Most of the quality lighting your going youre find out there is going to be designed around 12v systems since thats what cars and motorcycles use. Remember that instead of messing around with stepping voltage down to 12v from your original battery pack, you can always just add an extra battery pack of 3 or 4 18650 batteries in series and use this as your power source for lights/horn etc. I found that the Bird brain gps box that many scooters have mounted on top of the stem worked real well for this purpose. I glued two of those boxes together on my red scooter to make a big 12v battery box for all my accessories. It worked out well for me, looks good, is super easy to add new 12v accessories, and I can replace the batteries when they get weak. Best of all, it doesnt draw power from my scooter battery and decrease my range. Or you could even stuff the batteries in the neck and just stack em on top of each other in a small tube or something. You can buy a pre-made battery holder or just solder wire to the batteries to make your pack. Just dont get em too hot if you solder. You need to work quick.
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https://www.amazon.com/Battery-Holder-S ... way&sr=8-2