The increasing use of custom designed electronic devices in today’s vehicles allow the manufacturer to offer the driver unprecedented levels of safety, reliability, economy, performance, information and entertainment. Application Specific Integrated Circuits (ASICs) are the perfect choice for these high-volume applications as they combine many functions in a tiny device capable of sensing, controlling and communicating while surviving the difficult environments found in the engine bay, under the vehicle, within the wheel and so on.
As a result, and thanks to a combination of GPS, radar and a myriad of intelligent sensors, drivers of cars of the not too distant future will simply have to start the car, steer it and switch it off on arrival. But how will it be powered?
Electric vehicles are the main contender
Recent Government announcements here in the United Kingdom as well as other places have made it quite clear that vehicles powered by an internal combustion engine have no place on the roads of the future. And, as of today, the main contender to replace them is electric vehicles (EVs) – both pure and hybrid – with the development of fuel cells and hydrogen powered vehicles still very much on the drawing board.
Although still in their relative infancy, electric vehicle sales have started to ramp up as more and more people take advantage of the financial inducements offered by the makers and many Governments.
The most expensive “component” of an electric vehicle (EV) is its power source – i.e. the battery pack. And, not only are today’s EV batteries costly, they are still heavy so the next generation EV battery pack will need to be cheaper, lighter – even though they will be far more powerful – and generally more reliable over their lifetime if electric vehicles are to become the transport of choice for the masses.
As well as the anticipated future developments in battery size/chemistry which could well solve the cost and weight issues for the four or five-year in-situ life of an EV battery, the industry is looking at ways to make today’s EV batteries better for the vehicle and more readily reusable once they are removed from the vehicle. Could a wireless sensor device embedded in each cell be the answer?
The power cells found under the seats of today’s EVs provide a huge amount of energy storage and, when connected together in a large pack, they deliver many hundreds of volts. The EV manufacturers and their battery suppliers are already looking at automating battery pack production to reduce size and cost and reducing the size of each cell will make each multi-cell pack smaller and easier to “mould” to the car’s available space. However, these EV packs are rechargeable which makes them a complex electronics control application.
Why? The lead acid batteries that conventional cars employ give a good indication of state of charge (SoC) by simply measuring the voltage across their terminals. However, most EV powertrain packs use Lithium Ion (Li-Ion) cells which provide their SoC “information” in a totally different way.
Drivers of conventional cars are used to seeing a fuel gauge that details fuel left and a miles/kilometre range until empty. But, unlike measuring the position of a float in the vehicle’s fuel tank, determining the state of charge (SoC) of a Li-Ion battery pack is a more challenging task.
As a result, other techniques have to be used like ‘coulomb counting’ – which measures the current flow over time to gauge how much energy has gone into the cell or been taken out.
In addition to state of charge calculations, it is also necessary to determine a cell’s state of health (SoH) as we have all seen news of the consequences when Li-Ion batteries are overcharged or allowed to become too hot. Taking care of SoC and SoH is a battery management system (BMS) which comprises a number of devices that communicate with each other in a hierarchy to ultimately tell the driver how far he or she can travel before a recharge is needed. And this means managing each cell.
Traditionally, measuring the voltages on the cells has required hundreds of sensing wires and connectors all routed back to an electronics assembly that can calculate the load on the battery pack and report up to the vehicle electronics control unit (ECU). Not only do these connections present potential failure points under the sort of vibration conditions that a pack must withstand but they are also a way for electrical “noise” to be picked up that corrupts the very signal that they are measuring. And a battery pack might be running at many hundreds of Volts so not the safest thing in for technicians to wire up!
What if every single individual cell could have some built in intelligence allowing it to instantly indicate its state of charge and state of health from the moment it is made?
Miniaturisation makes it possible
With advances in miniaturisation this is possible with an integrated circuit designed specifically for the task. Size is very important to the EV battery pack makers since all available volume needs to be filled with the materials designed to extend the range and power of the vehicle. Reducing the volume of the electronics control system gives an immediate advantage in power density.
A typical cell for an electric vehicle is a ‘pouch’ where layers of anode, cathode and electrolyte chemical are stacked up and vacuum formed into a pouch the size of a notebook.
The terminals to this battery are made of a thin flexible foil that is ultrasonically welded to the neighbouring cells. A typical pack might contain 100 or more such cells, generally located under the floor pan of the car. A tiny battery management ASIC in each cell can perform all the tasks to keep the cell safe. Each cell management ASIC can connect wirelessly to the battery master which offers a key advantage of galvanic isolation between the high voltage pack and the low voltage rest-of -the-vehicle wiring electrics.
More than that, as part of the cell fabric itself, the ASIC can report on its status for the entire life of the cell, whether on the way to the factory by container ship, in the electric vehicle pack or in a second life application which could involve smoothing peak demand for the electric distribution grid. With full lifetime information on each cell, it becomes possible for the user – maybe an energy supplier – to “see” how each cell has behaved or been treated. Those cells or packs with a historical problem can be taken out of service before they cause future problems.
There is no doubt that electric vehicles will become part of all our lives in the very near future and that Swindon ASICs will be at the heart of their intelligent sensing. This technology will help us all to change our perception and our experience of them as they progress from simple golf buggies to high performance cars.