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Section 2.5 Shunt currents Shunt currents also occur for small charging and discharging currents and even if the battery is neither being charged nor discharged. Compared to the externally applied current, which varies from −150 A to 150 A, the equivalent shunt current only varies very little, as shown in Figure 2-9. Hence, shunt currents have a more severe impact on the efficiency if smaller currents are applied to the battery. This is because the ratio of shunt current to load current worsens for smaller currents. 2.5 2 1.5 1 100 80 60 40 20 Tank SoC in % -100-50 0 50 100 150 0 -150 Discharging/charging current in A Figure 2-9: Equivalent shunt current versus SoC and current for design 2.1 The electrode area does not affect shunt currents. The comparison of designs 2.1 and 4.6 demonstrates that it is possible to design a larger electrode with a lower equivalent shunt current. The large nominal current of a larger electrode is beneficial as it improves the ratio between externally applied current and equivalent shunt current. Operating a large electrode with a high current density is consequentially an effective measure to eliminate the impact of shunt currents on battery efficiency. Assuming a current density of 75 mAcm-2, cell design 2.1 carries a current of 150 A. With a shunt current of 1.95 A during the charging of the battery with 150 A at a tank SoC of 50 %, the Coulomb efficiency loss due to shunt currents is 1.3 %-points for a 40-cell stack. For design 4.6, we can double the externally applied current to 300 A. With an equivalent shunt current of 0.48 A, Coulomb efficiency loss is only 0.16 %- points and thus eight times smaller. Internal cell current In the model, the internal cell current refers to the current which passes the controllable voltage source, as shown in Figure 2-6 on page 27. Figure 2-10 illustrates the effect of the shunt currents on the individual cell currents in a stack. Herein, the internal cell current distribution on the individual cells is shown for a stack with 10, 20, 30 and 40 cells using the shunt-current-sensitive cell design 2.1. 32PDF Image | Model-based Design Vanadium Redox Flow Batteries
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