**2. The Test Bench**

In this section, the test bench is discussed. First, the interactions and functions of all subsystems are explained and afterwards the temperature control system is presented in more detail. Finally, the possible impacts of the test bench on the current distribution are demonstrated.

### *2.1. Interactions and Communication of the Subsystems*

Figure 1 shows the test bench for one cell with view-optimized modifications (a), a schematic view of the temperature control system (b), a detailed view of the position and orientation of the temperature sensors (c), as well as a detailed view of the electrical connection of the cells to the cell tester highlighting the voltage and current measurement of each cell (d).

The test bench for each cell (10) consists of two Peltier Elements (11), two aluminum plates (3), two CPU coolers (1), two speed controllers (2), a measurement device (4), one micro-controller (6) and a power supply unit (7). The housing is electrically grounded and consists of aluminum profiles (5). The cell tabs are connected to the load cable via a screwed aluminum union joint (13). The cell temperature is captured by two Pt100 sensors (12) of each cell side, *<sup>T</sup>*Cell,Center and *<sup>T</sup>*Cell,Top see Figure 1c. The temperature of the adjacent aluminum plates is measured with one Pt100 sensor *T*Plate. The cell voltage *U*cell is measured at the cell tabs, the cell current *ICell* is calculated via the voltage drop at the load cable (14) of each cell, see Figure 1d. The load cables of the cells are connected to a cell tester. This setup enables one to investigate any cell topology; the number of parallel-connected cells is only limited by the size of the climate chamber. The communication and interaction of these subsystems are presented in Figure 2.

The measurements are conducted in a temperature controlled climate chamber and the battery load is controlled by a cell tester. The temperatures of the lithium-ion cells are regulated by Peltier Elements. One side of the Peltier Elements is thermally coupled to the CPU coolers to keep the temperature of this side constant at ambient temperature. In order to ensure temperature homogeneity at the cells, the Peltier Elements are embedded in aluminum plates. In addition, the plates and the cells are isolated by Polystyrene (9). The assembly of these subsystems is presented in Figure 1b. The voltages of the Peltier Elements are adjusted by speed controllers, which in turn are controlled by micro-controllers. These are regulated by a PI controller implemented in Matlab.

**Figure 1.** Setup of the test bench. View-optimized modifications of the test bench with removed cooling tubes, cables and one aluminum profile (**a**). Schematic view of the temperature control system including the positions of the Pt100 sensors (12) (**b**). Detailed view of the position and orientation of the temperature sensors (**c**) and a detailed view of the electrical connection of the cells to the cell tester (**d**).

Measurement data, including six Pt100 temperature sensors (12), three for each cell side, as well as two voltage signals, are recorded with a frequency of *f* = 100 Hz. The signals are transferred by an interface via CAN to a PC and real time processed in Matlab. The power for the speed controllers, the measurement device and CPU coolers is provided by a power supply unit.

In addition, the pressure on the cells is kept constant by a spring construction and the mobility of one aluminum plate. A fluctuation of the pressure due to the correlations of the cell's thickness to SoC [33,34] and temperature [35] as well as the continuous increase of the cell thickness caused by lithium plating and gassing [36] could otherwise have a negative effect on cell aging [37]. The manufacturers and the corresponding types of the individual components are summarized in Table 1.

The functionality of the temperature control system, temperature homogeneity of the aluminum plates and the heating rate are explained in more detail in Section 2.2.

**Figure 2.** Schematic view of the communication and interactions of all subsystems.


