Electrochemical Systems (Task 10)
CHAPTER 4 ELECTROCHEMICAL SYSTEMS (TASK 10)
In a state-of-the-art system, the fast charging process involves the exchange of data between the vehicle and the charger. These data are used to limit charging to prevent damaging the battery. Although the details will vary depending upon the system, these controls normally limit fast charging at the maximum rate to the “middle” states of charge (SOC), e.g. between about 20 and 70% SOC. If the charging begins at a very low SOC, the rate may be limited at first. Once the SOC reaches somewhere between 60% and 80%, the charging rate will also be reduced in a taper to zero at full SOC. A generalization that captures this effect is that if one starts with a battery at 20% SOC, one can transfer electrical energy equivalent to x km of driving in the first 15 minutes of fast charging. If charging continues, one will be able to transfer energy equivalent to x/2 km (or less) in the next 15 minutes; and the rate of charging will continue to taper downward if charging continues beyond 30 minutes4. It is recognized that fast charging of a lithium-ion battery at low temperatures can lead to lithium plating onto the negative electrode. This plating can result in degraded performance and potential safety issues. Fast charging a lithium-ion battery at warm/hot temperatures (often defined as 40°C or higher) can result in an acceleration of the degradation of battery performance. To avoid such degradation, fast chargers and the vehicle control systems often limit the rate of charging at temperature extremes. For example, Tesla notes that Supercharging rates may be limited in certain climates. Data available from Idaho National Laboratory has documented how two specific vehicles control fast charging as a function of temperature.
Experimental data shows that fast charging as described above does not cause significant additional damage to a lithium-ion battery when compared with lower rate charging. The performance (both capacity and power) of any lithium-ion battery will degrade with time and with cycling. This degradation will be accelerated by higher temperatures. Typical goals for the performance of an EV battery are a 10+ year calendar life and over 1,000 full discharge cycles with retention of at least 80% of its original performance when operated at 30°C. Accelerated testing of advanced batteries designed for EVs has confirmed that these life goals can be met using published testing protocols.
Testing at Argonne National Laboratory (ANL) (located outside Chicago in the United States) was done on small commercial cells in two different chemistries that were cycled using a range of charging rates. The degradation of cells charged at the 2C rate is slightly worse than that observed in cells charged at 1C or less.
limit is reached. To fully describe actual charging or discharging rates, one needs to have detailed data on current flow and applied voltage as a function of time. The presentations from Idaho and Argonne National Laboratories include such data in graphical form.
2015 IA-HEV ANNUAL REPORT
Degradation increased with the charging rate. Warming of the cells, as measured on the surface of the cells, was observed even though the cells were located in a temperature chamber held at 30°C. Warming increased with the charging rate. The increased performance degradation at higher rates may be attributed to the increase in temperature. It has been proposed, but not yet confirmed by testing, that if the cells were actively cooled to keep their temperature at (or below) 30°C, little accelerated degradation would be observed.
Staff members from Idaho National Laboratory (INL) (located in Idaho Falls, Idaho in the United States) described data from a group of 4 EVs, 2 of which were charged at “normal” rates (Level 2, or a maximum of approximately C/4) and 2 were charged using modern fast chargers. Significant battery degradation was observed in both test samples, but the difference between the two groups was relatively small. Accelerated degradation could be correlated with other parameters such as elevated operating temperatures.
Studies reported by the National Renewable Energy Laboratory (NREL) (located outside Denver in the United States) showed that vehicle data supplemented with simulation calculations indicate that moderate use of fast charging, up to 10 times per month, does not seem to accelerate the rate of battery degradation significantly relative to Level 2 charging. In moderate climates (e.g. Seattle), the type of battery temperature management system does not have any significant effect on the rate of degradation. In hot climates (Phoenix), active cooling offers significant benefits relative to passive cooling. Batteries being charged in a high ambient temperature with only passive cooling can reach undesirably high temperatures. If these batteries are fast charged under these conditions, temperatures may reach very undesirable levels.
Automobile manufacturers’ statements about fast charging include Tesla’s statement that “Supercharging does not alter the new vehicle warranty. Customers are free to use the network as much as they like”. Nissan’s warranty on the Leaf guarantees 75% capacity retention for 5 years or 100,000 km for all forms of charging, including “QC”.
There are significant logistical and cost challenges associated with deploying a network of modern fast chargers. There are at least 4 different
configurations/designs of cables and plugs associated with modern fast chargers: CHAdeMO, Combo Types 1 and 2 (SAE), Tesla, and Chinese. Outside of China, CHAdeMO and Combo systems are most common, but they use different plugs and different charger/vehicle communication protocols. Many new fast charging systems are equipped with at least two different cables so as to be able to accommodate vehicles that use different protocols.
CHAPTER 4 – ELECTROCHEMICAL SYSTEMS (TASK 10)
At the meeting, it was mentioned that typical fast chargers cost about 20,000 USD. Cost quotations obtained after the meeting for dual port (SAE Combo and
CHAdeMO) chargers varied from about 25,000 to 35,000 USD depending upon the manufacturer. Installation costs can vary from 10,000 USD to over 100,000 USD. During and after the discussion, several participants remarked that they thought that 10,000 USD was a very low cost and that typical installation costs are much higher. A recent project in British Columbia5 worked to choose optimum locations (i.e. locations with moderate or low installation costs) for fast chargers and still had installation costs of 20,000 to 40,000 USD per site. In some locations, it can be very difficult and time consuming to get all of the necessary permits to install a fast charger.
The business model for fast chargers is uncertain. INL presented data on the use of a group of fast chargers as a function of time. As a part of a technology
introduction project, there was no cost to use these chargers. The number of charging events grew steadily over several months as users learned the locations of the chargers and more vehicles were added to the project. When the company operating the charger network instituted a fee per charge, the use of the chargers dropped significantly. Eventually the company went out of business. The small fast charging network in British Columbia charges for electricity on a kWh basis; the charges are about 3.5 times greater than the cost of electricity at a residence (0.35 USD vs about 0.10 USD). This network received significant government support, and the operators of the network view it as a “marketing tool” for EVs, a means to help mitigate “range anxiety” and encourage purchase and use of EVs. The operators do not expect the network to make a profit.
4.5
Next Steps
Discussion meetings on new topics are being planned for 2015. Possible topics include the Second Use of Vehicle Batteries, a Discussion of the Safety of Batteries in Vehicles from a European Perspective, a Summary of the Issues Related to Internal Short Circuits in Lithium-Ion Cells, and a Discussion of How to Handle a Vehicle if the Battery Has Been Damaged (First Responder Issues and Training).
2015 IA-HEV ANNUAL REPORT
4.6
Contact Details of the Operating Agent
Individuals interested in helping organize, host, or participate in a future working group meeting with a specific focus are urged to contact the Operating Agent. Mr. James A. Barnes, Ph.D.
Office of Vehicle Technologies, EE-3V U.S. Department of Energy
1000 Independence Ave. SW Washington, DC 20585 U.S.A. Phone: +1 202 586 5657 Fax: +1 202 586 2476 E-mail: [email protected]