// In progress · 2025–
Cell characterization bench
Built a four-wire Kelvin test jig rated past 120 A for characterizing 21700 cells before pack integration. Ran tabless cell research and cell selection, with a six-channel MAX6675 thermal rig logging temperature rise to a 50 °C cutoff.
Status
In progress. The bench is built. No results are published yet — the plot below is synthetic placeholder data, labelled as such, and stays that way until real runs replace it.
The requirement
Pack design starts from cell data. Vendor datasheets are measured under conditions that are not the conditions of an FSAE accumulator, so candidate cells needed measuring on one jig, under one procedure, at currents relevant to the application.
Cells in scope: Molicel P30B, P42A and P45B; EVE 50PL; Ampace JP40; Reliance RS50 and RS60.
Design decisions
Four-wire (Kelvin) sensing. At these currents, contact and lead resistance are the same order as the internal resistance being measured, so a two-wire measurement returns the fixture as much as the cell. Force and sense paths are separate.
Custom jig. Commercial 21700 holders target convenience at low current. This one targets repeatable contact pressure and a defined sense point on the terminal.
BK Precision 8601 electronic load, so discharge current is programmable and held constant as cell voltage falls.
Thermal cutoff at 50 °C to define maximum continuous discharge current. Current is stepped, the cell is allowed to reach thermal steady state, and the limit is the current at which it hits the cutoff.
Six-channel thermal monitoring — MAX6675 converters multiplexed on a shared SPI bus from an ESP32.