4S2P USB-C Power Bank

Design

The design was mostly taken from the 4s2p lithium pack. The bottom case part (TARRO.3mf) is used as-is, everything else is redesigned to fit a “120W two-way PD fast charging module” from AliExpress (bought from "Sep 14 Store", but other sellers have it as well).

The BMS is moved to the side of the cell holder pack. Cell holder is redesigned to have holes and grooves for the battery sense wires leading to the BMS. Any 4S BMS can be used, but I chose HXYP-4S-4060, a couple of which I had laying around from other projects. Coincidentally the power module seller also offers it as a package with the PD module.

The cover was made solid using various 2D projections of the original bottom case part and top cover. Not an easy task having only FreeCAD and OpenSCAD in my toolkit, and was probably the hardest part of the whole project. The source STL had geometry issues that didn't bother PrusaSlicer, but broke the CGAL renderer used in OpenSCAD. None of the STL fixers helped.

As a result the cover isn't chamfered properly near the mounting holes. Partly because of my skill issues, partly because FreeCAD was crashing when I tried. That's a thing I'll have to live with, I guess.

Anyone willing to fix/improve the models is free to do so using sources at GitHub.

2025-02-24: The PCB holder got redesigned:

• the PCB is fixed firmly in place using three M3x8 bolts;
• the cable hole got enlarged to pass an XT30 connector;
• the bottom part of the holder has tighter fit with the battery can;

The older STL without bolts can be found in GitHub commit history.

Printing

Everything was printed using PETG / “0.20mm Quality” preset. The bottom part can be made  in theory using PLA, since it doesn't heat up that much, but the PCB holder and the cover  parts must withstand moderate heating that may soften PLA.

The PD module has a single white LED that is lit either when connected to a charger or when discharging. The first pack was printed wholly in black, since I wanted to drill a hole near the port for the LED to be visible, but it wasn't necessary after all, as the light was visible regardless. Though the second version got printed in red for the LED to shine through more easily.

The PCB holder STL has to be split into four parts (the holder itself + three bolt spacers).

Post-processing & Assembly

After printing the PCB holder part it's better to clear the mounting holes of the front-facing holder posts using a 1.5mm drill bit.

The USB-C port hole may need some filing near its bottom depending on layer height, USB-C socket mounting position on the PCB, etc. Wasn't necessary for the first PCB, was necessary for the second. 🤷

When assembling the 4S2P cell holder it's best to place the NTC for cell temperature monitoring (if your BMS has one) at some cell near the middle before connecting all cells together.

The BMS is attached to the cell holder using double-sided tape after soldering the NTC and the wires already lead through cell holder holes. A layer of polyimide tape to fix it to the cells won't hurt.

I've used an XT30 connector pair to swap either the cell pack or the PD module independently of each other.

From what I can tell the PD module PCB is produced using panelized batches (i.e. MxN PCBs are etched, drilled and populated at once). It thus may need some filing near front, as breakaway tab leftovers may interfere with the cover.

The wires leading to the BMS should be no less than 16AWG and must be soldered before fastening the PCB, although the XT30 can be soldered afterwards in case it's used.

The front posts are designed to accept self-tapping M2 screws. Mounting the PCB should be started with the front posts. The back posts accept M3x8 bolts. For easier assembly screw the M3 bolts through the spacers, so that they protrude ~0.5mm.

Some rubber foam can be added between the battery pack and PCB holder, so that it doesn't rattle.

Notes

Although the PD module advertises 120W continuous power operation, testing at 20V/5A (100W) shows considerable heating after only 30min of operation. So although it's OK for a couple of minutes, the module may overheat and possibly fail if used at that power all the time. At 20V/3A (60W) the top cover is barely warm, which is enough for my laptop and soldering iron.

Testing with slightly used INR18650-35E cells showed between 2.72 and 2.84Ah capacity at 20V depending on current and temperature.

• Added holes for battery sense wires in the 4S2P cell holder
• BMS/XT60 retention plate swapped for USB-C converter PCB holder
• Cover made solid with a single hole for the USB-C socket