rhe actual amount of energy produced by those devices is actually pretty small. and Li batteries require a constant current to charge, which you won't get with these devices. if you've ever used one of the motion activated LED flashlights, you'll know it takes some very vigorous motion to create a usable anount of energy. and LED flashlights only use a few tens of milliwatts of power and cell phone power usage is around a watt or so during a call.
I'd bet the way they "spring loaded" it , and a bigger coil, would produce some good results, we are only talking about cell charging here,, take a look at those solar robots, they charge energy slow then switch on to burst it out,
The problem with having a kinetic charger in your rucksack is that you don't move up and down much when walking. The body naturally conserves energy by keeping you body fairly level as you walk.
A better place to detect significant motion would be at your knee or ankle.