any of three conditions: over-voltage, over-temperature or under-voltage. Other AD8280 features include:

• Extensive self-test enhances the designers ability to meet functional safety requirements, such as ISO26262 and IEC61508.
• Large, continuous range-of-trip point settings allow the flexibility to work with any Li-ion battery chemistry.
• Flexible, user-configured safety monitor settings.
• Daisy-chained implementations that minimize the need for isolators in a high-voltage cell stack.
• Low-power mode enables the user to minimize battery drain when the battery is not in use.
• Compliant with AEC-Q100 and EMI (electromagnetic interference) standards, making it suitable for automotive applications.

All the functions required for general purpose monitoring of stacked Li-ion batteries used in electric vehicles are provided by the AD7280. It has multiplexed analog input and temperature measurement channels for up to six cells of battery management. If cell voltages exceed an upper or lower limit defined by the user, the AD7280 generates an interrupt output signal alert.

The device's ADC has an internal 3-ppm reference and resolution of 12 bits with a 1 Msps throughput rate, with a 1µs conversion time. Plus, the AD7280 includes a built-in self-test feature that exercises the ADC.

The AD7280 operates from just one VDD supply that has a 7.5V to 30V range (with an absolute maximum rating of 33V). It provides six pseudo-differential analog input channels to accommodate large common mode signals across the full VDD range. Each channel allows an input signal range, Vin(+) to Vin(-), of 0V to 5V. Input pins assume a series stack of six cells. In addition, the AD7280 safety monitor can accommodate six external sensors for temperature measurement. The AD7280 includes on-chip registers that allow a sequence of channel measurements to be programmed to suit application requirements.

Cell-balancing provided by the AD7280 is an important feature for a battery stack with multiple cells. Prior to charging, the battery stack all cells should be at the same voltage, which ensures that charged batteries will all end up with the same voltage. It has balancing interface outputs that control external MOSFETs, and allow discharging of individual cells.

A daisy-chain interface allows up to 20 AD7280s to be stacked without the need for individual device isolation. The AD7280 requires only one supply pin which draws 7mA under normal operation, while converting at 1 Msps. It is housed in a 48 pin LQFP or 48 pin LFCSP (lead frame chip-scale package) operating over a -40°C to +105°C range, which is well within the controlled temperature range of the battery stack compartment.

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