I have a bike using a bottle dynamo. It works ok but I just use the light when it's absolutely needed because the dynamo really slows down the bike.
I would like to make the bike run with batteries, what is the easiest way?
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I solved this by attaching a USB connector to the existing light circuit, and power everything with a cheap cellphone power bank. You just need a USB (possibly broken) cable whose "computer" end is working, and a place to hold the power bank.
A saddle bag is a perfect place for it. The front basket can also do. Make sure the USB plug doesn't get too much water, it may get rust. For example leave it inserted in the saddle bag.
Assuming you have basic technical knowledge of electronics, it's a very easy process. You can attach the cable at the old location of the dynamo, or in parallel of one of the lights. Just keep in mind that usually on the bike the "ground" is the frame, and the "live" is the only cable. USB side, black is the minus and red is the plus.
Polarity doesn't matter with bulbs, but it may matter with some led lights. If it doesn't work, reverse the polarity.
LED lights are brighter and will last for much longer. There are very cheap replacement LEDs using the bulb form factor.
The light could be a bit weaker due to running at 5 Volts instead of the usual 6 that you get from a bottle dynamo, but most of the time it will be much stronger as it doesn't change with speed.