## 2 Preliminary Design
### Structure of the Cooling Plate  
To ensure a long life cycle, good capacity performance, and good power performance, the battery is designed to operate at a target temperature range (25 to 45 \({}^{\circ}\)C), and the temperature difference between the cells should be within 5 \({}^{\circ}\)C. Considering the overall cost, system complexity, heat transfer performance, etc., a liquid-cooling system was designed and is presented in Figure 3.  
The whole battery pack consists of two battery modules, and the cooling system also includes two cooling modules. The two cooling modules were connected in parallel, as shown in Figure 3a. Each cooling module cools one battery module, as shown in Figure 3b. The cooling module consists of one inlet pipe, one outlet pipe, and five serpentine pipes. Five serpentine pipes were connected in parallel, and the inlet was designed at a lower horizon than the outlet to facilitate expelling bubbles. A total of 24 battery cells were placed in contact with each serpentine pipe and cooled one by one in series, and thermal silica gel pads were added between the cells and cooling pipes to improve the thermal contact. This structure forms a mixed parallel-series cooling system.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Items** & **Cell** & **L5 Module** & **Pack** \\ \hline Number of cells & 1 & 5 & 240 \\ Normal Voltage (V) & 1.2 & 6 & 288 \\ Capacity (Ah) & 6 & 6 & 6 \\ Energy (Wh) & 7.2 & 36 & 1728 \\ DC-IR (m\(\Omega\)) & 2.5 & 12.5 & 600 \\ Discharge power 1 (W) & 185 & 925 & 40,000 2 \\ \hline \end{tabular}  
* 25 \({}^{\circ}\)C, 50\(\%\) SOC, 10 s. 2 The total power of the battery pack is smaller than the theoretical value because of the consistency of the battery cells.  
\end{table}
Table 2: Basic performance parameters of the NI-MH cells and pack.  
Figure 3: Liquid-cooling system. (**a**) A 3D view of the two cooling modules, including the in/out rubber pipes connected in parallel; (**b**) schematic of the cooling module; (**c**) schematic side view of the cooling module and the battery cell; (**d**) cross-sectional view of the serpentine pipe (with internal walls).  
In addition, because the cross-section was rectangular with a large width to height ratio (\(30\times 2.75\) mm), this structure required support walls inside to prevent deformation under stress. This stress occurs because of two phenomena: the first is that the straight pipe is extruded into the curved pipe during the production process, and the other is that the pipe is always under the pressure of the battery cells and the bracket during the operation process. According to the section size of the pipes mentioned in this paper, about 3-4 internal supporting walls can overcome this stress to prevent deformation. The internal supporting walls will divide the flow into several independent flow paths and form a harmonic-like structure. Parameter N represents the number of internal walls within one serpentine pipe (e.g., in Figure 3d N = 4). The number of internal walls will affect the heat transfer and pressure drop of the pipes. How to choose a suitable number of internal walls is of great significance and will be studied and analyzed in Section 4.