2.3.2. Cable Resistance

In addition to the requirement of low resistance, the load cables were also used as a shunt to determine the cell currents, which were calculated via the voltage drop at the cables. Since this drop correlates to the cable resistance, a trade off between the measurement accuracy and the influence on the current distribution arise. Therefore different cable resistances and their impacts on the current distribution were investigated by measurements with two parallel-connected cells. The cable resistances were examined by four terminal measurements using a multimeter (Keithley, DMM7510) and are given in Table 2.

**Table 2.** Investigated cable resistances.


**Figure 5.** Impacts of tab treatment, with the influences of surface cleaning and pressure (**<sup>a</sup>**,**b**) as well as the effects of air sealing (**c**) on the contact resistance at the cathode (aluminum) and anode (nickel-plated copper) tab.

In relation to the cell resistance, the cable resistances of the first type *<sup>R</sup>*Cab,Type1 range from 1.3% to 0.2% depending on the SoC and cell temperature and should have no significant influence on the current distribution. Type 3 *<sup>R</sup>*Cab,Type3 uses the complete measuring range of the measuring device, which should lead to the highest measurement accuracy. Type 2 *<sup>R</sup>*Cab,Type2 offers a trade off.

The current distribution of two parallel-connected cells with the discussed cables are presented in Figure 6. The temperature of the aluminum plates were kept constant at *T*Plate = 30 ◦C. The cells were discharged with a current load of *I* = −10.6 A, which corresponds to a C-rate of 1C. The discharge started with a SoC = 0.95 and lasted until the cell voltage reaches the voltage limit of *U*Cell < 2.8 V. After a break of 30 min, the cells were charged with a C-rate of 1C until the cell voltage reached the voltage limit of *U*Cell = 4.2 V. Thereafter the cells were relaxed for 30 min.

The current distribution is qualitatively the same for the three different cables with a shift to higher current differences within the parallel-connected cells with increasing cable resistance. However, this may also be due to the higher difference between the cable resistances. The differences are most obvious at the current rest and at the end of the discharge phase. This can be caused by the correlation of the time constant to the cell's resistance, as mathematically shown in previous work [8,9].

**Figure 6.** Influence of the cable resistances on the current distribution of two parallel-connected cells. With the current load (**a**), the cell and plate temperature (**b**) and the separated phases: Discharging (**c**), the first rest (**d**), charging (**e**) and the second rest (**f**). The relations of the cable resistances are *<sup>R</sup>*Cab,type3 ≈ 3 · *<sup>R</sup>*Cab,type2 ≈ 15 · *<sup>R</sup>*Cab,type1, with the exact values in Table 2.
