NiCad Water Bottle Battery

the finished product

Here's how to make a 6V NiCad water bottle battery. In the text below I refer to a single D-sized NiCad cylinder as a "cell", and refer to the result, the 5 cells soldered together in series, as a "battery".

First, buy a water bottle. It should be a widemouth, screw-top water bottle. Ours is made by Cannondale. It is the 21 oz. size (i.e., smaller as opposed to larger 28 oz. size). Make sure you check that 5 D cells will fit in your water bottle before you commit to using it.

Get 5 NiCad D cells. They should be the "tabbed" or "tagged" kind for easier soldering. You should get the highest capacity ones you can find/afford; they should be at least 4 amp-hours. You're putting some effort into making this, so don't cheat yourself by getting cheap, low-capacity cells.

If you want, buy a fuse and fuse holder to put in the water bottle with the cells. The fuse prevents the battery from completely draining itself if there is a short in the wires connecting it to the lighting heads. This can save you alot of money, as fully draining a battery could easily destroy it through cell reversal. Also, the rush of current that drains it could produce enough heat to start a fire. You should use the smallest effective fuse that will not blow with regular use. William Burrow suggests calculating the size (amp-rating) of the fuse you need by doubling the maximum expected current used by the lights and picking the nearest next larger size available fuse.

Solder everything up as in the diagram. The light blue rectangles are the cells, the red rectangle is the (optional) fuse holder, and the pinkish thing is the lid to the water bottle (shown in profile). The black lines are wires. Take two cells, solder a short wire between them, then tape them together to make a long cylindical stack. Do the same with two of the other cells. Put a long wire between the two stacks (a bit longer than the length of the stacks), then put the 5th cell in series with the others, connected by a wire again slightly longer than the length of the stacks. If you want a fuse holder, a good place to put it is after the 5th single cell. Attach longish wires to the ends of the series of cells (these will go out the lid).

In addition to taping the cells in the stacks together, it's a good idea to put electrical insulation tape around the ends of the stacks, the single cell, and the fuse holder. Make sure you pay close attention to the polarity of the cells, so they end up in series (+ end of one cell attached to the - end of the next).

Put the ends of the wires through the lid of the water bottle. It's neatest to put a hole in the nipple, feed the wires through, then put a liberal amount of glue in the area to prevent water getting into the bottle and to keep the wires in place.

To install in the water bottle, gently lower the first stack, then the second, then the single cell (and fuse holder if you have one) alongside the two stacks. Pack with foam (we used the sort of foam that cheap foam mattresses are made out of). Screw the top on, and solder a cord/connector onto the wires that protrude from the water bottle lid.