To investigate the ageing scalability and the OCV change at module level an ageing study at the module level is performed. Therefore, two identical battery modules were constructed.
The requirements included: a realistic size and capacity for an electrical vehicle, an exact temperature measurement, and the possibility to disassemble the series connection to obtain a deeper insight in the local ageing behaviour.
4.2.1 Topology
The modules were designed in an 8s14p cell interconnection topology, which is illustrated in Fig. 4.1a. This topology consisted of eight blocks connected in series, with each block consisting of 14 cells in parallel. This resulted in an operating voltage range from 20 V to
4 Experimental
33.6 V and a nominal capacity of approximately 40 A h. A total number of 224 cells was used to construct the two battery modules. These cells were selected out of 250 cells from the same production lot. The matching process is described in Section 4.2.3.
343 mm
182 mm65 mm
- +
Voltage measurement CMU and BSD Voltage measurement BMS
Negative cell tab Positive cell tab
Direct temperature measurement
Average temperature measurement (4 cells) Removable copper rail
b)
c)
2s 1s 1p
2p
14p
3s 4s 5s 6s 7s 8s
a)
Figure 4.1: Design of the modules in 8s14p topology: (a) schematic representation which illustrates the disas-sembly points (dashed red lines); technical drawings in top (b) and front view (c), illustrating the positions of the temperature and voltage measurements.
To gain a deeper insight into the ageing behaviour of the module, the series connections of the eight blocks are demountable. This is illustrated in Fig. 4.1a by the dashed red lines.
This allowed for separate capacity and resistance measurements for all eight cell blocks to determine interdependencies between the ageing behaviour and the position of the cells within the module. However, the interconnections between the 14 cells of each block were realised as non-removable joints, as removable connectors generally increase the contact resistances
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4.2 Design of the battery modules [135]. This would affect the equalisation processes between parallel cells and may distort the ageing behaviour.
4.2.2 Construction
Fig. 4.1b and Fig. 4.1c show the design of the modules. The 112 cells were placed in a frame that was 343 mm long and 182 mm wide, with a spacing between the cells of 1.6 mm and 4.6 mm, respectively. The blue circles symbolise the negative poles and the red circles the positive poles of the cells.
The cell connectors, depicted in grey, were made of 0.2 mm thin nickel-plated steel (Hilumin), which is a steel alloy that can be spot-welded onto the cell poles. The cell connectors were welded to 1.5 mm thick and 10 mm wide copper rails, depicted in orange. During the construc-tion process, the cell connectors were first attached to the copper rails by ultrasonic welding.
Afterwards, the produced "ribs" were spot welded onto the cell poles.
Due to the module design, with its removable series connectors, the entire current of the 14 parallel cells of each block had to flow through the current collector rails, depicted in orange.
To prevent high power losses and heat generation in the cell connectors, these rails were not made of Hilumin because of its high specific resistance, of approximately 0.1 W mm2m−1. In-stead, the current collector rails were made of copper, which provided a substantially lower specific resistance of 0.017 W mm2m−1. In this manner, the power loss of the connectors and hence the cell heating was minimised. Moreover, the module could be disassembled in blocks by untightening the copper rails on the front of the module (Fig. 4.1c).
To investigate the temperature behaviour, 25 temperature sensors were installed in each mod-ule. The locations of the sensors are also shown in Fig. 4.1b (cyan and green circles). The cyan circles symbolise PT100 sensors attached directly onto the cell surface by a thermal adhesive, and the green circles symbolise thermocouple sensors placed in the centre of a heat conduc-tive silicone spacer, which retained an equal distance from the four adjacent cells. The two groups of sensors thus enabled measurements of the cell temperatures as well as the average temperatures at different locations inside the module.
4.2.3 Capacity-based cell matching
To perform capacity-based cell matching, the capacity distribution of the used cells had to be measured to exclude outliers. For all 250 cells used in this investigation, the measured and normal distribution is given in Fig. 4.2a for capacities and Fig. 4.2b for ohmic resistances (measured at 1 kHz).
The relative coefficients of variationκ=σ/µfor the capacity and the resistance are 0.16 % and 0.72 %, respectively. The parameterσ is the standard deviation from the normal distribution [132]. The arithmetic mean value µ of the capacity is 2.88 Ah, and that of the resistance is 21.67 mW. Higher κ for resistances were also observed for LICs with other cathode materials
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[128; 132], which in turn could lead to a pronounced ageing behaviour for cells connected in parallel (Section 1.2.6 and [138]).
2.87 2.875 2.88 2.885 2.89 2.895 2.9 C / Ah
Figure 4.2: Cell matching: (a) normal distribution of capacities at the cell level; (b) normal distribution of ohmic resistances (measured at 1 kHz) at the cell level.
Despite the low tolerances, capacity-based cell matching was performed to guarantee a low balancing effort for the BMS. The objectives of cell matching were to select and connect cells to blocks such that the serial connections would result in a homogeneous capacity distribution in the module. In other words, the module capacities would not be reduced by a weak block, and the BMS would not require continuous balancing. Two (approximately) identical modules were created after the cell matching procedure.
The calculated and measured capacities of the eight blocks of both modules after matching are shown in Fig. 4.3a and Fig. 4.3b, respectively. Thereby, the capacity and resistance values for each block were calculated by:
Cblock =X14
On the basis of these results, the capacities at the module level were observed to be lower.
This was attributed to the length of storage time (approximately 4 months) between the single cell measurements and the measurements at the block level. The mean capacity losses caused by calendar ageing were 0.91 % and 0.88 % for module 1 (M1) and module 2 (M2), respectively.
The calculated and measured block resistances are shown in Fig. 4.3c and 4.3d. Here, an increase of the measured resistances compared to the calculated values was observed. The effect of a resistance change due to calendar ageing was considered to be negligible because the contact resistances were the dominant factor for increased resistances in the modules. In [135], the resistance of a spot welded contact (Hilumin to Hilumin) was given as 0.16 mW.
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