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The String Cell from Tanktwo replaces a "dumb" battery cell with one with multiple pressure contacts on its skin and a computer to make it "smart." The cells shown here have 6 external contacts.

Pour your next EV battery?

While electric propulsion appears destined to supplant the internal combustion engine at some point in the future, the laws of economics and market forces will likely keep ICEs around for decades. Batteries for EVs remain expensive. Packs are heavy, and recharging takes longer than pumping liquid fuel into a tank. Despite their many advantages, EV disadvantages are hard to ignore.

Enter Tanktwo and a truly novel approach to the basics of EV batteries.

The basis of EV batteries today starts with a small component collectively known as a cell. Typical cells are shaped as cylinders or rectangular slabs. What makes them attractive to electrified vehicle battery makers is they are widely used in consumer electronics. Cells are combined into modules, modules into a highly engineered, static battery pack.

Tanktwo’s vision is a battery pack composed of egg-shaped String Cells. The Finland-based company’s founders exploited their background in telecommunications to create 'smart' cells that contain small, programmed computers. Poured (literally) into an inexpensive passive container with contacts on the inside, they create a String Battery.

Conventional batteries too big

One of the biggest benefits of the String Battery lies in a problem with current battery concepts—capacity rigidly designed from the outset, according to Bert Holtappels CEO of Tanktwo which has an office in New York.

“These systems are inefficient because they need to be over dimensioned with significant margin, so that the likelihood of field failure is within an acceptable margin,” he said. Some EV makers are delivering cars with 100 kW·h battery packs when their owners might drive only 12,000 mi (19,311 km) per year, meaning that "most of the depreciation of the battery is coming from aging, not wear”—a costly loss of an asset, Holtappels explained.

Sizing the Tanktwo String Battery container for the maximum scenario requires filling only as many cells as needed for a typical usage, plus a healthy margin. The container would recharge the cells through the normal SAE J1772 plug as today. “This prevents underutilization of the pack,” he explained. “During the period he owns the vehicle, the same customer could visit the dealer and get an upgrade to the battery pack quite easily.”

The simplest version of the swapping device resembles a glorified Shop Vac. Holtappels recognizes that the more ambitious goal of convenient and fast battery swapping requires an infrastructure change that could inhibit adoption—think hydrogen fuel cell vehicles. “That might come naturally over time,” he speculated.

Efficient by design

There are some other potential efficiencies of the String Cell approach. Current battery packs need to level the charge between cells in each module. So, if one cell has deteriorated by, say 20%, the rest of the cells in the module must bleed off energy to balance the cells within the module. So, each module is limited by the deterioration of any individual cell.

This also causes traditional packs to need large safety margins. Often, this means a battery designed to maintain an 8 kW·h capacity over the life of the car might start out with 14 or 16 kW·h.

Because String Cells are ‘smart’, the pack is not limited by the least-charged cell. As Holtappels describes it, if an individual cell deteriorates by 20%, the cell is simply bypassed for 20% of the time when the pack is contributing power. Each cell contributes to the pack to the best of its ability without any need to bleed off charge.

He estimates this as providing 10% more energy efficiency when compared to traditional pack designs with the same rated energy.

“Our packs can last much longer on a practical level as well," Holtappels claimed. "Most companies [think they should retire their] pack from the vehicle even when it is still 70% usable. For String Cells, anything over 10% of initial capacity is OK for each cell.”

Dead or semi-dead cells that would be ruinous in a traditional pack can be bypassed in a String Battery, and easily replaced at a service or swap event, the company claims. So the packs are sized for the mean, not the worst case. The containers they go into are also fairly simple. Important components, such as bus communications and wire harnesses are already commoditized. Because of the unique character of the String Cells, there is flexibility in the shape of the container.

“This technique is agnostic about the battery cell chemistry: any improvement in the chemistry of cells would also improve the String Battery,” he said.

According to Holtappels, a couple of industry partners are already working with Tanktwo on initial deployments, including Firefly with an electric service vehicle. In these cases, the rapid battery replacement—3 minutes according to the company—was an important factor.

"This is definitely an out of the box idea, but it seems quite challenging," observed Dr. Menahem Anderman of Total Battery Consulting‘. Interviewed by the author for this article, Dr. Anderman would like to see future trials prove out the quality of the 'dynamic' electrical contacts between the cells, and examine complexity, cost and reliability in real life.

"I believe it will be quite a stretch to make a real viable system out of the concept," he said. There is no doubt the Tanktwo principals are just as aware of the challenges. Stay tuned.

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