In high-end, specialist motorsport, applications like eBoosting, brake energy recuperation (BER) and electric steering demand high power and fast energy delivery, plus an ability to recharge quickly using a lightweight energy storage system (ESS). Using batteries for the ESS poses several problems: you have to be very mindful of the depth of discharge (DoD), the state of charge (SoC), and monitor other vital signs, such as individual cell temperatures.
On top of that, there is a very real possibility of safety issues (think Boeing). If you charge a battery too fast, in particular a lithium ion battery, you can induce voltage or heating that becomes dangerous to the cell, leading to fire. And lithium ion batteries are self-oxidizing, meaning they are very hard to put out once on fire, as they continue to make their own oxygen during combustion.
Lastly, there are environmental concerns: who recycles lithium ion cells? The answer is, really no one does. The cost outweighs the recovered materials, leading to batteries in landfills, leaching lithium into aquifers and water courses over time.
The solution is ultracapacitors. They have tremendous advantages over batteries for these applications, but with one main problem: energy density. But while the energy storage of an ultracapacitor is much less than that of a battery, you can pair ultracapacitors with a high energy, safer battery. The resulting ‘hybrid ESS’ provides high energy, high power, high cycle, fast charge acceptance, in a safe, lightweight package.
Ultracapacitors offer a number of advantages, not least being their high power, high cycle life, high efficiency, fast charge acceptance and a life expectancy that is not influenced by DoD or SoC issues. On the other hand, they can be expensive in Watt-hour terms and, to be frank, they are still something of a mystery to engineers and there is currently little support for the technology at government level.
The construction of a cell directly affects the overall performance of a pack, whether it is battery or ultracapacitor based. Design features, such as cell termination, how the current collectors are connected to the terminals, casing design, thermal management, separator design and so on, all play direct, individual, roles in the performance of a cell.
For example, the terminal designed for the cells play a significant role in how you connect multiple cells together. Can you laser weld? Bolt? Solder? And does the method that you choose offer the proper benefits of thermal management, electrical balancing, and data communications?
Poor terminal/cell design leads to increase resistance (impedance or equivalent series resistance). This leads to increased heating in the cells, which leads to early life failures, improper electrical balancing, and in the case of batteries, fire hazard. Ultracapacitors have a much lower resistance than batteries (in the range of 0.2mO to 4mO depending on cell size/design), and the connection method chosen may double the resistance at each connection if not thought out properly. The good news is that ultracapacitors are highly abuse tolerant, and very safe if they are abused.
System integration within a race car is a major problem. There is often not enough space required for a properly sized ESS pack, and the weight of the ESS must be considered due to issues related to the centre of gravity for handling purposes. The cooling required incorporates liquid, and you need pumps, chillers, and airflow to maintain the cell temperature within the pack. The variance in temperature from cell-to-cell will directly affect the cell’s resistance, leading to additional concerns around cell voltage balancing.
This is much more critical and more challenging than cooling a centrally located internal combustion engine. Again, ultracapacitors are the safest energy storage technology in the world, with the capability to accept and provide very high current with lower cooling requirements, leading to the need for less cooling in applications such as eBoosting or BER.
Charge rate/time has changed dramatically in the past five years, with the introduction of lithium iron phosphate batteries (even more so with ultracapacitors). Every racing series that is moving towards electrification is pushing for faster charge capability, as this allows for a direct increase in the amount of energy recovered during a BER phase. Equally important is the ability to push that energy back out to get it onto the track. The holy grail of the ESS for these applications is a symmetrical charge/discharge capability, with a very high throughput capacity; being able to accept or provide hundreds of Amps at a time. Currently, only ultracapacitors can do this.
Ultracapacitors have a symmetrical charge/discharge capability, meaning you can charge them as hard as you discharge them. Typical cells in the 3,000F range can handle 100A rms continuously without much cooling. Voltage tolerance is fairly high for short periods, meaning the monitoring is very simple. Resistors alone can be used to ensure cell-to-cell balancing is achieved, and the ultracapacitors can be discharged to 0.0V with no degradation.
Charge controllers are required with all chemistries for racing and integration with the performance of the internal combustion engine. Electric motor/generators are used at the wheels to propel the vehicle and generate energy during braking, and dc-dc controllers are often required to provide a level output of energy to the supporting electronics. This area of electrical control is often the main cause of hesitation for designing any hybrid vehicle, but the technology is certainly available today and has been in use in passenger transport (hybrid buses, for example) for many years.
ESS packs are designed specifically for the end application, reducing problems from vibration and acceleration forces. However, the cell design chosen must be able to perform at the system level, and if the wrong cell is chosen, it will drastically affect the pack longevity.
Temperature for most ESS systems is extremely important.
For battery based ESS, if you intend to use the system below +3°C, then the system’s charge acceptance will be greatly diminished. At -20°C the battery systems become almost unusable (Lithium can plate out at that point, causing irreversible damage). Ultracapacitor based ESS systems can work at temperatures down to -40°C with almost no loss of energy or rate change required.
The material for most cells is typically aluminium. The choice of alloy can directly affect the design integrity when internal gas pressures increase. Most cells provide a one-time mechanical vent to allow gas to escape safely, preventing a catastrophic release. Seals are important to prevent electrolytes and other materials from escaping over time, which causes the cells to dry out. These materials have become fairly standard, as there are billions of cells produced annually.
For batteries, the power density and charge rate are the limiting factors. The high impedance due to the chemical reactions leads to heat generation, requiring more complicated thermal management systems. Cooling systems are often big, heavy, and expensive, so reducing the cooling requirements receive top billing during the design phase of a pack for transport, particularly motorsport applications.
The lifecycle of an ESS system is highly debated. The number of variables involved in battery based ESS system makes determining the expected cycle life a long and complicated exercise. For example, you have initial manufacturing variability, which can cause up to 15 percent variance in capacity from cell-to-cell; this will also affect the balancing tolerances.
Where the cells are placed in the pack is another variable, as the innermost cells tend to have higher thermal deltas than cells placed at the outer wall. The chemistry, termination, cell structure, and connections also play a role, as these all affect the sum of the impedance. Lastly, are the DoD/SoC and the rates used to charge/discharge, all having significant impact on cycle life.
A Lithium cell that is driven hard with improper cooling may have a life of only 100 cycles, whereas the same cell that has been placed in a properly designed system that only travels between 40 percent-80 percent SoC in a micro-cycling environment could hail upwards of half-a-million mini-cycles. By comparison, an ultracapacitor running at its heaviest duty cycle (100 percent-50 percent SoC with maximum continuous current), will give a life cycle of over 1 million cycles.
Ultracapacitors do store less energy, theoretically, than a battery. This is their single limiting feature. However, the gap of energy is often less than imagined. Consider that you can only often use 50 percent of the energy or less in your battery to maintain a proper cycle life, and the energy density gap becomes less. Over time, the ultracapacitor will develop much more energy per cycle than any battery made, due to the high cycle life.
Add in the ability to accept very fast charges and/or discharges, the lighter weight, wider abuse tolerance and temperature ranges, simpler packaging, installation, and reduction in safety/shipping requirements, and the ultracapacitor is less expensive up front to install than a lithium-based battery system. The ultracapacitor’s charge acceptance means you can store more energy, faster, than a battery, and in a racing system, this is a critical part of the ESS system.
Batteries have a round-trip-efficiency (RTE), or a full charge/discharge cycle, of approximately 65 percent to 80 percent, depending on the technology and chemistry. Ultracapacitors have a RTE of 98 percent, thus allowing for a much more efficiently designed ESS system. Because of the high power density of an ultracapacitor you do not need to oversize the design to meet power requirements. Batteries, however, have to be oversized to meet the power requirements, leading to a heavier, larger system than is necessary to perform the application.
The ability of a cell to vent is very important to prevent over-pressurization of the cell. If the cell has reached the designed end of life, or has been abused thermally or electrically, the cells are designed to release the internal pressure in a safe manner. This is easily done with an ultracapacitor. With a battery, you have large amounts of energy stored, and the venting is more critical to control. Developments are always being made in the internal gas generation of both chemistries, leading to lower requirements for venting; however there will always be a need to allow gas to escape safely due to the inherent abuse they will see during their life.
Fast inductive charging is currently common with ultracapacitors for a number of industrial applications. For vehicle use, the energy density of the ultracapacitors prevents them from being the primary energy storage device. Inductive charging is also common with batteries, but at a slow rate. If the power density/charge acceptance of batteries could parallel ultracapacitors, fast inductive charging would be the optimum solution. However, from a chemistry perspective, this is not a reality that is obvious in the near term.
The energy density of ultracapacitors is always increasing, while maintaining the power density and long cycle life that people expect from an ultracapacitor. Higher voltages, such as 3.0V technology, increases energy density by approximately 20 percent due to the E=½CV2 formula. Thus as the voltages of ultracapacitors increase, they will begin to invade on the traditional battery space.
However, the best way to approach an ESS is through the use of a hybridised system, which uses ultracapacitors for the power, fast charge, and fast discharge, while the battery is used for the steady state energy requirements. This method allows each chemistry to safely do what it was intended to do.
Phil Edwards is founder and managing director, Weald Technology Ltd
Chad Hall is co-founder and VP sales and business development, Ioxus Inc.
Weald Technology received the special Editors' Award at this year's e-Legacy Awards presentation in London for its tireless work inspiring the next generation of engineers and scientists. For more information about the company and, in particular, its work in the area of electric vehicle technology, click here.