As wearable all-day activity trackers compete to be smaller, more convenient, and more comfortable, there's one problem many of the newer entrants are running into: battery life. If users are supposed to be wearing a tracker all the time, when do they plug it in? And the point when a user takes off a wearable device to plug it into the wall is the point when they're most likely to lose interest or abandon it.

"The battery thing is really a big deal," Misfit Wearables CEO Sonny Vu told MobiHealthNews in a recent interview. His company's Shine tracker is expected to launch later this summer, and he says they're prioritizing battery life. "I'm not sure we're even within a generation of science to solve some of the power issues. ... I think we should be shooting for 2 to 3 years of battery life, and it's about being as miserly as possible with your power use to make it a great experience."

BodyMedia CEO Christine Robins agreed. Her company's Core armband is on the high end of the spectrum, with an hour of charge providing 7 to 10 days of use.

"Some of the other products take some charging or they don't hold a battery quite as long," she said. "It's a challenge in the market especially as you get into Bluetooth, Bluetooth LE. There's a lot of power consumption there."

But the solution, or a solution, might be no farther away than a quick scroll down the Kickstarter main page from the latest crowdfunded activity tracker. Pittsburgh-based startup SolePower is crowdfunding a removable insole that aims to charge electronics via the kinetic energy of the user's everyday walking. The company's goal is to fully charge an iPhone in 2.5 miles of walking, the average movement a person does in a day. And for the wrist, which is shaping up to be some of the hottest real estate for wearable trackers, watch maker Seiko already offers a self-charging watch called Kinetic that operates on a similar principle to SolePower.

Activity tracker customers are almost by definition moving more than average, so this technology seems like a natural fit, no pun intended. Conceivably, a shoe- or wrist-worn tracker could be designed that would never need charging, as long as the user kept their activity up.

In a crowded field where everyone is looking for an edge over the competition, it seems like it will only be a matter of time before we see an activity-powered activity tracker.