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I recently purchased a classic motorcycle, it's current factory electrics are designed to run at 6 volts.

I do have plans to change it later to 12 volts but due to regulations in my country and to have the paperwork first in order I will have to prove that the engine fires and that the various electrics work (lights, horn, etc...).

I did not want to purchase a separate 6 volt battery as I have 12 volt batteries lying around so I decided to purchase a buck adapter - specifically this one.

Basically my question is if I do manage to start the motorcycle then is it okay if the dynamo/generator will also start outputting current? As I understand then the dynamo would be outputting around 7+ volts.

EDIT: I do not plan on charging the 12 volt battery. It's more of a temporary solution to show to the local department of motor vehicles to set the paperwork in order to register the motorcycle to my name and to get new papers for the motorcycle. What I was meaning by the question is if the dynamo starts outputting current then will the buck converter be safe (not fry/burn out).

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    and how would you charge a 12V battery with 7V? You'd need a bidirectional conversion between these two voltages. That's expensive to build, and hard to build right. 6V batteries are cheap. – Marcus Müller Jun 22 at 20:43
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    Chances are that DCDC converter is by far not powerful to power the things on your bike. It can output 6 V, 5 A so that is 30 W. Chances are that the headlight and horn require much more than 30 W. As Marcus rightly points out, your 12 V battery will not be charged so you will be running it dry and you might have to re-charge it every time you drive the bike. It's a classic bike designed for 6 V, it's best to leave it at 6 V. – Bimpelrekkie Jun 22 at 21:00
  • @Bimpelrekkie You are likely right - I forgot how much current old incandescent bulbs can draw. – MasterKriz Jun 22 at 21:21
  • ...not to mention the current which a starter can draw. – Nick Alexeev Jun 22 at 21:52
  • @NickAlexeev it's kick-start only :) – MasterWaster Jun 22 at 21:54
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Chances are that DCDC converter is by far not powerful to power the things on your bike. It can output 6 V, 5 A so that is 30 W. Chances are that the headlight and horn require much more than 30 W. As Marcus rightly points out, your 12 V battery will not be charged so you will be running it dry and you might have to re-charge it every time you drive the bike. It's a classic bike designed for 6 V, it's best to leave it at 6 V. – Bimpelrekkie

I'm going to mark that one as the answer and just have to source a 6 volt battery. Thanks to all who answered.

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