Tesla Motors expects to consume two billion Li-ion cells by . Both the Tesla Model S and Model X electric vehicles (EV) get their electrical energy from the cell, a format that also powers laptops and medical devices. The cell measures 18mm in diameter and is 65mm long. A cylindrical cell in a metallic case is durable and has a high specified energy (capacity), but cylinders are heavy and have a low packaging density in a cluster compared to the prismatic architecture and the pouch pack. A battery pack for a Tesla vehicle deploys over 7,000 cells, and to get the desired voltage and amperage, the cells are connected in series and parallel. Figures 1 illustrates the popular cell.
For more information, please visit Godson Tech.
At 90kWh, the Tesla Model S has the largest battery in an electric car in terms of watt-hours; it also delivers the longest driving range between charges. In comparison, the Nissan Leaf comes with a 30kWh pack; the Ford Focus EV has 23kWh and the Chevy Volt 16kWh for corresponding shorter driving ranges. A 90kWh battery holds enough energy to provide a typical U.S. household with electrical needs for almost three days. But batteries must be charged and this draws heavily on the grid. They are also expensive; the EV battery alone carries the price tag of an economy car.
Lithium ion batteries come in many variations and Tesla chose the high-energy nickel cobalt aluminum chemistry (NCA) for the S-Model. Made by Panasonic, the cell is rated at 3,100mAh, a specific energy that is slightly higher than most contenders. Other advantages of the NCA are high specific power for exuberant acceleration and long life. The negatives are high cost and a lower safety margin than other Li-ion systems. Figure 2 outlines six of the most important characteristics of a battery in a spider web.
Batteries for the electric powertrain need high loading and a long life, and the NMC is another popular Li-ion system. NMC stands for nickel-manganese-cobalt and is also used in e-bikes, power tools and military and medical devices. The cathode may consist of one-third nickel, one-third manganese and one-third cobalt, but other combinations are also used to satisfy special requirements. These blends lower the raw material cost due to reduced cobalt content. Figure 3 demonstrates the characteristics of the NMC.
Another popular Li-ion system for electric powertrains is the Lithium Iron Phosphate (LiFePO4). Its strength lays in long life and superior safety, but it has a lower capacity than cobalt-based Li-ion systems. A further tradeoff is the lower nominal voltage of 3.3V/cell rather than the customary 3.6V/cell of other Li-ion systems. Figure 4 summarizes the attributes of Li-phosphate.
Lithium-ion has much improved. In , the capacity of an cell was 1,100mAh at a manufacturing cost of over $US10 per cell. In , the price dropped to $2 and the capacity rose to 1,900mAh. Today, high energy-dense cells deliver over 3,000mAh and the costs have gone down further. This, however, does not come without compromise. Newer cells are more delicate than older ones and this can affect the cycle count.
A Swiss manufacturer of upscale e-bikes did a comparison on older and newer cells. They use the NMC cell from Panasonic and LG Chem. The early version rated at 2Ah still delivered 80% after the onboard Battery Management Systems (BMS) indicated cycles. Then came the 2.2Ah NMC and the capacity dropped to 70% after cycles. The modern 3Ah NMC used today drops to 60% after cycles. It should be noted that the end-capacity of the newer cells is still higher that the older ones; the 3Ah cell retains 1.8Ah after dropping 60% whereas the 2Ah cell has only 1.6Ah after a 20% capacity drop.
EV batteries must carry an eight-year warranty. To achieve this, a new battery may only charge to 80% and discharge to 30%. As the battery loses capacity with age, many BMS gradually increases the charging bandwidth to maintain equal driving range. Once operating at full bandwidth, the battery gets stressed more, reflecting in accelerated performance drop and reduced driving range.
Cold temperature causes the performance of all batteries to drop. Bitter cold also makes charging more difficult, especially with Li-ion, as charging is more delicate that discharging. The ability to use a battery at low temperature does not automatically permit charging under these same conditions. Carless charging at low temperatures can inflict permanent damage to the battery.
Li-ion should not be charged below zero degree C (32°F). Some battery manufacturers permit charging down to -10°C (14°F) by reducing the charge current to a tenth of the battery rating, or 0.1C (see C-Rate), a charge that would take 12–15 hours on an empty battery. Charging too fast at low temperatures could lead to dendrite growth, reflecting in higher self-discharge and compromise safety.
The battery stress is highest at 4.20V/cell when the battery reaches full charge. Keeping a lower voltage also protects the battery during cold-temperature charging and some BMS limit the voltage and current accordingly. Many EV batteries include a heating blanket to protect the battery when charging at cold temperature. Energy to heat the blanket is readily available from the grid.
EV owners want ultra-fast charging and technology is available to do so. Although convenient, fast-charging is harmful to the battery. If at all possible, avoid charge times that are less than 90 minutes, or charge rates above 1C. The onboard BMS keeps record of stressful battery events and historic data can work against a warranty claim. This was the response of a large European EV manufacturer when the question of ultra-fast charging came up at a recent EV battery convention in London.
Safety is a further concern, but this applies to all batteries. A one-in-200,000 failure triggered the recall of almost six million lithium-ion batteries in . Sony, the manufacturer of these cells, said that on rare occasions microscopic metal particles may come into contact with other parts of the battery cell, leading to a short circuit than can cause venting with flame.
Li-ion has improved and the failure rate has been reduced to one-in-10 million. This is reassuring, but the formula of one-in-10 million could cause 200 cells to fail in the batch of two billion that Tesla plans to consume. It is likely that the failure rate has gone down further but caution is in place when storing tons of batteries in one place. Fires with battery manufacturers and in warehoused storing batteries are common.
Relatively little is known when Li-ion batteries are exposed to harsh environmental conditions. Internal shorts and rapid disassembly are of concern, an event that no safety circuit can stop once in progress. The fault occurs inside the cell and the battery must burn out.
The Li-ion battery of Boeing 787 Dreamliner may have failed due to an electric short; the modified battery enclosed in a metal housing will provide a safeguard should a short recur. All batteries are subject to failure and there is also a reported incident where the battery circuit breaker of a Boeing 777 had to be pulled because of an overheating NiCd battery. In the early s, the National Transportation Safety Board reported several battery incidents per year involving the then new nickel-cadmium ship-board battery on airplanes. Improvements eventually made NiCd safe; this will also happen with Li-ion.
Transporting batteries by air remains a concern. There are regulations as to how much metallic (or equivalent) lithium can be included in an air shipment. Some content may go unregistered and the United Arab Emirates General Civil Aviation Authority found with reasonable certainty that the fire aboard the UPS 747-400 freighter was caused by a lithium battery. The aircraft went down on September in the Dubai desert about an hour into its flight to Cologne, Germany.
New air cargo containers are being tested with materials that can withstand intense fires for up to four hours, enabling an emergency landing on most flights. The fire-resistant panels of these air cargo containers consist of fibre-reinforced plastic composite that snuffs fire by depriving it of oxygen.
A fire is easier to put out in the cabin than in the cargo bay and since January people can no longer pack spare lithium batteries in checked baggage. (See Li-ion travel restrictions.) Airlines allow them as carry-on where fire extinguishers are available. A coffee pot served as an extinguishing device of a flaming laptop battery in one incident. Travelers are reminded of how many batteries can be carried on board in portable devices and as spares. This also includes primary lithium batteries and the maximum weights of lithium (or equivalent) are:
Effective , lithium-based batteries can no longer be carried as cargo in a passenger aircraft. In addition, Li-ion in cargo must have a state-of-charge of 30 percent. All packages must bear the Cargo Aircraft Only label in addition to other required marks and labels. This limitation does not affect lithium ion batteries packed with or contained in equipment.
While Li-ion is being scrutinized for safety, other chemistries also exhibit problems. Nickel- and lead-based batteries cause fires too, and some are being recalled. Reasons for failure are defective separators resulting from aging, rough handling, excessive vibration and high-temperature.
Examining 113 recorded incidents of transporting batteries by air over a 19-years period reveals that most failures occurred due to inappropriate packaging or handling. Damaged battery packs and electrical shorts due to careless packaging were the main culprits. Most incidents happened at airports or in cargo hubs. Problem batteries include primary lithium that contains lithium-metal, as well as lead, nickel and alkaline systems, and not just lithium-ion, as is perceived. Modern consumer products have very few failures involving Li-ion batteries today.
Batteries International Magazine, Issue 90, Winter /14
Portable Rechargeable Battery Association (PRBA)
Boston Consulting Group (BCG)
E-One Moli Energy (Canada) Ltd
National Transportation Safety Board
United Arab Emirates General Civil Aviation Authority
Batteries in a Portable World, Cadex Electronics: Isidor Buchmann
Balancing a lithium battery pack correctly is perhaps the most important function of a BMS system. This process is crucial to ensure maximum efficiency and the highest capacity throughout the battery’s entire life cycle.
Flash Battery’s proprietary Flash Balancing System acts both actively and passively with a balancing power that by far exceeds conventional BMS systems (20A), not only at the end of the charging cycle but also in active mode during charging and discharging. This translates into ultra-quick balancing times and maximum run time for Flash Battery lithium batteries.
But there’s more to the Battery Management System. By now widely accepted as the “brain” of a lithium battery, it can manage the safety and range of an electric vehicle by measuring and analysing key functioning data.
If care and meticulous attention is put into its design, it ensures stable performance over time, prevents faults, and performs self-diagnosis and preventive maintenance, providing a comprehensive check of the battery pack.
To recap, the other main functions of a smart BMS system in addition to balancing are:
1. Real time monitoring of every battery parameter
The electronics must be able to check the temperatures and voltage of every single cell in real time, measuring the battery’s incoming and outgoing flow of current. According to the readings, the BMS will take the strategic decisions needed to optimally manage the charge and discharge stages or simply, an extended period of inactivity.
2. Sending information to the vehicle control unit, motor control or on-board display
With lead batteries, the electronics of the vehicle defined the status of the battery, reading only the overall voltage. With lithium batteries, things are different: the battery sends its own data.
Various different data are sent but the most important are:
For more information, please visit Lithium Battery for Electric Devices.
3. Controlling the battery charger
In lithium batteries, the battery charger is simply a power supply that delivers the current the battery requires; the charging curve is decided by the battery based on the requirements of each single cell. The CAN bus communication protocol is commonly used to transfer information to and from the battery charger and battery. If, instead, automotive standards are used, such as, for example, CCS Combo charging (the European electric vehicle charging standard), the protocol takes advantage of Power Line Communication.
4. Heating and cooling the lithium battery pack
Depending on the application and the place of use, the battery may be equipped with heating or cooling systems. The BMS system inside the battery activates or deactivates them as required by the battery and vehicle status.
5. Performing predictive analysis throughout the life of the vehicle
In addition to the functions described above, more advanced BMS systems also gather the most significant data, send them to the cloud and analyse them. Flash Data Center, a proprietary platform developed by Flash Battery, analyses every charge and discharge cycle of the interconnected batteries on a daily basis. Using artificial intelligence algorithms, Flash Data Center detects faults before they develop into costly breakdowns by sending automatic alerts to Flash Battery’s service centre.
We have listed the characteristics for the “standard” operation of a good BMS system in a lithium battery but everyone is well aware that most industrial machines and electric vehicles are special applications with very specific requirements.
That’s why turning to a lithium battery manufacturer that produces the control electronics in-house is crucial. Going to an assembler would mean tremendously lengthening the time to develop the prototype and later, that for servicing when needed.
But let’s talk about a real case:
With the purpose of establishing sustainability as one of the guiding values for the development of its products, CIFA S.p.A., a leading manufacturer of concrete mixer trucks, pump trucks and mixing plants since , chose to rely on the expertise of Flash Battery for the electrification of its vehicles, including the hybrid concrete mixer Energya and the first plug-in hybrid truck pump mixer in the world, the Magnum MK28E.
Both projects required important collaborative work with the customer to develop a battery custom made for the requirements of the application, with high-performance yields, quick-charge capabilities, and high capacity to withstand climatic factors not to mention the mechanical stresses that occur in specialty equipment during daily operation.
In addition to the dedicated mechanical part, the Battery Management System was, too, custom made to meet the requirements of the vehicle. The battery, in fact, uses the BMS system to control the on-board battery charger and, at the same time, the charge of a faster, automotive standard charging station at ground.
What’s more, the lithium battery pack communicates back and forth with the vehicle’s control unit, which in turn, sends out the information for the display and the power management of the traction/specific function motors (for example, to operate the mixing drum or pump the concrete). Lastly, the battery controls the generators, the integrated heating system and the insulation measurements, to ensure the safety of the entire vehicle.
A smart Battery Management System definitely plays a critical role in properly managing the performance and life of a lithium battery, but let’s not forget that its work goes hand in hand with another important ally: chemistry.
A good BMS can definitely make the difference in making the best use of the chosen chemistry, which alone counts for over 70% of the definition of the correct performance of a lithium battery.
Thanks to the BMS system, the chemistry can in fact be improved, ensuring the same reliability and performance over time. This translates into a vehicle with lower losses in terms of state of health and range percentages, maintaining the performance at different temperatures intact because the software is able to manage and control every device revolving around the battery and, if it is well-designed with the hardware part, can help cut down charging times, as is the case for our Flash Balancing System.
Contact us to discuss your requirements of LED emergency driver power supply. Our experienced sales team can help you identify the options that best suit your needs.