Power Electronics

Protecting Lithium-Ion Batteries for Electric Vehicle Applications

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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CURRENT MEASUREMENT

The AD821x accurately amplifies small voltages in the presence of large common-mode voltage, provides high bandwidth, as well as level shifting and bidirectional capability. Excellent dc and ac accuracy from -40°C to 125°C minimizes errors in the measurement loop without sacrificing cost and package size.

The ADuC7032 is a complete system solution for battery monitoring in 12V automotive applications. The device integrates all of the required features to monitor, process, and diagnose 12V battery parameters, including battery current, voltage and temperature over a wide range of operating conditions. Typical monitored functions include the power for lighting, radio, navigation, air conditioning, windshield wipers, etc.

ADUM140X ISOLATORS

The ADuM140x* are quad-channel digital isolators based on ADI's iCoupler® technology. Combining high speed CMOS and monolithic air core transformer technology, these isolation components provide better performance than optocouplers. These iCouplers eliminate the need for external drivers and other discrete components and consume one-tenth to one-sixth of the power consumed by optocouplers at comparable signal data rates.

The ADuM140x isolators provide four independent isolation channels in various channel configurations and data rates. All models operate with the supply voltage ranging from 2.7 V to 5.5 V, providing compatibility with lower voltage systems as well as enabling voltage translation functionality across the isolation barrier. In addition, the ADuM140x provides low pulse width distortion (<2 ns for CRW grade) and tight channel-to-channel matching (<2 ns for CRW grade). The ADuM140x isolators have a patented refresh feature that ensures dc correctness in the absence of input logic transitions and when power is not applied to one of its power supplies.

BATTERY MANAGEMENT SYSTEM

The Blackfin BF50x series delivers better performance than most comparably priced processors with integrated analog-to-digital converters (ADCs) and flash memory. This enables significant gains in signal conversion and computational precision, and applies advanced power control techniques that yield greater energy efficiency.

Blackfin BF50x processors deliver up to 400MHz of performance at a price point where 150-200MHz clock speeds have been the norm, extending high-performance digital signal processing capabilities to a wider range of applications, including those previously serviced by high-end microcontrollers. Designers are also enabled to utilize more advanced software tools and libraries for code generation, which helps shorten product development cycles and speed time to market without compromising on processor cost.

These processors combine industry-standard interfaces with a high-performance signal processing core to ensure that applications can be developed quickly and cost-effectively, without the need for expensive external components. With optional integrated dual-SAR (successive approximation register) 12-bit ADCs for more accurate data conversion and 4MB of on-board executable flash memory, Blackfin BF50x processors minimize off-chip components that lower overall system costs.

Other semiconductor companies have products intended for battery monitoring and protection of multiple Li-ion cells.

LTC6801 BATTERY FAULT MONITOR

Linear Technology's LTC®6801(Figure 3) is a high-voltage battery stack fault monitor that operates without a microprocessor, and without the need for optocouplers or isolators. An LTC6801 can monitor up to 12 series-connected battery cells for over-voltage and under-voltage conditions. You can daisy-chain multiple LTC6801s, providing a method to monitor each individual cell in very long battery strings. When connected in a daisy-chain, a single differential clock output confirms that all cells in the stack are within the defined operating range. This clock interface provides high noise immunity and ensures that fault conditions are not hidden by frozen bits or short circuit conditions. The result is a reliable and simple design that can serve as a complete monitoring or redundant circuit.

The LTC6801 is a low-cost companion to the LTC6802 precision battery measurement and cell balancing IC, providing a backup circuit for hybrid electric battery packs, battery backup systems, and other high powered Li-ion battery systems.

A wide range of over-voltage and under-voltage thresholds can be set via pin connections and the LTC6801 offers selectable threshold hysteresis and adjustable update rates. The LTC6801 is fully specified for operation from -40°C to 85°C and two temperature sensor inputs are monitored for over-temperature faults.

LTC6802 BATTERY STACK MONITOR

Linear Technology's LTC6802 is a building block IC that allows constructing battery modules with a minimum of components, while providing BMS (Battery Management System) performance requirements. It is a multi-cell monitor IC that provides accurate direct 12-bit digitizing of up to 12 series-connected battery potentials, cell balancing controls and even a pair of additional ADC inputs for temperature readings or other metrics.

The LTC6802 ADC does not depend on resistor networks and provides a uniform, light load on each cell and automatically assumes a low-power standby condition during idle periods to reduce power. A Serial Peripheral Interface (SPI) digital connection to a local microprocessor comprises the means for command and data communication. The LTC6802 serves as a standard slave I/O device to the µP, enabling all BMS algorithms to be software-coded and controlled exclusively by the developer. One version contains has an SPI port that can be daisy-chained without the need for isolation, further reducing cost and complexity in the module.

A second version of the LTC6802 (LTC6802-2) has an SPI that is individually addressable. This eliminates the primary disadvantage of all daisy chain topologies; namely, that a fault in one module results in a loss of communications with all modules above it. The primary trade-off for this scheme is that the data buses on the modules must be isolated from one another.

MAX11080 BATTERY MANAGEMENT SYSTEM

Maxim Integrated Products MAX11080 is a high-voltage, 12-channel battery-protection IC for high-cell-count lithium-ion (Li+) battery stacks (Figure 4). This IC provides redundant cell monitoring to prevent Li-ion batteries from thermal runaway. Typical battery protection circuits incorporate three- and four-channel fault monitors with costly galvanic isolators between the monitors and an assortment of active and passive components. These circuits and bulky, costly, and time-intensive to design.

Up to 31 of the MAX11080 ICs can be daisy-chained together to monitor as many as 372 cells. This capability prevents cascading electrical failures and eliminates the expensive isolation components required by discrete solutions. In a typical hybrid car, Maxim's solution reduces the cost of the BMS by up to 80%, according to the company.

The MAX11080 simplifies the design of high-cell-count battery packs. A 12-channel fault monitor employs a proprietary capacitor-isolated daisy-chain interface to minimize component count and cost. This unique architecture allows up to 31 devices to be connected in a series stack to monitor as many as 372 cells. Meanwhile, the capacitor-based interface provides extremely low-cost isolation from one bank of batteries to the next, eliminating cascading electrical failures.

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The company's high-voltage, small-geometry BiCMOS process enables high voltage tolerance (80V), excellent ESD performance (±2kV, Human Body Model), hot-swap capability, and reliable performance over a wide temperature range. To protect against battery thermal runaway, the MAX11080's overvoltage detection guarantees less than ±25mV error over the full AEC-Q100 Type 2 temperature range (-40°C to +105°C).

Additionally, the MAX11080 offers a 10x reduction in power consumption (80µA, operating mode) to conserve battery life. A unique built-in shutdown feature reduces consumption to an ultra-low 2µA leakage, allowing the pack to be stored for many years with very little battery drain The MAX11080 has 16 selectable overvoltage thresholds, as well as eight selectable under-voltage thresholds. The under-voltage detection feature can be disabled if desired. The device includes a programmable detection-delay feature that allows the user to filter out transient events in the battery pack to eliminate false overvoltage or under-voltage alarms. The alarm line operates using a 4kHz heartbeat signal, the absence of which indicates a valid overvoltage or under-voltage event. These features are critical for discriminating between legitimate and false alarms, preventing the application from shutting down unnecessarily.

The MAX11080 has built-in self-configuration and self-diagnostic modes. On power-up, the device automatically detects the presence of batteries and can be configured from two to 12 cells in any connection sequence or installation pattern. The device also self-tests the internal comparator circuitry to ensure proper functionality on power-up. It can detect the open or short of any pin on the package and constantly monitors the pins for such a failure.

The MAX11080 is packaged in a 38-pin TSSOP and is specified to operate from -40°C to +105°C .

ELECTRIC VEHICLE BATTERY GLOSSARY

AEC-Q100 STANDARD

The AEC-Q100 standard includes customer specific requirements (CSR). ISO/TS-16949. It is an international standards Automotive Quality Systems technical specification. It defines the purpose of establishing common part-qualification and quality-system standards for automotive components. It contains detailed qualification and re-qualification requirements, and includes unique test methods and guidelines for the use of generic data. Components meeting these specifications are suitable for use in the harsh automotive environment without additional component level qualification testing.

BATTERY PROTECTION

Battery protection and monitoring are a necessity with Li-ion battery packs. At the higher operating voltages experienced in electric vehicles, an overvoltage 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.

BATTERY STACKS

For electric vehicle transportation applications they can typically involve hundreds of cells, providing several hundred volts. Although these stacks are inherently dangerous, they must still communicate with the cell monitoring electronics, which are usually located within the battery enclosure. Thus, the communications method must be safe and reliable.

C20, C6, C1, ETC

An expression describing rate of discharge. The number indicates the number of hours to completely discharge the battery at a constant current. So C/20 is the current draw at which the battery will last for 20 hours, C/1 is the current at which the battery will last 1 hour. The useful capacity of a battery changes depending on the discharge rate, so battery capacities are stated with respect to a particular rate. For instance, a battery is rated at 42 Ah (ampere-hours) at the C/10 rate of 4.2A, but only 30 Ah at the C/1 rate of 30A.

COLD CRANKING AMPS

A performance rating for automobile starting batteries. It is defined as the current that the battery can deliver for 30 seconds and maintain a terminal voltage greater than or equal to 1.20 V per cell, at 0°F (-18°C), when the battery is new and fully charged. Starting batteries may also be rated for Cranking Amps, which is the same thing but at a temperature of 32°F (0°C).

CYCLE LIFE

How many charge/discharge cycles the battery can endure before it loses its ability to hold a useful charge. Cycle life typically depends on the depth of discharge (DOD). For example, if a hypothetical battery pack will propel your car for a maximum range of 100 miles, and you drive 50 miles between charges, (50% DOD) then you may get 600 trips before replacing the pack; but if you drove 80 miles between charges, you might only get 400 trips before the pack wears out.

DEPTH OF DISCHARGE

The amount of energy that has been removed from a battery (or battery pack). Usually expressed as a percentage of the total capacity of the battery. For example, 50% depth of discharge means that half of the energy in the battery has been used. 80% DOD means that 80% of the energy has been discharged, so the battery now holds only 20% of its full charge.

DIGITAL ISOLATOR

Digital isolators provide isolation so that the high voltage output from an EV battery can be monitored at a lower voltage.

ELECTRIC VEHICLES

This can refer to a vehicle that employs only batteries (EV) that are rechargeable, or a hybrid electric vehicle (HEV) that uses a car's internal small gasoline engine to recharge the battery, or a plug-in hybrid vehicle (PHEV) that can be recharged form a power line or from a car's internal gasoline engine.

ENERGY DENSITY

The amount of energy that can be contained in a specific quantity of the fuel source. Typically quoted in watt-hours per pound, wh/lb, or watt-hours per kilogram, wh/kg. Battery technologies such as NiMH and li-ion are in the 80-135 wh/kg range.

LITHIUM-ION (LI-ION)

A type of rechargeable battery in which lithium ions move from the negative electrode to the positive electrode during discharge, and reversely when charged. Different types of lithium-ion batteries use different chemistries and have different characteristics. The typical output voltage of a Li-ion battery is 4.0V.

LITHIUM-ION POLYMER (LI-POL)

Rechargeable batteries (secondary cell batteries). Normally these batteries consist of several identical secondary cells in parallel to increase the discharge current capability.

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NICKEL CADMIUM

Newer rechargeable technologies, such as nickel-metal hydride (NiMH) have mostly replaced NiCad, because they have better energy characteristics and don't contain toxic cadmium, a carcinogen. The battery has a nickel-hydroxide cathode, a cadmium anode, and aqueous potassium hydroxide electrolyte. Average battery output is about 1.3 V.

NICKEL METAL HYDRIDE

NiMH batteries are common in laptop computers and cellular phones. NiMH is similar to nickel-cadmium but uses a metal hydride anode; a variety of metal alloys are used. Average battery output is about 1.3 V. These batteries are also used in electric cars, like the Toyota Prius.

RESERVE CAPACITY

A performance rating for automobile starting batteries, it is the number of minutes at which the battery can be discharged at 25 A and maintain a terminal voltage higher than 1.75 V/cell, on a new, fully charged battery at 80° F (27°C).

SLI BATTERY

Starting, Lighting, and Ignition battery, a battery designed for use in a conventional gasoline automobile. An SLI battery is designed to give a lot of current during starting, but then to be recharged immediately by the car's alternator. Electric cars usually have a portion of the total battery capacity set aside for SLI use.

STATE OF CHARGE

The amount of electrical charge in the battery, expressed as a percentage of the difference between the fully-charged and fully-discharged states.

TRACTION BATTERY

A battery designed to be used to provide the power to move a vehicle, e.g. to be used in an electric car. An electric car can have a battery pack consisting of 100 Li-ion cells.

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