ALAMEDA, Calif. — The new Whoop fitness tracker straps around the wrist, a lot like any other health monitor or smartwatch. But you can also buy a sports bra or leggings equipped with this tiny device, which can be a sliver of electronics stitched into the fabric of clothes.Squeezing a fitness tracker into such a svelte package was no small feat, said John Capodilupo, Whoop’s chief technology officer. It required a whole new kind of battery. The battery, built by a California start-up, Sila, provided the tiny fitness tracker with more power than older batteries while maintaining the same …
