As battery technology becomes more sophisticated so must battery management systems

4 min read

Pure electric car sales have grown by more than 50% in the year to date and, by some counts, that's ten times faster than the overall car market. The number of plug in cars in the UK, according to research from the RAC Foundation, more than tripled over the past 12 months, while figures from the forecaster LMC Automotive suggest sales of hybrid, electric and hydrogen fuel-cell vehicles across Europe could top 360,000 units in 2015.

While demand may be strong, albeit coming off a low base, it remains the case that the single most important issue that is holding back the development of the market is battery technology.

"The battery is the single biggest contributor to the purchase price of an electric vehicle," says Sabine Jud, marketing manager for mobility sensors with ams, "and the performance of the battery, whether in terms of range, lifetime and power output characteristics, will largely determine whether the consumer sees an electric vehicle as an acceptable alternative."

Mike Kultgen, design manager, battery management systems with Linear Technology, agrees: "The electric vehicle market remains in its infancy, but if we can increase battery energy and drive costs down, then we'll start to see demand increase."

Vehicle safety is paramount, as the risk of fire as a result of more complex battery system structures is a concern. As a result, the battery management system (BMS) is a key component in electric and hybrid vehicles and needs to be able to provide accurate and precise battery status monitoring and assessment, balancing and charging and discharging control.

"BMS technology is essential to the safety and performance of electric and hybrid vehicles," says Kultgen, "and is necessary to protect the battery from damage, prolong its life and maintain its fitness to perform the functions of the application.

"From a designer's perspective, safety is more important than performance," he believes. "We need to know what's happening in the battery pack, especially as we pack in more energy. So that means the exact state of the battery (gas gauging) and how much energy it has left (monitoring). The more accurately we can measure that, the better."

Among the basic BMS functions are: measuring the pack voltage; the current flowing into (when charging) or out of (when discharging) the battery; and monitoring individual cell voltages and temperature.

Other functions include disconnecting the battery when necessary; balancing the charge stored by each cell in a stack; checking the operational status of system components; calculating the state of charge (SOC), state of health and state of function of the battery and then communicating these data to the vehicle.

The ability of the BMS to perform these functions depends on the accuracy of its sensor inputs.

The large lithium ion battery packs used in electric vehicles tend to rely on analogue integrated circuits to measure voltage, current and temperature.

"The major components of a measurement IC are the high voltage multiplexer, an A/D converter and the voltage reference," explains Kultgen. "The limitation on measurement accuracy is the voltage reference, while the rejection of inverter noise is limited by the A/D converter."

Large lithium ion battery packs are comprised of a series of connected cells and these can be segmented into groups of cells, or modules and a monitoring circuit is connected to each module.

"Batteries are expensive," explains Kultgen, "and all it needs is one cell to fail for the whole battery to fail."

Cells have to remain in a balanced state of charge to maximise energy, power and functional lifetime and the major sources of imbalance are caused by the current drawn by the analogue electronics and the mismatch in the cell manufacturing or the cell operating temperature.

Battery packs larger than 48V require isolated data transfer between the analogue integrated circuits and the digital controller.

"When it comes to the engine of a car, you are dealing with a very noisy environment and you need to be able to transfer data cleanly from the battery if you are going to be able to undertake accurate measurement," Kultgen explains. "Any sensor technology has to be robust and provide cost effective data communications – and that is certainly a challenge."

Making large Li-ion batteries modular has many advantages, including scalability, serviceability and the ability to alter the form factor. If the battery pack is modular, then the data bus must be isolated. If the modules are physically separate, then the data bus requires a wiring harness and any wiring in a battery pack must tolerate high levels of electromagnetic interference.

"As a result, a robust two wire isolated data scheme is necessary," suggests Kultgen.

Isolated CAN is the traditional solution for data transmission in modular battery packs, but that can be large and expensive, requiring many transceivers and isolators.

Recently, manufacturers of battery measurement ICs have designed alternatives to isolated CAN. One example is an isolated SPI scheme – or isoSPI. In the isoSPI approach, the clock, data in, data out and chip select signals are encoded into differential pulses. The pulses are then transmitted over two wires using transformers for isolation.

An example of this is the LTC6804 from Linear Technology that has been designed to receive and transmit isoSPI pulses. The IC can convert any SPI bus to an isoSPI bus.

According to Gernot Hehn, an application engineer in the automotive business unit at ams, one of the most challenging design aspects of an automotive battery sensor is the need for very precise measurement over a very wide current range – 1mA to 1kA – and this requires a sensor interface with a measurement range of more than 100mV and with a resolution of better than 1µV.

"Batteries are expensive and all it needs is one cell to fail for the whole battery to fail." Mike Kultgen

"Our solution is the AS8510, a highly integrated sensor interface that can provide accurate current and voltage measurement, offering two independent data acquisition channels which can simultaneously measure current and voltage signals of both polarities and with no offset.

"SOC data are obtained through the accurate measurement of the current flowing in and out of the battery over time and this is aided by calibration cycles based on similarly accurate measurements of no-load battery voltage, together with the battery's temperature."

Accurate SOC data capture requires precise current measurement over the entire signal and temperature ranges as well as an exact time base. "SOC data informs the driver's 'remaining range' indicator and enables the BMS to prevent damaging over-discharge events," Hehn explains.

Current is measured through a 100µ? Manganin shunt, an extremely precise and stable resistor. With highly linear 16bit sigma-delta A/D converters, a zero offset architecture and an A/D converter reference which is temperature trimmed in the factory, the AS8510 can offer an accuracy of 0.2% over the full automotive temperature range, input range and lifetime.

Voltage is another key parameter that needs to be measured and with the AS8510, the externally attenuated battery voltage is directly digitised, either simultaneously with the current measurement, or at a different sample rate if so configured.

Improvements in battery sensor technology are helping to deliver important improvements to the performance of the drive-train battery and, in turn, are helping to make battery powered electric vehicles ultimately more appealing to consumers.