From personal practice:
how to assemble lithium-ion cells into a single battery pack?

Welding of lithium-ion battery packs for VOLTS energy storage devices, electric scooters and even Tesla
electric cars 

Batteries, which consist of many individual battery cells, are used in energy storage devices, both domestic and industrial, in UPS systems and in the engineering industry for electric vehicles. The applications of batteries include virtually all electrical devices that must operate autonomously.

Disassembling an electric scooter, you can find in it a battery made of similar cells as in Tesla electric cars, but the cells in the former will be hundreds of times smaller.

"A battery cell - (a single cell consisting of an anode, cathode and electrolyte) has a standard voltage due to its chemical composition. This voltage is quite low: no more than 5 volts. For practical use such voltage is not always convenient, so the cells are connected in series in a battery and the total voltage of the battery is equal to the sum of voltages of individual cells. Also one cell has a limit on the maximum current output, but this parameter can vary greatly for different types of cells and their chemical compositions. When it is necessary to get more current from the battery, cells are connected in parallel. Thus, by varying the number of cells connected in series and parallel, it is possible to obtain from standard cell sizes a battery with the required characteristics (voltage, maximum current and capacity) for use in a particular device"

"Batteries are a source of direct current that is designed to store and accumulate electricity. Most models of modern batteries operate on the principle of cyclic conversion of chemical energy into electrical energy and guarantee many charging and discharging cycles"

In order to create a rechargeable battery, it is necessary to achieve a reliable and durable electrical connection of the cells into a common electrical circuit. For this purpose, different types of connectors and connection techniques can be used.

Let's talk about the most used for lithium-ion batteries.  

Welding methods for lithium-ion battery packs

Wires and busbars are traditionally used as current conductors in electrical engineering. Wires can be useful for connecting two remotely located points when a complex geometry of the route is required. Busbars, on the contrary, are used to connect a large number of similar electrical elements located close to each other.

In batteries, wires are not usually used and the cells are connected by means of busbars. In order to connect the bus bars and contact pads of lithium-ion batteries, there are several technologies that have already been actively integrated into the industry. 

1. Mechanical screw connection 

In this connection method, the battery is connected to the busbar with a bolt or nut. For this purpose, the manufacturer must equip the cell with a stud or a threaded hole in the terminal of the cell. Usually such terminals are found in relatively large prismatic cells with a capacity of 40 Ah and higher. 

Mechanical threaded connection is easy enough to perform even without special equipment, you only need to control the tire tightening force.

If the tire is tightened too loosely, the contact resistance will be high and the connection area will heat up, resulting in accelerated corrosion of the metal, deterioration of cell life, melting of the insulation, or even fire. Excessive tightening may result in thread breakage or damage to the battery cell casing. The optimum tightening force is set by the cell manufacturer and is specified in the cell technical documentation.

Important: The tightening may loosen during operation, so it is advisable to use spring washers. Even with the maximum allowable tightening, this method of connection does not guarantee good contact and is not recommended for high-current batteries. It is also necessary to consider the chemical composition of the materials to be connected when selecting connectors to avoid electrochemical corrosion.

2. Electric spot welding

Electric spot welding can be called one of the most popular ways to connect rechargeable batteries based on 18650 cells. This technology can be found in electric scooters, power tools, and portable equipment.

The essence of electric welding is to connect battery cells with a relatively thin (0.1 - 0.3 mm) nickel or nickel-plated (coated with a layer of nickel) tape.  The tape is welded to the cell by passing a short current pulse of several hundred amperes using a special spot welder. The current allows to melt the metal of the tape and the upper layer of the cell contact area at the point of contact of the welding machine electrodes, and the short duration of the pulse (from hundreds of microseconds to tens of milliseconds) prevents the battery from overheating.

The advantages of this welding method include its relative cheapness - manual welding machines are freely available. For battery repair, for workshops and small productions there are manual models of machines, for large batches there are automatic models with CNC, designed for continuous work and large volumes. To the minuses - the need to use only thin connecting bars (tapes), which are not able to pass large currents. That is why this method is usually suitable for connecting 18650, 21700 and similar cells, but not for assembling a battery from prismatic batteries with operating currents per cell of 40 or more amperes.

Note: there are also machines that allow welding with copper bars, but their cost is much higher. Copper bars allow to conduct more current than nickel bars and even more so than nickel-plated bars.

3. Laser welding

A popular welding method of both small 18650 cells , and powerful batteries based on relatively large prismatic cells with capacities up to several hundred ampere-hours.

Laser welding involves heating the tire metal with a laser. The equipment for such manipulations should be automatic or semi-automatic, to maximize the quality of the connection.

Laser welding is considered to be of higher quality and more versatile than spot welding, but the cost of minimal equipment will be higher and the repairability will be lower. Often, such equipment can afford only specialized production . Laser welding can be used to weld thick conductor bars to the cell terminals, allowing currents of hundreds of amperes to flow.

4. Ultrasonic welding (used in VOLTS energy storage batteries)

An improved process for joining metal surfaces by friction. In ultrasonic welding, a machine presses a thin aluminum wire against the contact terminal of a lithium-ion battery cell, causing it to vibrate at ultrasonic frequencies (about 100 kHz). As a result, the metal atoms of the two surfaces mix and fuse together in a few hundred milliseconds. After connecting the wire to the cell, the machine does the same procedure to connect the same piece of wire to the current-carrying busbar. Depending on the operating current of the battery, there may be one or more of these connecting conductors.

In this way, each cell is connected to the busbar not directly, but via thin conductors. This allows the method to provide an additional function of overcurrent protection for the cell. In essence, the connecting conductors act as fuses (fusible links). This is to ensure that in the event of an internal short-circuit in an individual cell, cells that are serviceable and connected to the same busbars do not increase the short-circuit current through the damaged cell. In this mode, the connecting wires of the faulted cell burn out and the faulted cell is disconnected from the rest of the circuit, preventing the development of an emergency and reducing the risk of battery fire.

Ultrasonic welding provides a reliable and high-quality connection, is suitable for automated conveyor assembly and does not require a molten phase. This method is considered to be the most technologically advanced and safe way of manufacturing lithium-ion battery packs. Therefore, we use this type of welding in our VOLTS energy storage battery packs.

Nuances: Ultrasonic welding requires a perfectly flat and clean joint surface. Also, there are limitations on the current that can be passed through the materials to be joined - usually up to 30A. 

A small caveat: we strongly discourage the use of soldering to directly connect a conductor or bus bar to the lead of prismatic and cylindrical cells when self-assembling Li-Ion batteries.

Li-ion battery cells are very sensitive to overheating. When soldering, a good heating of the parts to be joined is necessary for a reliable connection, which can destroy the cell immediately or after several charge and discharge cycles. In case of severe overheating, the cell may catch fire. It is recommended to use special equipment or contact a specialist.