Ever wondered why your Petzl Stop gets so hot when abseiling? The amount of potential energy which it converts into heat as you descend is 127 kilojoules for a 72 kilogram caver abseiling 180 metres. If you are fatter than this, then its more. (use Energy=mass x g x height).
If you take six minutes to abseil this distance you will generate 353 joules per second, ie. 353 watts of power, more if you are less cautious at abseiling.
By a remarkable coincidence, the amount of energy needed to charge a FX2 is also about 127 kilojoules. (Charge at 700mA for 16 hours, with a terminal voltage approaching 3.2V) So instead of using a friction brake, you wind your rope round a pulley attached to a generator, and charge your cell as you fall. The principle is quite simple: if you use a dc servomotor as the generator, then it has a defined torque/speed and back-emf/speed behaviour, and it can be shown that the motor limits your speed to a certain terminal velocity without the need for any other brake. The speed is governed by the voltage of the cell you are charging, your mass and the diameter of the pulley. Unfortunately there are two problems. Firstly, if the cell became disconnected you would plummet; and secondly you usually need to charge a cell slowly, over 16 hours and not in 5 minutes.
However, you could use the power to drive a 350 Watt halogen lamp which would illuminate the pitch as you descended. This would be ideal for cave photography, and allow you to fully appreciate the austere grandeur of the cave.
By another remarkable coincidence, 127 kilojoules is the calorific energy in a small digestive biscuit. This means two things:
i) You need to eat a biscuit each time you climb a big pitch.
ii) A biscuit could replace an FX2 If Only We Knew How.