Just like the tiny lithium-ion battery packs which power every modern day consumer electronics device from portable games machines to mobile telephones, laptop computers and tablet PCs, the lithium-ion battery packs in electric cars have to endure hundreds of charge and discharge cycles, are heavily abused by their owners, and asked to operate in all kinds of temperatures and conditions.
As a consequence, lithium-ion battery packs, the modules which make those packs up and the cells which make up the modules, are created in the kind of environment you’re probably more used to seeing in a science fiction movie. A world where silent, fully-protected boffins in clean room suits glide between workstations, talk in hushed tones, and take continual measurements.
This week, we were lucky enough to be among the first journalists in the world to enter into the inner sanctum of Nissan’s lithium-ion battery production facility in Sunderland, England. The facility, which makes battery modules for the Nissan LEAF electric car and Nissan e-NV200 electric van, takes raw materials in at one end of the factory and spits out fully-assembled battery modules and battery packs at the other.
On Tuesday, we explained the careful steps Nissan took to build its battery factory and the high level of cleanliness demanded of anyone who enters into it. Yesterday, we followed the raw materials as they were processed and turned into battery cells for the first time, leaving the inner sanctum of the clean room for the rest of the battery ageing area.
Today, we’re picking up the story as the sealed, aged cells head on through the plant, meeting up with other cells to become battery modules and then with other battery modules to become fully-assembled packs.
Coming of Age
As we explained yesterday, each newly-filled and sealed battery cell must age in a special part of Nissan’s facility in order to allow the electrolyte and electrodes to interact for the first time. The length of time for this, says Nissan, has been chosen specifically for its particular cell chemistry and specifications, and while not every battery manufacturer does this, Nissan believes it helps ensure a long, unstressed life for each cell.
On leaving the ageing room, the each cell is tested by a specialist automated machine which examines both the cells external dimensions but also its electrical connections. Like the test applied earlier in the cell’s life, this consists of a high-voltage, low current pulse sent through the cell to check for short-circuits and other faults.
Once passed by this machine, cells are passed on a conveyor belt to an inspection station, where a highly-trained worker has approximately three seconds to visually inspect each and every cell for visible defects.
Nissan says the reason for using a human worker at this point is simple: while computers are good at spotting particular problems, they find it far harder to identify more general defects. A human eye meanwhile, is far better at spotting bulges or inconsistencies which shouldn’t’ be there, and Nissan says this final level of cell quality control is essential in order to maintain quality later on down the line.
The Power of Two
With cells electronically, electrically and physically inspected — and any faulty units already rejected — the completed individual battery cells pass through a conveyor belt system into a laser-cutting unit, where each cell tab is laser-cut to the correct shape.
From there, the cells move into a special machine where bus bars — the electrical interconnects between each cell — are ultrasonically welded onto each cell. Although the cells are no-longer in clean-room conditions, Nissan takes great care to ensure there’s no human contact between the bus bars and the cell tabs. Even the slightest contact with human hands could leave grease on the bus bars or tabs which would ultimately corrode the connections, leading to a poor electrical connection between the cells. As a consequence, workers who handle this part of the process still wear latex gloves to ensure the cells are protected from human contamination.
With tabs welded on and macro-melt injected along the outside of each cell pouch to provide additional rigidity to each cell, the cells are then sent to a stacking machine. Here, two cells are first stacked on top of one another to produce a ‘2 stack,’ and two ‘2-stack’ cells are stacked together to produce a ‘4-stack.’ Throughout this stacking process — which is completely automated — super-fine measurements are taken to ensure that the cells are carefully and accurately stacked on top of one another, each cell is suitably ‘matched’ to the other cells in the stack, and all mechanical and electrical connections are carefully made.
With the battery cells now stacked in groups of four, each stack is transplanted into a battery module case. Consisting of an aluminium bottom and top case, each 4-stack is glued into its own battery module case, sealed, and sent on its way to join other battery modules in a battery pack.
From here, each module is given a final electrical check and visually inspected to ensure there’s no electrolyte leak. As with earlier points in the process, any modules which fail this test are sent back for thorough strip-down and analysis.
It’s at this point in the process where battery modules are sent different ways. While each Nissan LEAF battery pack and each Nissan e-NV200 battery pack contain the same number of cells and modules, the two packs are a slightly different shape from one another.
Because of this, some of the modules leave the production line at this point and are boxed up ready for shipment to Spain, where Nissan produces its e-NV200 electric van. In order to keep shipping costs down, Nissan complete final battery pack assembly in Spain for the e-NV200, with battery modules arriving from Sunderland in crates ready for appropriate stacking and assembly into an e-NV200 battery pack.
Those modules destined for LEAF battery packs however, carry on through the production line for stacking and pack production. And wherever the battery modules head, the huge amount of digital data amassed on them — detailing the exact day and time each module was created, the individual cells in each module, the results of each and every test the modules and cells went through and the rolls from which those cells were made — follows.
With modules now sealed and assembled, the plant splits into three separate assembly lines, each responsible for assembling the three strings of modules which make up the LEAF’s battery pack. The two assembly lines responsible for the front most strings — which live under the front seat and floor respectively — are assembled and lifted into position in the battery pack frame by hand. The rear most string — which is much larger and heavier and fits under the rear seat — is lifted into place by a robot.
With the modules laid in the bottom of the battery pack casing, a team of workers then gradually tighten up the electrical and mechanical connections which hold the battery pack together, torquing each bolt to the required tension as detailed on overhead monitors.
At this point in the production line, workers have to wear insulating gloves to protect them from electrical shocks. In fact, this is the first point in the battery production process where workers are protected from the battery packs and not vice-versa.
Once all the mechanical and electrical inter-module connections have been made, the battery pack is considered live, and the battery management module is installed along with the master disconnect switch.
Then and only then, can the top of the battery pack case be installed and sealed, and the battery pack as a whole can communicate with the outside world.
With the battery pack sealed the whole unit is checked for air and water-tightness using compressed air, and the pack is sent down the production line for its first charge.
While the entire pack sits at around a 5 percent charge throughout final assembly, it is then rapid-charged to 70 percent full using equipment similar to Nissan’s CHAdeMO DC quick charging station. Instead of an external CHAdeMO connector of course, the power is fed directly into the battery using the battery pack’s external DC power connector.
From there, the completed battery packs are given one final electrical check and sent over to the trim and chassis line, where they will be united with a vehicle on the production line.
Other packs are sent as required to dealerships for battery exchange and replacement, although Nissan told us on Monday that so far, only three battery packs have been replaced in the UK out of many thousands of vehicles. While Nissan declined to detail what these replacements were for, it did say none were for battery capacity loss.
At the start of our series, we called the lithium-ion battery cells in an electric car battery pack under-appreciated. And after seeing how much it takes to build an electric car battery pack, we think you’ll agree.
Nissan provided train fare and hospitality to enable Transport Evolved to bring you this first-person report. Due to restrictions on bringing external ‘dirty’ equipment into the clean room area, all photographs in this article were taken by official Nissan photographers.
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