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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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    Easiest or cheapest - throw it all away an buy a new light is the easiest, especially if the lamp is not LED. – mattnz Mar 31 '16 at 19:32
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    Have you considered getting a decent dynamo? Particularly with bottle dynamos, there is a huge variation in quality. – sleske May 31 '16 at 14:32
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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.

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    If you're still running old-school incandescent lamps in your lights, replace them with LED based ones. The old fillaments use 90-95% of their power to create heat and only single-digits to make visible light. LEDs are brighter not because they use a tenth the power, but because they use almost all it of it for light instead of heat. Personally I'd just fit two LED battery-based lights front and another two on the rear, one each flashing and one steady. – Criggie Apr 1 '16 at 3:22
  • @Criggie added reference to LEDs to the answer – kiwi Apr 1 '16 at 8:18
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    I didn't downvote since you are answering your own question with a solution that works, but this is a pretty poor solution. You can get a modern LED light setup with a proper waterproof battery fairly cheaply these days. Even in a saddle bag the powerbank won't be waterproof, and may not supply enough current to power a good LED. – Andy P Apr 1 '16 at 11:38
  • @andyp its not a terrible solution - any USB battery will put out 500 mA minimum, and most will do about 1-1.2A at 5v for faster charging. Those USB batteries that can fast charge an ipad will be putting out 2.1 or 2.4 Amps at 5V. From memory, the Euro standard for incandescent bulbs is about a 6 Watt front and a 0.5W rear lamp, at 6V that would be ~1.1 Amp from the dynamo. So its about right electrically. – Criggie Apr 1 '16 at 22:38
  • @Criggie I was thinking more of the current requirements of a modern multi-LED head lamp - I have one for example that pulls 2A @ 7.4V, and is far from the most powerful on the market (although still overkill a lot of the time). – Andy P Apr 4 '16 at 8:23

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