Cell test bench

Cell Test Bench

Single-cell characterization bench for 18650 and 21700 Li-ion cells (DMEGC INR45E, generic 3.5 Ah 18650). Part of the Solaris final-year project.

• 0–10 A linear electronic load (constant current), down to 2.0 V
• Charging up to 5 A from a 30 V / 10 A bench PSU through an XL4016 CC/CV module, voltage set by a DAC
• Bidirectional current measurement on one 10 mΩ 4-terminal shunt (Ohmite LVK25)
• Cell held at about 10–45 °C in an insulated pocket cooled/heated by two TEC1-12706 Peltiers
• STM32F103 Blue Pill, USB CDC to a Python UI
• Hardware protection (OV, UV, OC, OT) that works without the firmware

Repository layout

Status (v0.2)

Opening in KiCad 10

The files are saved in KiCad 7 format. KiCad 10 opens them and upgrades on save. The project's own fp-lib-table points to lib/cell_bench.pretty, so the footprints and 3D models travel with the repository.

After opening, run these once in KiCad 10:

1. Schematic → Inspect → ERC. Expect pin-type warnings on the module headers, which are drawn as connectors.
2. PCB → Edit → Fill All Zones (B), then Inspect → DRC.
3. Tools → Update PCB from Schematic should report no changes.

Layout notes

• Every connector sits on a board edge, with wire entry facing outward.
  • Left edge: PSU input, buck modules, XL4016 charger, thermostat.
  • Bottom edge: cell, series resistor, IRFP260N, NTCs. The MOSFET's back faces outward so the heatsink bolts on beyond the edge.
  • Top edge: Peltier driver, fans, Blue Pill USB.
• High-current path: 3 mm tracks on both layers, plus pours. The 10 mΩ shunt pad necks down to 1.6 mm because the pad itself is only 1.4 mm wide.
• Ground: the bottom layer is kept as clear as possible so its GND pour stays continuous. Each SMD GND pad has its own via to it, and stitching vias tie the two pours together.
• Plug-in modules (Blue Pill, 2× ADS1115, 2× MCP4725) have keep-out areas where the module body overhangs the board.
• The layout was autorouted by hardware/tools/router.py. It is electrically complete, but a hand pass in KiCad (or a run through Quilter) will make it tidier.

Sheets

Blue Pill pin map

The Blue Pill plugs in with its USB end at the top board edge. The silkscreen marks the B12 and VB corners: check the orientation before soldering the headers.

Key design numbers

• Load setpoint: DAC 0–3.3 V → divider 29.4 k / 1 k → 0–108 mV → 0–10.8 A.
• Current monitor: I_MON = 1.65 V + 0.12 V/A.
• Protection trips: OV > ~4.29 V, UV < ~1.95 V, discharge > ~11 A, charge > ~6.4 A, heatsink or cell NTC > ~62 °C.
• Charger: set the XL4016 CV pot to 4.25 V before connecting the FB tap. The DAC can only lower the voltage from there.

Known limits

• The XL4016 module's own current limit is bypassed, because its IN− and OUT− share system ground. Charge current is regulated by firmware and limited by the OC comparator.
• UV protection also trips when no cell is inserted, and prevents charging a cell that is below ~1.95 V.
• The comparators have no hysteresis yet. The firmware is expected to latch faults.
• Check that the BTS7960 module accepts 3.3 V logic inputs.
• Module pin orders (MCP4725, ADS1115, BTS7960) vary between sellers. Check them against the silkscreen before soldering the sockets.
• Check the relay footprint against the Panasonic ALFG2PF121 datasheet. KiCad's footprint is drawn for the ALFG series.

Regenerating

The schematic, placement and routing are reproducible from hardware/tools/:

python3 design.py ../kicad                 # schematic (overwrites edits!)
python3 makelib.py ../kicad                # project footprint library + 3D models
kicad-cli sch export netlist --format kicadsexpr -o net.net ../kicad/cell_test_bench.kicad_sch
python3 pcbgen.py net.net ../kicad         # board outline + placement (overwrites the PCB!)
python3 router.py in.kicad_pcb out.kicad_pcb

Once you start editing in KiCad, those edits are the source of truth. Don't re-run the generators after that.