From cell to powerhouse: The journey of battery pack production

2024/08/08 –The battery pack makes or breaks the performance of modern electric cars. They need to store as much energy as possible in as little space as possible in order to deliver generous ranges. However, to ensure that the large high-voltage lithium-ion battery packs provide as much storage capacity as possible not only at the beginning of their life cycle, but over their entire service life, high-tech production is required in which the individual cells are assembled under strict quality requirements to form a ready-to-use traction battery. In the following article, DRIVEN describes how battery packs are manufactured at SVOLT across a complex production chain and what needs to be considered in the process. An animated video, linked at the end of the article, also explains the production process.

Two alternative manufacturing processes

There are two basic processes for manufacturing battery packs. Either the individual cells are bundled into modules in an intermediate step and then assembled into a larger pack (cell-to-module-to-pack) or the cells can be assembled directly into a battery pack without modules (cell-to-pack). Although the current trend is moving toward cell-to-pack because this process saves time, weight, space, and enables higher energy density, the standard is still the module-based construction method, which is explained in detail below using prismatic cells as an example.

From cell to battery modules

The module manufacturing occurs mainly in five steps, where individual battery cells are combined and connected either in series or parallel. The finished modules can have different sizes, weights, and shapes depending on the requirement.

Even before the first production step, the delivered battery cells undergo a thorough quality check: both open-circuit voltage (OCV) and direct current resistance (DCR) are measured. Then, the surface cleaning and adhesive application follow. After that, the cells are stacked in the desired arrangement and tensioned in a specific fixture.

For electrical connection, the module geometry is checked using a dimension check and the isolation between the cells is tested with high voltage. Then, the connectors (“bus bars”) are installed. These are welded to the cell contacts, the terminals, for electrical connection. Then, the welds are cleaned and checked, completing the module.

From the battery module to the finished battery pack

The assembly of several modules along with other electrical, mechanical, and thermal components into a battery pack or battery system takes place in four steps:

First, cooling plates are mounted at the bottom of a housing, followed by applying thermally conductive adhesive on top. This serves to fix the modules and create a thermal connection. Then, the battery modules are inserted, their position is checked and fixed. Next, the high-voltage connectors are mounted on the modules, and the battery control unit, which contains the control software, is installed and connected to the cell electronics via a cable harness. Finally, the battery housing is closed with the top part of the housing.

Leak test and final checks

After assembly, the housing and cooling system are tested for leaks using suitable test gas and leak detection equipment. Then, the battery pack is connected to an end-of-line test stand to update the battery management system (BMS) with the latest software, a process called “flashing.” The battery is undergoing various safety tests, such as a voltage test.

Throughout the entire assembly process, data is collected and stored for quality assurance. All processes are carried out under the highest technical cleanliness and compliance with all requirements to ensure a high-quality and safe battery system that can be sustainably recycled after many years.

After the final check, the battery pack is set to delivery charge state and packaged for transport to the vehicle manufacturer. There, the battery pack is "married" with the vehicle – for a long, sustainable life without harmful emissions for the climate and nature.