Because batteries do wear out – and you need to keep going and going and going.
That is not to say that they are perfect, they still have their issues. There is after all no such thing as free energy. The power in the lights will be put there by your leags, and the brighter you need them the more tired your legs are going to be.
A dynamo bike light is really just a mini-turbine and it works in exactly the same way that a windlill does. It involves magnets spinning around inside coils of wire. The faster they spin the more energy is converted to electrical energy.
The negative asoects of a bike dynamo are:
- They don’t work when stationary
- They make your tired, jelly legs even more fatigued.
- Slow speed = dim lights
Dynamo lights are entirely unsuitable for use on mountian bikes where slow speed negotitations are all part of the fun – and it’s not nice to be peering into the gloom as you carefully pick your way up and over rocks.
Times move on and the technology improves – but the general pricipales will always remain the same. Your energy becomes electrical and then light energy. It is the advent of hi-qulaity, low-energy LED bike lights that makes it all work as bit more efficiently.