4 NiMH cells sometimes just aren’t enough to power the arduino and sensors that I want. 5 might do.
I designed this 5 AA cell holder to hold rechargeable cells and power my Arduino projects.
I first tried using this well designed battery pack – Thing 5079 – but had problems with the clearance necessary to allow for the bolt head contacts. The bolts and nuts are additional components that I don’t want to have to use for batteries.
My design uses solid copper wire for the battery contacts and interconnects. Soldering the wire ends is optional and I imagine will increase the physical lifetime of the pack. The wires should be wrapped twice through the wall to give good contact with the battery. The wire channel design has a land to cause the copper to extend out to press against the battery. There are wire channels on the inside and outside of the box to correctly align the wires and keep the package neat.
The alternate design has some corners built into the top to center an arduino above the battery box and hold it steady. It was designed for this Arduino holder, which I use to protect my furniture from scratches by the pins at the bottom of the Arduino, and it also keeps the contacts from short circuiting. With this design, you can even stack the battery packs to make a split power supply or a 10 cell pack (12 volts).
My initial design was too thin and flimsy. The walls bowed out more than I wanted, especially at the center. This could cause a disconnect between the battery and contact. I also erred by adding too much in slop space for the batteries which caused them to move around and not stay aligned.
The next design was successful. I added 1mm walls between the batteries which is just about perfect. I added a few channels for the interconnects. The wires as contacts give enough spring to hold the batteries in.
The next iteration includes mounting posts to slot in an Arduino and holes in the bottom to make it easier to stack packs. This is a picture in-situ.
I used solid cat-5 twisted pair, since I have a box of it sitting around. Cut a bit more than foot and pull out the twisted pairs.
Pick a color. Untwist it about half way and tighten the twist at that point. Cut the light colored wire in half and strip it to the twist. Poke the dark strand through a pass-through hole in one side of the pack. Pull the wire through and press it in the wire channel. Strip the wire at the point of the farthest contact hole (2nd hole from end). You’ll want the wire to be insulated up to the point where it enters the box.
Then thread the wire through the holes twice, carefully pulling it taut. I used some needle-nosed pliers to help. It’s possible to pull apart the copper wire after it’s been bent a few times, so be a bit gentle with it as you work with it. To make it stronger, we’ll tack the wire to itself with a bit of solder when we’re done making the rest of the contacts.
Poke the stripped end of the light wire through the opposite corner as the first. The second hole in from the where the dark wire enters the pack. Wrap the copper wire twice around as you did the first wire. Pull it taut.
Strip some more of the wire – maybe 8 inch pieces. Make a U and pass the two wires through the next two wire holes – at the bottom. On the inside, pass it up and through the top holes. Pass it down the outside, and back up through the inside again, making a double loop.
Repeat for the remaining pairs to complete the interconnects. When they’re all done, dab a bit of solder on each of the bottom holes where the wires are entering the pack. This will physically solidify and improve the connection. Take care not to burn the insulation on the ground wire. This could cause a short circuit. Also, don’t heat the wire too long, as the plastic could melt.
Trim the wires close to the wall on the inside and you’re done. Be careful not to clip the battery contact wires. You can optionally connect a power connector, or just strip the wires and poke them into a breadboard or Vin/Gnd. I snap off a pair of pins and solder them to the wire ends to keep the wires apart and make a clean, easy connection to Vin/Gnd on the Arduino.