Modular user-populated 18650 UPS with mains inverter output — 37 W to 1.5 kW on one PCB design, scaled by how many cells you populate.
An open-source UPS built around a hard constraint: the shippable product contains no lithium at all. Cells go into SMD holders on both faces of the board, so the thing that crosses a border is an ordinary PCB — no UN38.3, no dangerous-goods paperwork, no air-freight restriction. You buy 18650s locally, new or salvaged from laptop packs.
Scales by cell count37 W to 1,500 W on one PCB design. Populate fewer holders for less power and runtime — no jumpers, no BOM variants, no firmware rebuild.
One constant runs itDeriving from a 0.3 C cell limit, each populated cell contributes 2.86 W. Firmware computes the power ceiling from the cell count and applies it as foldback, not a trip.
Silkscreen population mapEvery holder carries a digit 1–8. The instruction is one sentence: fill every holder numbered ≤ N. Row ordering is interleaved between faces so any fill level stays balanced.
Finds its own cellsModules auto-discover over CAN and measure per-group internal resistance, cross-checked against what you declared. Disagree and it refuses to run.
One bidirectional converterA dual active bridge charges the pack and discharges it. Ships line-interactive, becomes online double-conversion in firmware alone — no board respin.
Auditable deratingEvery capacity and rating traces to one written chain. Catching a dropped term in that chain moved the maximum build from 16 modules to 17.
Phase 0 is documented — architecture, derating basis, a thirteen-entry hazard register, and an NFPA 855 configuration-versus-code table so a build can be sized to stay outside fire-code scope. No schematic capture yet. The first board is a $50 test coupon answering the two questions this construction can't look up: what a holder's contact resistance does over fifty insertion cycles, and whether a heavy bottom-side holder survives a second reflow.
Stack — 18650 SMD holders, BQ76952, STM32G474, KiCad 9, CERN-OHL-S
