Electric Car: The Power Comes from the Cell
Author: Joachim Geiger
Range is the holy grail of electromobility. It determines whether an electric car is perceived as a fully-fledged vehicle. In the competition for best energy source, the lithium-ion battery currently holds the best cards. Speaking in favor of this battery type is its good ratio between weight and range.
It’s a founding myth of electromobolity that the success of an electric car depends on its range. The legendary Lohner Porsche, for example, would certainly have had the potential to shape the future of the automobile. Unveiled at the Paris World Fair in 1900, the front-wheel-drive car with wheel hub motors carried a 410-kilogram lead-acid battery that provided an impressive range of 50 kilometers. The outcome of the story is well-known. The internal combustion engine quickly and permanently put an end to this vehicle concept. More range, cheaper to buy and run, cheaper fuel – the e-car didn’t stand a chance. Today, the electric car is once again standing at the beginning of an era. And once again, the focus is on range. This time, however, the electric car might have the better end of the deal. Battery technology is making huge strides and pushing the limits of capacity and range ever further. So it’s time to take stock.
What are the arguments in favor of lithium-ion batteries as an energy source for e-cars?
“The range of an electric car depends not only on the size but also energy density of the battery,” explains Andreas Richter, an engineer at the DEKRA Competence Center for Electromobility. The unit for energy density is kilowatt hours per kilogram (kWh/kg) – it describes how much energy can be stored in the battery per kilogram of battery weight. Typical values for lithium batteries are between 0.1 and 0.3 kWh/kg. One argument in favor of the Li-ion battery is that it has a good ratio between weight and range. Compared to the lead-acid battery, it weighs only one-third as much for the same amount of energy.
What turns lithium-ion batteries into powerful chemical factories?
The smallest unit of the battery is a cell – all electrochemical processes take place at this level. Individual cells are connected together to form battery packs, which are then connected together to form modules. The typical structure of a cell consists of two electrodes – an anode and a cathode. The powdered active materials for the electrodes are usually graphite (carbon) at the negative terminal and a combination of nickel, cobalt, manganese, and lithium at the positive terminal. Processed with liquid and binder to form a thick paste, the chemically active substances are applied to thin metal foils made of aluminum (cathode) and copper (anode), which serve as current conductors.
The structure is completed by a separator layer permeable by lithium ions, and a liquid electrolyte. During charging, the lithium electrons move from cathode to anode via an external circuit. At the same time, the lithium ions, which are positively charged by the missing electrons, move through the electrolyte and separator to the anode. Together with the electrons, they seek a place in the graphite, which serves as pure storage medium. When discharging, the particles take the reverse path. The electrons give up their energy to the motor on their way back to the cathode. The more lithium there is on the cathode, the greater the storage capacity of the individual cell – the energy density is higher.
What role does the battery management system (BMS) play for the battery?
“The BMS is part of the power electronics’ control system and is responsible for ensuring optimum working conditions for the battery at all times,” knows DEKRA expert Richter. It ensures that all cells have the same voltage and provide the required power. During charging, the BMS communicates with the charging station or wallbox to optimize the charging current. Thermal management is a special task. To prevent damage to the battery, the system can reduce charging speed or power output during driving. At low or high outside temperatures, it switches on heating or cooling.
Gross or net? Why information on capacity isn’t always clear.
The capacity indicates how much energy the battery can provide the electric motor. The higher the capacity, the greater the range of an electric car. The capacity is specified in kilowatt hours (kWh). However, the capacity slowly decreases over the course of the battery’s life. Volkswagen, for example, gives the guarantee that a new e-car’s battery will still have a usable capacity of at least 70 percent after eight years or 160,000 kilometers, if used correctly. But how can a manufacturer ensure that its batteries actually achieve the promised service life? The trick in this case lies in the little word “usable”.
As DEKRA expert Andreas Richter explains, manufacturers always specify a safety reserve for their batteries to prevent harmful overcharging and deep discharging. The means to this end is to control the voltage in the battery cells, which is usually between 3 and 4.2 volts ex works. Manufacturers now set themselves the highest voltage when the battery is full (end-of-charge voltage) and the lowest when the battery is empty (end-of-discharge voltage). Battery management prevents these fixed values from being exceeded or undercut. A fully charged battery is therefore never completely full, an empty one never completely empty. As a result, however, only a limited amount of energy is available to the user – the usable capacity. VW specifies a usable energy quantity of 90 percent. However, the usable capacity of different electric cars can only be determined if a manufacturer clearly refers to the net capacity of the battery in the technical data sheet.
How reliable is the manufacturers’ data on range and consumption?
“When the data is determined according to the current WLTP procedure for type approval, it’s much closer to reality than the previous New European Driving Cycle (NEDC),” explains Erik Pellmann, Head of Powertrain and Exhaust at the DEKRA Technology Center in Klettwitz. In the Worldwide Harmonized Light Vehicles Test Procedure, or WLTP for short, the test vehicle starts with a fully charged battery. As a rule, it runs through the test program until the usable part of the battery is almost completely discharged. The distance driven corresponds to the range. Because the WLTP includes a dynamic driving profile with accelerations and decelerations, phases of recuperation automatically occur. The achieved additional range is included in the result. DEKRA experts determine consumption by documenting the required recharging energy with the help of an electricity meter. The advantage is that it also records energy lost by the battery during charging. The energy consumption per 100 kilometers is then just another calculation. In practice, however, the determined values can shift higher or lower. High or low temperatures, for example, can lead to significant deviations from the WLTP values, which are determined at a standard of 23 degrees Celsius. In addition, driving style, driving profile and load, the use of power consumers such as air conditioning and heating, but also the condition of the battery itself play a role in consumption.
Big or small battery – in the end, is less more?
A smaller battery scores points in the life cycle assessment due to savings in raw materials, weight, and consumption. But it also follows that a smaller storage capacity means less range. So how small should a battery be to use the car without serious restrictions? Is a range of 150 or 200 kilometers sufficient? If you can charge at home, at the office, or at a public charging station at any time, you should be on the safe side. On the other hand, the performance of a battery may suffer in the cold season – sub-zero temperatures can significantly limit the range of smaller models. Very frequent charging can also take its toll on a small battery. In addition, e-cars with a small energy storage system will probably fetch less money when resold in the future. Conversely, people who value flexibility and long-distance capability will hardly be able to avoid a larger drive battery. In this case, economic and ecological considerations could determine how high its capacity ultimately has to be. Large batteries score points at electric charging stations due to high charging capacities, which save a lot of idle time at fast charging stations. And because a large battery needs to be charged less often than a smaller model with the same mileage, its service life is also extended.
Charging and discharging – why can this stress the battery?
A battery delivers top performance for a long time. But even super athletes eventually show their age – the performance curve can then drop rapidly. “A significant factor in this process is recurring high currents during charging and discharging,” reports Andreas Richter. As the DEKRA expert explains, large currents mean increasing temperature inside the cell, which can increase the internal resistance over time. A high internal resistance can severely disrupt the flow of electrons. In addition, there’s the phenomenon that the cathode material’s structure can change at high currents, which also has an unfavorable effect on capacity.
High charging currents are involved, for example, when charging at fast charging stations – in general, the goal here is to charge as much power as possible into the battery in a short amount of time. This usually isn’t a problem because the battery management system intervenes in a regulating manner. Nevertheless, regular quick charges stress the battery. This is also true if you always charge the energy storage unit to its maximum capacity or frequently discharge it deeply. Another influencing factor is driving style. If you drive your electric car like a race car and continue to deliver top performance even when the charge level is low, you’re not doing the battery any favors. If, on the other hand, you mostly keep the charge level between 20 and 80 percent, this can extend the service life. Of course, it’s a good idea to fully charge the battery to 100 percent if you’re planning a long-distance trip.
What about battery costs and why are they considered problematic?
Drive batteries count among the most expensive components in an electric car. Major cost items are the raw materials procurement and energy provision for the battery cells’ production – depending on the car manufacturer and model, the share of total vehicle costs can be between 25 and 40 percent. A current “Powertrain” study under the umbrella of management consultancy PricewaterhouseCoopers assumes that by 2030 battery costs can be reduced from the current 90 to 68 euros per kilowatt hour. One adjustment for cheaper batteries could be the use of low-cobalt materials. The production of rechargeable batteries can actually be accompanied by somewhat problematic conditions. These include environmental pollution and precarious working conditions in the mining of lithium and cobalt in countries such as Chile, Argentina, and the Democratic Republic of Congo. The use of alternative materials and a sustainable supply of raw materials could provide a remedy. Production with renewable electricity could also reduce CO2 emissions, especially for heat drying the coated metal foils. The use of used batteries in stationary energy storage systems and recycling are also likely to contribute to greater sustainability in the future.
What growth potential do Li-ion batteries have?
The next stage of evolution is already emerging in the form of solid-state batteries. Manufacturers such as VW and BMW are already working hard on this technology. It’s expected to have twice the energy density of conventional lithium-ion batteries and significantly shorter charging times. As the word implies, solid – instead of liquid – electrolytes separate the positive and negative poles. In this concept, the solid electrolyte acts as a tool to replace the graphite on the negative pole side with pure lithium. A quantum leap in lithium-ion technology could also be the process developed jointly by the Fraunhofer Institute and The Netherlands Organisation (TNO), a Dutch research institution, which uses nano-coating to manufacture cells. Spatial Atom Layer Deposition (SALD) technology is said to increase the battery surface area of current systems by a factor of three – the bottom line is that this should enable ranges of over 1,000 kilometers and five times the charging speed. SALD is said to work with liquid electrolytes as well as solid-state batteries. Scientists at Chalmers University of Technology in Gothenburg are working with a cathode based on lithium iron phosphate to develop a so-called structural battery that will be installed as part of the load-bearing structure of an electric car. Carbon fibers are used, which serve simultaneously as electrode, conductor, and load-bearing material.