Temperature characteristics of test during charge and discharge. (US Army CCDC Ground Vehicle Systems Center) Figure 4. Current and voltage characteristics of LFP 2.3 Ah under test during charge and discharge. The cell under a 120-A pulse for 3s shows a similar profile, with an initial capacity of 1.95 Ah. The initial capacity with this profile was 1.84 Ah. It can be seen that the cell can sustain the pulse for six minutes before it reaches the 2V discharge limit. Resultsįigure 3 shows the cell’s voltage and current response to a load profile, as shown in Table 2, with a 120-A pulse for two seconds. The cells were then placed into a thermal chamber for environmental control at 10 ☌ for automated lifetime testing. The cells were attached to an A&D/ BITRODE electronic load with thermocouples affixed to the cell negative tab and cell skin surface. (US Army CCDC Ground Vehicle Systems Center) Experimentalīased on the use of LFP cells in commercial pulse power applications, such as power tools with a long lifetime, a 26650 LFP cell was selected. The desired electrochemical reaction is the lithium intercalation in graphite but lithium can also react with components of the electrolyte to form a solid-electrolyte interphase. As shown in Figure 2, the electrolyte reacts and consumes lithium to form an insoluble interface that decreases cell capacity. Models have been proposed based on empirical and physics-based aging mechanisms.īased on previous work, the cell degrades due to the consumption of active Li material via solid electrolyte inter-phase (SEI) growth. ![]() For the LFP cells tested at 15 C discharge rate, the rapid cell capacity decay was attributed to the increase in cell resistance.Ĭell degradation theory and prediction is critically important to multiple commercial applications and is an active area of research. tested LiNi xCo yAl 1-x-yO 2 (NCA) and LiFePO 4 (LFP) for pulse at high rate. ![]() There has been limited published experimental work on high-rate discharge. In these pulse power applications, the high-power pulse duty cycles can have discharge rates that are significantly higher (>10 C) than commercial HEV ESS systems, resulting in increased thermal and electrical stress. However, as the platform size decreases, the ESS discharge rates for DE capabilities increase significantly beyond standard HEV ESS solutions. This discharge rate can be met using existing HEV ESS solutions. Thus, the discharge rate for the silent mobility capability is constant across different platform sizes. The silent mobility power requirement has been normalized for a hybrid-electric combat vehicle platform weight (3.9 kW/t) and the battery pack is proportionally sized (0.6 kWh/t). By definition, a 1 C-rate discharge is equivalent to a discharge current that will discharge the entire battery in one hour.) (Note: Standard industry practice is to define charging/discharging by C rates. The discharge rates for silent mobility - a 30-kW DE and 100-kW DE capability using a Hybrid Electric Vehicle (HEV) configuration - are shown in Figure 1 along with examples of commercial systems. (US Army CCDC Ground Vehicle Systems Center) ![]() Battery capacity normalized with platform size to provide silent mobility and directed energy capabilities. Army’s pursuit of vehicle electrification is to realize benefits of significant fuel savings/range extension, increased silent watch/mobility, and new capabilities in Electronic Warfare (EW), high-power sensors, and Directed Energy (DE) systems. To mitigate the thermally induced degradation, the use of different thermal management systems and alternative cell design is discussed and recommended. The decreased lifetime is attributed to increased lithium loss due to the increased temperature during pulse discharging. Although these cells were thermally managed in a convective chamber at 10 ☌, the 2-second pulse showed a 31 ☌ temperature rise and the 3-second pulse a 48 ☌ temperature rise. For two-second and three-second pulse duration tests, the observed degradation is 22% and 32%, respectively. Results are reported of high-power pulse duty cycles on lithium-iron phosphate cells that show a dramatic loss in lifetime performance. However, as the vehicle platform size decreases, the Energy Storage System (ESS) pulse power discharge rates (>40 C-rate) to support system requirements can be significantly greater than commercial ESS. The benefits include new capabilities that require high-power pulse duty cycles. Army has been pursuing vehicle electrification to achieve enhanced combat effectiveness.
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