Friday, July 5, 2013

Walking your way to a full battery

Chris Gammell, of Chris Gammell's Analog Life and The Amp Hourtweeted about this gizmo. Not surprisingly, it set off his bullshit sniffer, and mine too. So, let's do a tiny little bit of physics and find out if their claim to a fully charged iPhone battery in 2.5 miles is legit.

First, the assumptions:
1. My mass is 75kg. I wish, but that's close enough.
2. My stride length is about 6 feet, so 3 feet per step. A little long, but probably close enough. That gives me about 4400 steps in that 2.5 mile walk.
3. I lift my foot about 10mm with each step.
4. An iPhone battery has about 1500mAh of battery life, which, for an average LiPo battery voltage of 3.7V, translates to 5.5wh, or approximately 20kj of energy required.

For a standard gravity, 10mm step height, 75kg person, and W = f x d, we get 7.35j per step. 4400 steps gives us 32kj total generated; we need to capture 20kj of that to charge the phone. That's 62.5% of the energy generated.

That strikes me as a high, but not *totally* unattainable goal.

Call this one "I'll believe it when I see it", which is better than "flim flam and snake oil".