Universal Bike Light Battery

An 8 ounce Kraft Parmesan Cheese container happens to fit into a standard bicycle water bottle cage very well. Run to your local supermarket (or perhaps your refrigerator) and pick up a container or two of this cheese.
Make several Italian dishes over the next few weeks (or empty contents in to another dish) and then clean out the cheese container. Remove the label or leave it on to confuse the competition.

You'll need a few more items to build this battery pack. First, pick up two battery holders capable of holding four C-cell batteries each. Make sure the battery holder arranges the cells two-by-two as pictured here. There are other battery holders that arrange all four batteries side-by-side. The side-by-side style will not fit into the container.

Purchase two connectors that are compatible with your bike light. In this case I used Size N Panel Mount Coaxial DC Power Jacks from Radio Shack. You'll want a connector that you can secure to a hole drilled into the container. The power jack pictured here has a nut and flange to make mounting in the hole possible.

The Parmesan cheese container fits snug into a standard water bottle cage. However, you may wish to modify the container slightly to overcome any fitting issues unique to your bottle cage. In this case I need to shape the container to better fit the end of the bottle cage.

One suggestion to customize the container is to use a heat gun. The heat will soften the plastic so that you can mold it to fit your bottle cage. Be careful not to overdo the heat as the container may shrink or catch on fire. Burning plastic is not a welcome step in this Instructable.

Solder lead wires to the connectors. I recommend installing braided 18 AWG wire as opposed to 20 or 22. The 18 AWG will reduce resistance from the batteries to your light and result in better light output. Use heat shrink tubing to prevent short-circuits.
Drill two holes in the bottom of the container. The size of the holes depend on the connectors you are using. Feed the leads wires and connectors into the container and secure with a nut from the inside. Long needle-nose pliers may come in handy when tightening the nut.

You'll find that the plastic in the bottom of the cheese container is thicker and more rigid than the sides. This is a nice feature as it makes connecting the battery to the light much easier. Pushing on the connector won't bend the plastic as much as it would if it were mounted on the side.

The battery holder will likely come with red and black leads. Remove these and solder the connector leads from step #7 directly to the battery terminals. Consider beefing up the stock terminals by soldering in some pieces of sheet copper.

Of course make sure you leave enough length in your wire to remove and install the batteries. In this example I am wiring two separate four-cell battery packs. Each battery pack will have a separate connector. This will allow me to run two lights from two separate battery packs, or I can run a single light and have one battery pack as a back-up. Having a back-up battery and light is important when you are out in the woods at night. All of the 24-hour races I've competed in require that riders have two completely independent lights.

Now you're ready to go. The battery pack is complete and ready to accept your choice of "C" cell batteries.

I picked up Radio Shack rechargeable NiMh "C" cells for this bike light. Each cell is rated for 1.2V and 3000mAh. This is a good capacity for bike lights. Four of these cells connected in series gives you 4.8V and 3000mAh. With all eight cells installed, the battery pack has 6000mAh capacity - good for hours and hours of night riding. Higher capacity cells are available for less money . Since the batteries represent the majority of the cost and control nearly all of the performance of this Instructable, do some comparison shopping before settling on a model.

The assembled battery holders slide into the container back-to-back. You may have to insert one pack at a time to get them both inside. Small variations in the battery holder dimensions and cell diameter make each build unique. You may also need to twist the battery packs to coil the leads wires for a better fit.

It is a tight fit, but that is the idea. This battery pack puts eight C cells into a container that is about the size of a small water bottle. Plus, the batteries are serviceable. Not many commercial bike light batteries are serviceable.

Install the lid. The lid on these cheese container flips open so that you can shake the cheese out. I thought of using that feature to run the wires out, but the battery pack wouldn't be as splash resistant or dust proof.

Snap the lid down and you're ready to ride!

The battery pack mounts with the lid facing down. The wire runs up the down tube to the handlebars where your light is mounted.

Here is a variation on the position of the battery connectors. In this case I mounted them on the side of the container. Depending on where your lights are connected to your bike or how you want to run the power wires, you may wish to use this location.

There is at least one benefit to this connector position. That is the wire that you solder to the connector extends across the bottom of the container rather than straight into the center and interfering with the space for the battery holders.

Connectors mounted on the side of the container give you a different position to run the power leads. Your decision.