Seemingly a simple solution, the battery powered electric vehicle presents a range of challenges to vehicle designers and to the electronics engineer. Erik Soule, vice president of Linear Technology's signal conditioning products business, said: "There is a lot of power involved in an electric vehicle and bad things can happen if you don't manage it properly."
Linear has addressed the issues with the LTC6802, launched a couple of years ago. "But the electronics have proved to be tougher than people thought for a number of reasons," he noted. "For instance, there are high voltages and the process needs extreme precision, as well as noise immunity. It's a problem when inverters switch at 20kHz because they radiate emi with a large number of harmonics, so you also have to filter all that out."
Other requirements include reliability, fault tolerance and diagnostics. "All of this is being driven by the ISO26262 automotive safety standard," he continued. Lithium ion is now the preferred battery technology for electric and hybrid vehicles. It offers, amongst other things, the ability to support more charge cycles, higher energy density, a better self discharge rate and a higher cell voltage. But lithium ion comes with a 'health warning'.
"Lithium ion batteries must be treated with respect. Fires have occurred in notebook computers because overvoltage peaks were not monitored correctly," said Steve Sockolov, director of Analog Device's precision signal processing group. "Although the quality of battery fabrication has improved, guarding against higher temperature conditions in any energy, industrial or automotive application is critical." "There are a number of issues relating to the batteries in an electric car," Soule reflected. "When the battery is charged, for example, you want to know as a driver how much energy is left. But from the system point of view, you want to know whether all the cells are balanced. We're looking at 1mV precision over a range of temperatures."
Prius' power pack and drive train.
Linear product marketing engineer Greg Zimmer explained in more detail. "Li ion battery performance depends on battery temperature and age, battery charge and discharge rates and the state of charge (SOC). These factors are not independent. For example, Li-ion batteries generate heat when discharged, which can increase discharge current. This has the potential to create thermal runaway and catastrophic failure.
Meanwhile, charging a Li ion battery to 100% or discharging to 0% will degrade its capacity, so it needs a restricted SOC range, such as 20% to 80%. This means the usable capacity is only 60% of the specified capacity. Because a 1% change in SOC may only be indicated by a few mV, the battery system must monitor this cell voltage accurately." Accuracy, in Soule's opinion, starts with a voltage reference. "Less obvious, but equally important, factors include thermal hysteresis and long term drift. Remember, the electronics in an electric vehicle may be on '24/7' for 15 years."
The LTC6802, which can monitor and control 12 lithium ion cells, is more like an analogue front end, said Soule. "We're looking at dealing with 100V in the analogue domain, while using fine cmos technologies for the digital side." Analog's offering is the AD8280, a hardware only safety monitor for lithium ion battery stacks. While the part has inputs to monitor six battery cells and two temperature sensors, it can be daisychained with other AD8280 devices to monitor hundreds of cells.
Information about the status of the alarms on the entire daisychain, as well as input signals that enable the part and initiate self test, are communicated via a master device. Intersil is also interested in this area, recently launching the ISL78600 battery management system. Each ISL78600 features a 14bit temperature compensated data converter that scans 12 channels in less than 250µs, measuring cell voltage to within 2mV.
A high noise immunity and transient tolerant communication scheme is used to link devices. This fully differential daisychain architecture allows the use of low cost twisted pair wiring for multiple battery packs. The part can link with a microcontroller using spi or i2c interfaces.
Custom system for electric vehicles
Nuvation Engineering, a US based provider of electronic design services, has developed a custom battery management system, fuel gauge and driver interface specifically for electric vehicles. The system is currently featured on the OptaMotive 'E Rex' – the electric equivalent of OptaMotive's T Rex.
"E Rex is a three wheeled electric car with six times the efficiency of a Prius, yet more torque per pound than a Porsche 911 Turbo," claimed Nuvation's ceo Michael Worry. "The E. Rex is capable of 0 to 60mph in 5s and has a top speed in excess of 100mph. The E. Rex proves going green can be fun and efficient." E Rex is powered by a battery pack featuring 106 lithium iron phosphate cells.
This 35kWhr pack is said to have a driving range of 200 miles and can be recharged in four hours from a 220V/40A supply. Power is supplied to the wheels using brushless dc motors. Working with Maxim, Nuvation engineers have designed the battery management system around the MAX11068 to provide voltage monitoring, temperature monitoring and balancing. Fuel gauging has been implemented using Maxim's ModelGauge, which combines voltage measurement with coulomb counting to support continuous automatic calibration.
"The advanced battery management system monitors each cell's voltage and temperature to ensure safety, balanced cells for long range and long battery life and to optimise vehicle performance," said Worry. According to OptaMotive, the E Rex will have a range in excess of 100 miles and will return a fuel economy equivalent to 100mpg.
The MAX11068 is a programmable 12 channel battery monitoring data acquisition interface optimised for use with batteries used in a range of applications, including automotive systems. It integrates a simple state machine and a high speed i2c bus for laddered serial communication. The analogue front end combines a 12 channel voltage measurement and data acquisition system with a high voltage switch bank input.
All measurements are performed differentially across each cell. A high speed 12bit SAR a/d converter is used to digitise the cell voltages and all 12 cells can be measured in less than 107µs. A two scan approach is used to collect cell measurements and correct them for errors. Firstly, the voltages of all 12 cells are acquired, after which the a/d converter input is chopped to remove errors. Information is supplied to the driver of E Rex via a touchscreen panel, providing a high level view of what the car is doing, including how much charge is left in the battery. The system controller is a single board computer running Linux.
