I'm from the Netherlands, so I mean a Dutch style bicycle, as in:
Then I want to attach a 12 volt horn to it. I could just put it in the basket. It doesn't necessarily need to be charged while pedaling, or it can just be partly charged, but just that it runs out of power a bit slower. Then I'd like to attach a horn to it.
Would this be possible?
I don't see why you couldn't do this. It's just a matter of attaching a small generator to the bike, usually driven by the wheel, and then making sure you get the right voltage output. This guide looks to be a good place to start: USB Bike Generator
I think you're on the wrong track looking at batteries because they're heavy, stealable, fragile, and bad ecologically.
Firstly, you already have an airhorn installed in the middle of your face. Its steerable so it always sounds in the direction you're looking. Plus it needs no batteries, and best of all it plays a customised noise, from "OI!" to "LOOK OUT!" to "STOP!"
That was a serious suggestion, but if you want a horn consider an airhorn something like http://deltacycle.com/airzound-horn It refills from a bike pump and is lighter than a battery, plus no drag from a generator/alternator.
My own experience - I put a small 1.2AH 12V battery inside a disposable 600 ml bottle, and used a small car battery charger to charge it. I also put a car 12V-->USB adapter in the mouth of the bottle to run my camera and cellphone. This worked well, but loses a drink cage.
My electric bike has 3x 7AH SLA batteries. They are in a frame built from light angle aluminium and rivets. This replaces the main drink bottle cage and works fine, but probably overkill for your requirements.
Here's the setup I have on my commuter bike:
The motorcycle horn is pretty loud and I mostly use it to get the attention of drivers who are putting me in danger. I also have bright lights and cycle defensively, but sometimes you just need a loud horn to cut through a driver's oblivious daze. The battery and horn are pretty light weight and considering all of the other stuff I'm carrying to work, doesn't make much difference in pedaling effort.
The 1800 mAh battery is plenty for my commute (15-20 minutes each way) and will definitely last through several loud honks. Due to the rather obnoxious nature, I haven't tested exactly how long it will power the horn. The cctv batteries have a two nice features: separate power input and output so I don't have to unplug the horn to charge the battery, and an off switch so the battery doesn't run down in storage. I plug in the battery at night, same as my light batteries.
The momentary switch is a little red push-button model. I drilled a hole in an old plastic bike light mount to attach it to the handle bars. I can reach it pretty easily with my thumb when I need it. The wiring is pretty simple, just old speaker wire soldered in place with a little silicone caulking over it to keep the connections dry.
The charging bit is not easily done. Most bicycle dynamos / dynohubs produce 6 VAC (the actual voltage varies depending on speed; if they follow the German StVZO requirements, they produce at least 3 watts). The nominal 6 VAC is relatively easy to rectify to 5VDC for a USB charger, but in order to charge a 12VDC battery, you would need to rectify and step it up to 13.2 VDC. You'd most probably have to build the circuit yourself as I do not know of any off-the-shelf devices that will do this (there are IC chips that will do this, but you'd have to build the circuit yourself).
What you could do is to charge a 5VDC USB lithium-ion battery pack (using a variation of the USB Bike Generator circuit) and then use a 5VDC->12VDC step-up to power the horn (or to get a 5 or 6 VDC horn). It would be much smaller and lighter than a 12VDC lead-acid battery.
Alternately if for some reason you're set on lead-acid, you could get a 6VDC lead-acid battery. The charging voltage for these is 6.6 VDC so it'd still be a bit tough to get your dynohub to charge it safely and efficiently, but it's doable. There are as I said, 5VDC horns.
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