Battery Cell Connections

Date: 13 Oct 2018 

In a battery pack, the lowest denominator or energy storage is the cell. Cells come in various different geometries, such as cylindrical 18650 cells used in Tesla's Model S or flat pouch cells used in Nissan Leaf. When many of these cells becomes electrically connected , they become a module; the building block of the battery pack. So it is crucial in a vehicle application that the method of electrical connection between the cells are able to conduct the required electrical load, as well resist the vibrations of the road. Like MSXII, MSXIV will use 18650 batteries as its energy source.

This page will document the different types of cell connections researched, and go over the design decision of cell connections for MSXIV.

Connection Method

Solder

Used commonly in DIY Powerwall communities (HBPowerwall on YouTube) as a means of connecting fuse wires to busbars. It is easy, affordable, and can be accomplished without the need for complex machinery. In general, soldered connections can become 'cold' (loose/bad connections) when exposed to high temperature variations, of vibrations. In terms of pricing, soldering has the advantage over other options. However in the application of designing a solar vehicle, soldering is proven to disadvantageous. As the batteries are lithium-ion chemical reactions, introducing heat to them via soldering can damage the battery chemistry and shorten battery life-cycle as a result. Solder joints also suffer mechanically, as they perform poorly under stress. This fact is crucial for building an automobile, as vibration and movement is constant on the road.  In terms of manufacturability, soldering takes much longer than spot welding as the area being connected much first be heated up in order to stick to the solder.

Mechanical Connection

Another option is to join the cells together via a mechanical connection using bolts to connect a large copper plate to each end of the 18650, thus forming a parallel connection across all cells. This would prevent minimal damage to the cells during assembly, but due to low reliability and manufacturing complexity, this idea was discarded.

Spring Loading Contacts

Reliable for low power stationary connections, but for high power connections that could be vibrating a lot, they are not ideal. Due to the materials they are made of, these types of contacts have higher resistance than spot welding and soldering, so more power is lost in the connections. The Vruzend system is a good example of this, and there have been numerous complaints of cells becoming loose and not creating solid connections between the cells. This method is easier, quicker, and safer to manufacture than spot welding and soldering as it does not require specialised tools, high currents, or heat. It is also easier to remove individual cells.

Spot Welding

Spot welding a type of welding where high current electricity is used to melt and join two pieces of metal together. This method has a very low resistance and thus is ideal for high power applications (less power lost as heat in the connections). A machine is required for spot welding, costing anywhere from hundreds to thousands of dollars, depending on the machine's quality. While spot welding does introduce heat to the battery, it does so at a fraction of the time it takes to solder a cell, making spot welding a safer option for assembling battery packs. Spot welding is used frequently in the automotive industry to join the body frame of cars together, so vibration is not a major concern for the strength of the cell connections. Spot welding is also widely supported over soldering in the e-biking community, a very similar application of batteries as Midnight Sun's. While spot welding is the most costly option, it is also the safest, which is why MSXIV will continue to use spot welding as the method to connect cells together.

Connectors

Fuse Wires

Inspired by the Tesla Model S battery pack, fuse wires is a connection type that acts both as protection against over-discharge and a connection wire. The fuse blows when a cell is shorted or too much current is running through the cell, protecting the battery system. This idea is very attractive as it provides an extra layer of protection for the battery pack, while doubling as its electrical connection system. However, there are multiple reasons that this idea was not ideal. 

While, having an extra layer of protection for the battery pack was enticing, it was not needed. For MSXIV, the maximum predicted discharge rate will be 0.9C, meaning maximum discharge rate for 18650 batteries would be ~3 amps. Using fuse wires as our connection method would introduce unnecessary design complexity while providing no advantages to the battery pack's safety, as the discharge rate in the cells would never reach a level that would cause the fuse to blow. Manufacturing with fuse wires would also be a manufacturing nightmare, as the narrowness of the fuse wire makes spot welding them very time consuming and tedious. On top of that, fuse wires present an unexpected challenge in the connection of the battery modules. As fuse wires are very fragile, they must be given assembly clearance in the assembled pack. Having this space would complicate our bus bar design and waste precious space in the battery box. 

Model S fuse connection	A spot welded fuse wire

Nickel Strips

Nickel strips is another widely used method of joining cells together. Compared to fuse wires, nickel tabs have a much larger area for spot welding, making the connection much stronger, simplifying manufacturing complexity, and reducing manufacturing time. Nickel can be ordered in thin sheets, giving much more design flexibility as it could be cut to meet design requirements. The flatness of nickel tabs also reduces the module volume compared to fuse wires when assembled, thus saving precious space in the battery box. 

In conclusion, nickel sheets gives much more design flexibility and greatly simplifies manufacturing, hence MSXIV will use nickel tabs its way of cell connection.

EDIT (by Micah Black):

Just wanted to put this here as a note. Here is an article claiming that clamped cell connections can have a much lower contact resistance than spot welded connections. A clamped connection also allows easier assembly, but can be prone to failing with vibration and mechanical shock.

https://www.sciencedirect.com/science/article/pii/S2352152X16301529