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The bottle contains both the battery pack and the regulator board. It provides a safe environment for the batteries and regulator without weighing too much.

The regulator board sits on a layer of bubble wrap. The battery sits on top of another layer of bubble wrap on top of this. The battery is at the top, so it can be removed easily (theoretically!) for charging or replacement.

The bottle is a 'White Stuff Bottle' from ProBikeKit. It's a bit too tall for this application; the battery sits right at the top and makes it top-heavy. I've had no problems with it coming loose, however.

The design of the bottle

The regulator board was assembled on Veroboard (again). I drilled a hole to allow access to the adjustment potentiometer on the BuckPuck. The Power and Active LEDs point out to the sides, using the bottle itself as a diffuser. This worked out really well - it gives off a noticeable glow and uses an insignificant amount of power. I also drilled some holes and cable-tied the cables (battery and LED) to the board to provide some extra mechanical stability.

The power switch mounts in a hole drilled in the bottle. I was surprised at how well this worked - the bottle is made of very soft plastic, but it's more than strong enough to support the switch. The softness actually worked to my advantage, since the switch is mounted low enough to hit the bottle cage (oops!) You should mount it fairly high to avoid this problem. The LED power cable is routed out of a hole drilled in the bottle; the socket needs to be soldered on after the bottle is assembled.

I bought a 1000mA BuckPuck, and the Cree LEDs require 700mA (oops again!) I used the calibration pot to fix this. It's very sensitive and fiddly, and so I'd recommend buying the correct BuckPuck if you have the option. The BuckPuck datasheet contains instructions on how to calibrate the drive current.

The regulator board: Note the load resistor soldered in where the LED connector would normally be. This allows the LED current to be calibrated.

The battery is thirteen NiMH cells, soldered in series. Tagged cells make this a lot easier, but the tags can be a bit brittle. Thirteen cells gives just enough voltage to run the three LEDs - the Cree LEDs have a 4.5V forward voltage drop, instead of 3.5V for the Luxeons. It also happens to be the absolute maximum that would fit in the bottle - once the rim of the bottle is trimmed back a bit.

The battery pack

I soldered the tags of the cells together and wrapped the lot in duct tape. I found that the best thing to do with the tags was to bend them so that there was plenty of slack between the cells. Once they're duct-taped, they're pretty secure, but you don't want a big bump to break the solder joints. I also made sure that the cable was taped down securely - it'd be easy to yank the cable and break tags off the cells otherwise. I used two-pin Molex connectors for the battery; they're secure and handle high currents well. Under extremely high currents (like a short circuit or halogen lamp power-on) they tend to weld together and become difficult to separate.

The assembled bottle: Note the LED power cable, power switch, and battery Molex connector inside the bottle.

Note that there's no fuse in this circuit. You should probably have one in series with the battery pack. NiMH cells are more than capable of causing a fire if mistreated; in three years of the Sydney 24 Hour race, I'm aware of three separate battery packs being destroyed through mistreatment (not mine!) I didn't put a fuse in in, mostly out of laziness. I've had a waterproof fuse holder hanging around for three generations of bike lights and haven't bothered with it yet...