How Whoop landed a fancy new battery for its fitness trackers

Thanks to Sila's energy-dense battery, Whoop shrunk its device 33% while maintaining battery life.
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6 min read

For you and I, battery selection is a matter of AA, AAA—maybe C or D here and there. For a wearables startup with a valuation of $3.6 billion and looking to rival Fitbit, the process is a little more complex.

In September, Whoop became the first company to put next-gen battery-maker Sila Nanotechnologies’ new silicon-based battery in a commercial product. Sila’s energy-dense battery gave Whoop the ability to make a 33% smaller tracker with a host of new features, all without sacrificing battery power from the previous model.

  • The battery will go in the Whoop 4.0 fitness tracker, which it uses in its wristbands as well as in Whoop Body, its line of body apparel, like boxers, bralettes, leggings, and performance tops, which include pockets to hold the tracker.
  • The Sila-enabled devices started shipping in late September.

For Sila, which has raised over $880 million since its founding in 2011, this is the commercial start of (literally) bigger battery plans: Sila has joint ventures with BMW and Daimler, and it’s shooting to have its batteries in a commercially available EV by 2025.

We talked with Whoop cofounder and CTO John Capodilupo about how the partnership with Sila Technologies came to be.

This interview has been lightly edited and condensed for clarity.

How did the partnership with Sila begin?

We reached out to Sila Nano, and it was perfect timing for both of us. We were looking for the next-generation battery. Sila had been developing their new technology for quite a few years, but were ready to go to mass production. About two and a half years ago, we reached out to them for partnership, and said, “Let’s do it.”

Why is Sila’s battery such a big deal for Whoop?

Batteries are by far the largest component by volume inside of any wearable. Really there hasn’t been too much advancement, maybe 2% to 3% extra energy every year. For us, we wanted to make the Whoop 4.0 smaller, so that it accommodates more wrist sizes, looks sleeker, but also enables things like Whoop Body, making it easier to wear throughout the body. We knew a major constraint was finding the power to run all the things.

What incentive did Sila have to start out with a wearable?

Sila actually has quite a few Whoop users already. They enjoy the product, and I think they really saw the potential and the growth of Whoop as strong signals for them to get involved. From a business standpoint, I think they were excited.

Then also it was the sizing and staging of both companies. Sila has plans and ambitions to go into electric vehicles and really put batteries everywhere. But those batteries are so much larger than the batteries inside of Whoop, that it’s just not feasible for them to output on day one the amount of secret-sauce material that they make into those huge batteries. They wanted to get their product in the world, show it worked, and things like that.

Did you look at other companies besides Sila when you were searching for the next battery maker for the 4.0?

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We did evaluate quite a few other partners, including traditional graphite cells who may be doing more advanced things, but it was for graphite. [Sila] had the perfect combination of the energy-density increase we were looking for, cost-benefit analysis, and also the people.They’re a large company, but still a startup, just like Whoop, and there was just a really strong bond between us that we knew would serve each party.

What separates Sila’s approach from other battery makers?

A lot of other next-generation battery-technology developers are making their own factories to produce their own batteries. Sila is very unique in this regard that their chemistry is actually, you can think of it as like a drop-in for traditional chemistry. They don’t produce the entire battery cell.

We could still work with our tier-one battery manufacturers that we’ve worked with in the past, but instead have Sila’s secret sauce inside of it to give us the extra energy density.

When you’re evaluating something as critical as a battery for both performance and a safety standpoint, taking a risk on a next-generation technology is one thing, but taking a risk on a next-generation technology, plus the next-generation manufacturing line from an unproven company producing batteries is a big jump to make. I really think Sila has the correct approach in this regard. We were able to work with both Sila and our trusted battery manufacturer to produce the cells that are inside of that 4.0.

How has the new battery allowed the 4.0 tracker to be different from previous models?

First and foremost, it enabled us to reduce the form factor. Whoop 4.0 is 33% smaller than the 3.0, but it doesn’t sacrifice battery life. To do that was 1) a feat of electrical engineering and embedded engineering, to reduce our base power consumption, but 2) just having more juice in the battery, basically—and that’s where Sila came in.

But it wasn’t just the shrinkage. We actually added a whole bunch of new things to the 4.0, compared to the 3.0. We added skin temperature. We added a haptic alarm, so it actually vibrates, and moving mechanical parts take up a lot of energy. We’ve put in a lot more power hungry parts and new sensors inside of a smaller form factor. We’re using all that extra energy density to help us accomplish that without sacrificing battery life.

Has working with Sila’s new tech affected your manufacturing process or supply chain at all?

They work with the battery supplier themselves. It was kind of nice actually, from a supply-chain perspective. That’s another reason we went with them: Sila has a relationship with the manufacturer that makes our cells, and they kind of handle that for us. It was, “Hey, we want these cells with the Sila material inside of it.” It’s the same as if we were buying a normal cell from that manufacturer. That was really nice from our perspective. We don’t have to budget out forecasts independently over materials and different things like that. It’s just one source for us.

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