You’ve spent a beautiful day hiking up and down mountain trails, and as you stop to set up camp for the night, you take off your backpack and plug your smartphone into it to recharge. Or you can use that electricity to power wearables or other health tech devices.

That’s the goal of a team from Georgia Tech and Tsinghua University in Beijing who have developed an energy-harvesting backpack. They created a box for the payload that rides on vertical rails attached to the backpack frame, suspended by flexible plastic. The weight slides up and down in response to the shifting center of gravity of the wearer, causing the plastic to stretch and contract. This vertical motion is converted into electricity using triboelectric nanogenerators in the rails. The system produces enough power to light an array of LED lights.

Now, you may be thinking of TANSTAAFL at this point: There Ain’t No Such Thing As A Free Lunch. This must be cannibalizing some extra energy from the wearer to produce this power. According to the researchers, it’s exactly the opposite. By letting the weight’s inertia to keep it steadier as the walker’s body moves up and down, the system actually reduces the amount of effort require to carry the pack. The researchers found that it can reduce the vertical force on the wearer by more than 20%.

So if you’re healthy enough to take a stroll, you may be able to power your electronics while the backpack lightens your load.