**Measuring Test Bench with Adjustable Thermal Connection of Cells to Their Neighbors and a New Model Approach for Parallel-Connected Cells**

### **Alexander Fill 1,\*, Tobias Mader 1, Tobias Schmidt 1, Raphael Llorente 1 and Kai Peter Birke 2**


Received: 28 October 2019; Accepted: 20 December 2019; Published: 26 December 2019

**Abstract:** This article presents a test bench with variable temperature control of the individual cells connected in parallel. This allows to reconstruct arising temperature gradients in a battery module and to investigate their effects on the current distribution. The influence of additional contact resistances induced by the test bench is determined and minimized. The contact resistances are reduced from *R*Tab+ = 81.18 μΩ to *R*Tab+ = 55.15 μΩ at the positive respectively from *R*Tab− = 35.59 μΩ to *R*Tab− = 28.2 μΩ at the negative tab by mechanical and chemical treating. An increase of the contact resistance at the positive tab is prevented by air seal of the contact. The resistance of the load cable must not be arbitrarily small, as the cable is used as a shunt for current measurement. In order to investigate their impacts, measurements with two parallel-connected cells and different load cables with a resistance of *R*Cab+ = 0.3 m Ω, *R*Cab+ = 1.6 m Ω and *R*Cab+ = 4.35 m Ω are conducted. A shift to lower current differences with decreasing cable resistance but qualitatively the same dynamic of the current distribution is found. An extended dual polarization model is introduced, considering the current distribution within the cells as well as the additional resistances induced by the test bench. The model shows a high correspondence to measurements with two parallel-connected cells, with a Root Mean Square Deviation (RMSD) of *ξ*RMSD = 0.083 A.

**Keywords:** lithium-ion battery; parallel-connected cells; measuring test bench; current distribution; tab contact resistance
