**1. Introduction**

Large-scale battery applications like electric vehicles (EV) have to meet high power and energy demands, which is mostly realized by the parallel-connection of lithium-ion cells, e.g., Tesla Model S (74p96s (The abbreviation xpys corresponds to a cell configuration with x cells in parallel and y cells in serial connection)), Tesla Model 3 (46p96s), VW eGolf (3p88s), Nissan Leaf (2p96s), BAIC EU260 (3p90s), Renault Zoe (2p96s) and Audi etron (4p108s) [1,2]. Caused by production-induced distributions of cell resistances and capacities [3,4], inhomogeneous cell currents arise within these parallel cell configurations [5], further leading to State of Charge (SoC) [6,7], Open Circuit Voltage (OCV) [8,9] and temperature gaps [6,10] within the parallel cells. Since the cell currents must be maintained in their corresponding operating window and cannot be measured for space and cost considerations, correlations of the current distribution to cell parameters and cell states are essential for an optimal and safe battery operation.

There is a variety of articles focusing on modeling [11–15], aging [16–18], safety [19–21], state estimation [22,23] and measurement [24–26] of parallel-connected cells. Mostly qualitative

effects like OCV [27], SoC [13], temperature [13,26,28,29] and current differences [13] are demonstrated but quantitative relationships are missing, especially with regard to the thermal connection of the cells to neighboring cells and cooling. The cell temperature, as well as the temperature difference between parallel cells, can have a high influence on the current distribution both due to the high sensitivity of the cell resistance to the temperature [30,31] and due to the correlation of heat dissipation to the cell current [32].

Therefore, in Section 2, a test bench with a flexibly adjustable thermal connection of the individual cells to their neighboring cells and cooling is presented. Thus, both the formation of temperature gradients due to the current distribution and the impacts of design-induced temperature gradients in a battery module on the current distribution can be investigated. In Section 3, an extended dual polarization Equivalent Circuit Model (ECM) is introduced and compared to measurements. This simulation model takes into account the influences of the test bench on the current distribution and the parallel-connected cell layers within the cells. In Section 4, the influence of temperature gradients induced by inhomogeneous thermal connections of the cells to neighboring cells and cooling on the current distribution of two parallel-connected cells are investigated. At the end, a conclusion is given.
