Nickel Strip Selection
- Micah Black
The thickness of the nickel strip should be selected to optimize a number of items
- Power loss and heat dissipated
- Manufacturability
- Safety
Power Loss
The power loss across the nickel strip is proportional to the current flowing through, and the resistance of the strip: P = I2R. Our maximum pack current is expected to be 100A, and we can not change that, but we do have control over the resistance of the nickel strip.
The resistance of the strip is proportional to the resistivity (p) of the material used, the length of the strip, and the cross sectional area of the strip.
The length of the strip in our design is 8mm.
The cross sectional area (thickness and width) of the strip can be controlled based on which strip that we buy.
Resistance = p x (Length) / Cross Sectional Area
To decrease the power loss, we will want the lowest resistance, and thus the thickest strip possible.
Manufacturability
The nickel strip is being spot welded twice, in two places, on each side for every cell in every module that we are making. This means that the number of spot welds is: 30 modules * 48 cells/module * 2 * 2 * 2 = 11,520 spot welds.
These welds must be able to be done quickly. This means that a low welds energy is required, as our spot welder supply cables can heat with excessive weld energy and low rest times between welds. A low weld energy requirement means that the nickel strip must be thinner, as it takes less energy to heat up to the point that it welds.
To increase manufacturability, we want the thinnest nickel strip possible.
Safety
It is commonly known that heating up any battery can be dangerous. Even localized heating in the spot welding process can vaporize some of the electrolyte inside the cells at the location of the spot weld, leaving behind reaction products which can contaminate the rest of the cell and create internal shorts, etc. (The section "From LFP" on this page explains the details: https://www.electricbike.com/introduction-to-battery-pack-design-and-building-part-3/)
To increase safety, we want the minimum amount of heating possible, and thus the shortest welding time, which means the lowest weld energy and thus the thinnest nickel strip. Note that this concern only applies to the negative end of the cells, and the positive end is virtually unaffected, so a thicker nickel strip could be used on the positive end.
Strip Thickness Choice:
We first looked at thicknesses available from the manufacturers, and we found that 0.1mm, 0.15mm, and 0.2mm are commonly available from Aliexpress.
From the Safety and Manufacturability standpoints, we wanted the thinnest nickel, so we decided to see how much the power loss actually matters. We used a python script to calculate the power losses in the nickel strip at different pack currents, and produced the following graph using MatPlotLib. The pack loss is the loss in the Nickel Strip compared to the loss due to the internal resistance of the 18650 cells (38mOhms each).
As shown in the graph, we have only a 2% increase in % of pack power loss, and 10(ish) watts of extra power dissipation going with the thinnest strips available. Also note that at our target cruising current (15A), there is less than 1W of difference between the thinnest and thickest strips.
We chose the 0.1mm strips as the extra safety and manufacturability is totally worth the tiny bit of extra power loss.
In future packs, alternative methods to connect the negative cell can should be evaluated to increase the safety of our packs (if we are still using 18650/21700 style Li-ion cells).
The graph was made with this messy but functional python script.