I'm from the Netherlands, so I mean a Dutch style bicycle, as in:

enter image description here

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?

  • Sounds like a lot of effort just to get a horn. Most 10 Volt batteries are going to weigh around 10 pounds, possibly more. There's lots of options for loud bicycle horns that don't require carrying around such a large battery.
    – Kibbee
    Aug 21, 2015 at 19:26
  • 2
    What are you going to use for a generator? Aug 21, 2015 at 21:53
  • If a battery is not secured to the bicycle, what is there to take a charge? A capacitor bank?
    – Kaz
    Sep 18, 2016 at 4:41
  • A horn is only used for a second or two at a time, so it doesn't actually use that much power. In my experience with an 8 Amp horn, the batteries showed barely any voltage drop after a week of usage. A 27 WH battery will give you 97200 watt seconds worth of power so it will last a very long time. Aug 22, 2019 at 5:04

4 Answers 4


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

  • Thanks for your comment. What kind of accu/battery would you recommend?
    – stenlan
    Aug 20, 2015 at 17:07
  • Link-only answers are not good, because when the link breaks they become useless. Can you please edit your comment to answer the question.
    – Móż
    Sep 16, 2016 at 21:46

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:

  • 12v motorcycle horn
  • 12v cctv backup battery (1800 mAh lithium, ~150 grams)
  • momentary switch mounted on the handlebar with an old bike light clamp
  • 5.5mm power plug and a couple feet of wire

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 problem with horns is that it takes conscious effort to use, and in that very-compressed time you're better served by taking avoiding action instead of making a noise and then expecting someone else to take action. The hand on the horn button is also going to brake slower/worse. My horn is my voice - it works just as well, never needs charging, and always functions.
    – Criggie
    Sep 16, 2016 at 22:08
  • 1
    @Criggie If it's a true emergency you can brake first and then honk later to express your displeasure! Aug 22, 2019 at 6:06
  • @Benjamin since that posting I've ended up with a loud airzound air horn on each bike. Its less useful than I expected, only of value if I as the rider can see it all about to go wrong with enough lead time.
    – Criggie
    Aug 22, 2019 at 9:44

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.


  • most hub dynamos will actually generate considerably more than 6VAC unless there's a voltage limter in the dyno. Mine will happily run a 10V/500mA white LED, for example. I'm not confident they'd go to 13.5V to charge a 12V SLA though.
    – Móż
    Aug 24, 2015 at 1:01
  • Note that LEDs will light at lower than their nominal voltage. It's also AC so the peak voltage is higher than the RMS voltage and you're losing most of the energy if you only take the highest peaks. And yes, there are fluctuations in the peaks (most are current limited, not voltage limited), etc. etc. In other words, trying to charge a 12VDC battery from a 6VAC source is going to be an exercise in frustration.
    – RoboKaren
    Aug 24, 2015 at 1:23

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