Berlin is way far North, which means it gets very dark very early in the winter. Thus, anyone riding a bike must have lights for his/her bike. A nice old lady made this perfectly clear to me a few weeks ago when she screamed at me "Mann muss Lichter haben!!!" Ach so, alles klar...
The thing is, I had a light on my bike. It just happened that at the time it decided to stop working. This is because my bike has a dynamo on it. For those of you not in the know, a dynamo is a little generator that is powered by running along your tire. The idea seems nice; with a dynamo, you are self sufficient and not dependent upon batteries that will run out, so you should have a light that always works. But guess what? Dynamos are a pain in the ass. Let me explain.
Dynamos essentially require degrees in mechanical engineering and electrical engineering in order to be properly maintained. Mechanically they must be in exactly the right position against the tire so that it is optimally turning as the tire turns. Otherwise it doesn't turn and instead rubs a hole in the side of your tire. Also, your tire pressure has to be exactly right. And then the dynamo can't be too dirty, otherwise it won't turn properly. Which is so easy to do on the ultra-clean streets of Berlin.
Then these things require a wire from the dynamo to the back and front lights. To complete the circuit, electricity then runs through the frame of the bike itself! So, here you have an electrical circuit running over the whole of your bike through a flimsy wire and the bike frame exposed to the elements. Sure, nothing could go wrong there!
If it's working, great! You get a nice bright light (except only when you're peddling). But if it's not, then how do you figure out what's wrong? Is it the light bulb? Is the wire broken someplace? Is the dynamo working properly and making good contact with the tire? Has the rain shorted out the bike? At the time when the nice old lady was informing me that my light wasn't working, it would work (temporarily) if I bounced up and down on my bike. So what the heck could be wrong to explain that?
Guess what? I don't want to find out. A light with a battery is just much easier to figure out, and it's not distributed over the whole bike. And LED technology has made batteries go for much, much longer without compromising brightness. I've noticed that nearly all the old bikes in Berlin have dynamos, but that many of the newer bikes have battery-powered lights. So even the slow-to-adopt-new-technology Germans are catching on.
Do yourself a favor: ditch the dynamo!