Incoming lithium-ion cells are measured for capacity and internal resistance and sorted into closely matched groups. A weak cell in a pack drags the whole pack down, so matching here saves trouble later.

Thin nickel strips are laid across the cell terminals and fused in place by short bursts of current. The cells end up in parallel or in series, depending on the design, as one solid block.

Solder paste is screened onto the pads of a bare circuit board through a metal stencil. Pick-and-place machines then set down the charging chip, the voltage converter, the protection parts and the indicator LEDs.

A reflow oven warms the loaded board slowly and then melts the solder particles in the paste. As the solder cools, every part is fixed to its pads in one pass.

Workers solder the cell block to the board's battery terminals and test it as a unit. Its converter steps the cells' voltage up to USB level, and its protection circuit cuts power on overcharge, deep discharge or short circuit.

The pack and board drop into a molded plastic or extruded aluminum case with openings for the USB ports and lights. The halves are clipped, screwed or glued shut.

Finished units are charged and drained on racks, sometimes more than once, to settle the cells and confirm the delivered capacity. Automated testers then check output voltage and current before labeling and packing.