Rechargeable Guitar Pedal Power Supply

Introduction

This is a rechargeable power supply, designed to deliver 9VDC to power standard guitar pedals. It is powered by a pair of 13400 LiPo cells (can be recovered from used ‘disposable’ vapes), together with a combined charger and voltage boost board (based on the TP4056 safe charger chip). I am currently testing how long this supply will power my chromatic tuner pedal - it's at least a few hours, I will report back when I have a more accurate figure. UPDATE: the pedal is still running after fifteen hours connected to the power supply (on battery alone). No doubt other pedals might draw more power than a simple chromatic tuner pedal, but for my purposes, I'm declaring victory. Please let me know of your experience with other pedals!

You Will Need

• The printed parts (see Printing below)
• LiPo cells: the model is designed to accommodate two 13400 LiPo cells, easily harvested from used ‘disposable’ vapes. While the sizes of the cells used vary considerable, the ‘Elf Bar’ brand vapes use 13400 cells. If you have different-sized cells, you can adjust the parameters in the OpenSCAD file supplied, which will generate the right size STLs for you. The cells are connected in parallel (2P) for longer battery life.
• Combined charge/boost board: this does all the heavy lifting. The board I use is extremely cheap (18p) from AliExpress, and is called “Lithium Li-ion 18650 3.7V 4.2V Battery Charger Board DC-DC Step Up Boost Module TP4056 DIY Kit Parts”. In spite of the silly name, it is a very handy board, including a USB-C charge port, TP4056-based safe charge/discharge circuitry, and adjustable-output DC-DC boost circuitry, all on a little (23.5mm x 18mm) PCB. Given the cheapness relative to shipping cost, it's worth buying a handful for projects. This is the link I bought it from, though you can doubtless find the same board elsewhere - see close-up picture of the board to ensure you're getting the right thing. 
• Output cable: I happened to have some cables with moulded-in DC jacks snipped off of wall-wart PSUs left over from another project. Feel free to make up your own cable, or snip one off and old wall-wart PSU. The cable hole and strain relief part are sized for a round cable with an OD of 3.5mm. This can be adjusted in the supplied OpenSCAD file if required. The barrel jack is a standard 5.5mm x 2.1mm.
• Hookup wire: you'll need a little bit of hookup wire to wire up the batteries to the board. Given the short lengths and low current, gauge is not too critical, but it will help if it is flexible. I used some stranded hookup wire I had handy; in a pinch I expect that nipping the ends off a couple of DuPont-style jumper cables would work fine.
• Screws: The lid is fastened to the case body using M3 self-tapping screws. These are cheap and work well for projects where the case does not need to be opened too frequently (eventually the self-tapped threads will wear out if it is frequently opened and closed). I used M3x8 self-tapping screws, but anything up to about M3x12 should work fine. You might be able to get away with non-self-tapping M3 screws, but it will be hard work.

In addition to these components, you will need a few tools: soldering equipment, a digital multimeter, a hot glue gun and small Phillips-head screwdrivers.

Printing

None of the objects should require support if printed in the supplied orientation. There is a small bridge for the USB-C port, but most printers should manage this without support. Print the following items:

• Case Body (pedalpsu_case.stl): the main body of the case
• Strain Reliever (pedalpsu_strainreliever.stl): strain reliever for the 9VDC output cable
• Case Lid (pedalpsu_lid.stl): lid for the case
• Wiring Junctions (pedalpsu_junction): these are optional but rather handy. I twist all the positive wires together, solder them, then squirt some hot-melt glue into the junction and shove it over the exposed ends. It ends up looking like a small wire nut. Neater and better than tape, and less fuss than heat-shrink tube. You'll need two of them if you are going to use them.

Wiring and Assembly 

Before Starting: fully-charge each of the LiPo cells before connecting them together - otherwise you will not obtain the full battery capacity

• Solder a short wire (approx 40mm long) to each of the B+ and B- terminals. Ideally, you would use a red wire in B+ and a black wire in B-. Colour doesn't matter, as long as you are consistent. Be careful not to reverse the polarity of the battery connections! Feed these wires through from the top of the board (the side with the USB port), and solder on the underside. Trim the excess wire with a flush cutter or similar (scissors, nail trimmer).
• Solder wires (ideally red) to the positive terminals of the batteries
  • Note: I usually put a bit of kapton tape over the exposed connection once I've soldered it. This isn't strictly necessary, but is a good habit and can save you from accidentally letting the magic smoke out of your parts.
• Solder wires (ideally black) to the negative terminals of the batteries
  • See kapton tape note above.
• Position the cells in their slots (see interior photo). Gather the three positive wires together (one from each LiPo cell, and one from B+ on the board), and trim them to the same length with a little slack. Strip about 10mm of insulation from each end. Double-check the polarity of the cells before connecting. Twist the ends together, and secure by soldering. Insulate the connection (either by using one of the Wiring Junctions as described under Printing, or a piece of tape or heat-shrink tube.
• Gather the three negative wires together (one from each LiPo cell, and one from B- on the board), and trim them to the same length with a little slack. Strip about 10mm of insulation from each end. Double-check the polarity of the cells before connecting. Twist the ends together, and secure by soldering. Insulate the connection (either by using one of the Wiring Junctions as described under Printing, or a piece of tape or heat-shrink tube.
• Adjust the voltage : switch your multimeter into DC voltage mode (if not auto-ranging, selecting the 20V range will work best). Position the probes on the - and + terminals, and adjust the trim screw until you reach 9V. Rotating clockwise should decrease the output voltage, anticlockwise should increase it.
• Glue the strain reliever in place - a small amount of hot-melt glue on the upper surface of the flange, and then press it through the hole from the inside.
• Run the DC output cable through the strain reliever from the outside. Strip approximately 30mm of the outer insulation from the end, and approximately 10mm of the insulation from each of the internal wires. Feed the wire from the centre pin of the barrel jack through the - terminal on the board, and the wire from the outer pin of the barrel jack through the + terminal on the board. The PSU I had snipped my cable from was center-positive, which means I had to put the red wire into the - terminal, and the black wire into the + terminal. Highly unsatisfying.
• Secure the board: squirt some hot-melt glue onto the plinth provided for the charge/boost board, staying clear of the hole for the USB port. Press the board into the plinth so that the USB-C port lines up neatly with the hole. A few squirts of hot-melt glue around the edges of the board will secure it in place, though be careful not to get glue in the USB port.
• Secure the DC output cable: pull the DC cable out through the strain reliever, leaving just a little slack inside the case. Squirt a blob of hot-melt glue over the cable in the inside of the case, such that it also touches the case wall and strain reliever. This will prevent the cable from accidentally being pulled out and damaging your board.
• Secure the LiPo cells: position each cell snugly in its slot (see interior picture), and ensure nothing is standing proud of the top edge of the slots. Squeeze a little hot-melt glue in either side of each cell to secure it in place.
• Secure the cable joints: I am paranoid, so I position these in the voids on opposite sides of the battery back from each other, then glue them in place. This helps prevent any possibility of a battery short, and also reduces the possibility of cable movement.
• Pop the lid on, and screw it down.

You're now ready to jam!