Tanktwo says that its smart swappable cells will save big on battery pack costs

TankTwo string Cell cutout

Tanktwo’s smart cells represent much more than another concept for battery swapping. The system is based on a tank full of “string cells,” which contain lithium-ion battery materials as well as electronics for sensing, routing, and communications.

Understanding Tanktwo’s system requires throwing out any preconceptions of how battery packs are supposed to work. Tanktwo CEO Bert Holtappels told Charged that the simplest way to get the point across is to emphasize the upfront savings an EV manufacturer can pass on to its consumers by using intelligent string cells. “What will really make this thing work is the business model for our industrial partners.”

Traditional battery packs are designed with a lot of extra capacity that is not accessible while driving, in order to allow for degradation and to protect cells from safety hazards and premature wear. Tanktwo claims it can reduce that extra capacity significantly.

“In a traditional pack, module or cell variance is the enemy, to be avoided at all costs,” Holtappels explained. “Tanktwo string batteries, however, work efficiently even if cell performance varies widely. When higher variance can be tolerated while retaining most of the efficiency, the state-of-charge (SOC) limits can be widened.”

“Not only can the spread be wider, but a significantly deteriorated cell, like one that has lost 70% of its design capacity, can still contribute to pack capacity. This means that string cells, even when employing identical battery chemistry, have a longer useful life or can be pushed harder, hence a pack can be smaller.”

TankTwo full tank

“Every string cell contributes to the best of its ability at all times,” said Holtappels. “Old and new string cells, cells of different capacities – and even chemistries, in theory – can be mixed without limitations. And we make capacity restoration very easy. So instead of over-dimensioning to prevent warranty liability, you can restore capacity in a few minutes only if and when it happens. We make a shop-vac-like analyzer that will remove string cells that do not meet certain criteria. Replacing the lowest performers with new cells is done in minutes, restores pack performance to the required level, and costs a fraction of a total pack.”

Doesn’t adding intelligence and switching capabilities to every cell increase pack overhead and cost? Holtappels explained that the potential savings outweigh additional costs. “The numbers work out in our favor for a few reasons. The components added to make cells intelligent cost only cents per cell in volume, as the CPU and memory footprints are very modest by today’s smartphone standards. But the immediate efficiency gains I described are on the order of tens of percent. So from day one our customers win.”


The full interview with Tanktwo’s CEO is available in Charged – Issue 21.

  • dogphlap dogphlap

    I followed the link but I still don’t get it. As far as I can tell these cells rely solely on the contact of one cell on another to form chain(s) of series connected cells with a final terminal voltage of anywhere between 100V to 600V. How many chains form within a tank and if more than one how is this handled (just saying the cells are smart is not a real answer) and how is the connection to the two (or more ?) chain end points found and made ? An EV battery supplies current measured in the hundreds of amps, will these inter-cell connections be able to pass that level of current without arcing and or going high resistance ?

    If someone came to me with this concept I’d dismiss it without some seriously, convincing information on how the obvious problems had been overcome and yet TankTwo’s CEO appears very confident and seems to be getting little in the way of wary scepticism in the pieces I have read so far. The more I think about it the less credible this seems.

    • Bert Holtappels

      Skepticism is good – it is the only way to true progress. I’m in a good position to answer your questions, as I happen to hold the patents for the system. To address your main questions:

      1) many strings are made in parallel, and many small DC-DC converters boost varying string voltages to a common bus voltage. The converters operate in constant source or constant load mode, depending on certain factors. The bus voltage is set by the customer.
      2) shorting is not an issue, the routing algorithms take this into account. Although perhaps not intuitive, there are many, many options for short-free routes, even with 6 contacts per cell, while still maintaining a very high utilization rate. This can exhaustively be simulated for any tank size and shape.
      3) Surface contact resistance is sufficiently low, because of the fact that the ellipsoid shape of the cell allows for minimal degrees of freedom and physical force is applied onto the cells.
      4) several detailed patents can be read from going to espacenet, and search for Tanktwo.
      5) Why don’t you give me a call at +1-212-321-0630 if you have any specific questions or concerns?
      6) If you are a potential customer, demo units are available.

      Kind regards,
      Bert Holtappels

      • dogphlap dogphlap

        Thank you for replying to my comment.
        While I don’t fully understand how you have made this work I’m aware that many many people are much smarter than me so if you say you have got this to work I’m happy for you and for TankTwo. I see from one of the patent sketches on espacenet that contacts for charge and load are arranged on the inside of the tank, which answers one of my questions though just how this works when the battery units are randomly arranged in the tank is beyond my present understanding. You are in a much better position than me to know if this novel arrangement makes economic sense, either way I wish you luck.
        I purchased a Tesla Model S 70D a couple of months back and that has taken all the money I currently wish to spend on EVs so unlikely I’ll be a customer of yours in the near future. Best regards.

      • Gaskilla

        Have you provided Tesla Motors with a sample of your product? They are always looking for new battery technology and they would be a huge customer. I’d be interested to hear what they would say about your product.

        Get a free Subzero Weather Package or $1000 off your Tesla Model S order with this link http://ts.la/tom9993

    • Clint Steele

      They are good questions:
      * It seems to me that the chains are found not by the cells but by the control system external from the cells.
      * I think the current issue is resolved by the fact that the current will be shared across the random strings so no one cell will carry that amperage.
      * I would guess that each cell has enough intelligence to tell if it is in a closed loop and can reverse polarity as needed.
      * I reckon good cell design can provide good contact to stop arcing.

      • dogphlap dogphlap

        Thanks for the comment Mr Steele.
        I’m assuming the external control system would need some kind of a 3d map to determine the best arrangement for interconnecting the individual battery units (which I referred to as cells) but I concede that to make such a smart controller work is beyond my abilities. Mr. Holtappels seems confident. Maybe there is a simple algorithm that could do that but it eludes me.

  • OpenMind

    How do they stop the cells rattling awy causing poor connectivity between the cells and anoying the driver?

  • Paul Zigouras

    I just don’t understand how you would prevent arcing during a full-load disconnect (such as when going over a bumpy road). Would the voltage and current between each cell be the same as the entire pack?

  • Electric Bill

    I could be 100% convinced these little footballs are nothing but unicorns and sasquatches, but all it would take to make this the new standard would be to put dozens of EVs on the potholed streets of L.A. and see just how long they last, and whether any of them eventually go critical. The longer they last, the more converts you’ll have. Have at it, Tank Two.

  • John_B

    The wasted space in-between the cells, the expense, weight and wasted space of electronic gadgetry inside and the contact coating around each cell…. Hey! I know..What about instant swappable saltwater electrolyte? Gallons of salt water would be a little cheaper than each individual one of these things right?. Imagine of they had that? OH! they do… for years now. A whole car is built and being tested: Google Quantino. We still have to try to re-invent the wheel till it’s made out of lithium though. signed… Coal strip mining about to lose it’s jobs.