The heart of it is a power-bank boost module with a digital charge readout and fast-charge protocol support (about ¥10), paired with 8 × 18650 (¥2–3 each). Test the cells’ internal resistance, stick fish paper on the positive ends, load them into a single-row holder and wire everything in parallel (the module is a boost type, so it only needs 3.7–4.2V); 8 × 2000mAh = 16000mAh. Wrap in fish paper, spot-weld, insulate the positive and negative ends again with fish paper, measure 3.6–3.7V, solder leads onto the positive and negative pads and wrap the joints with filament tape. Two wires go to the module; press the switch and it shows the charge level. The case is hand-cut from PVC sheet (a 3D printer would look nicer), openings cut with a home-made fret saw and a small electric saw, glued with strong adhesive, with cut-outs for the display, the USB / Type-C ports and a small button. The Type-C port does input and output; with a powerful enough charger it goes into fast charge (green lightning icon), the USB port also fast-charges, and it will keep a phone going for two or three days.