Why cold = high R_int Li+ ion mobility in electrolyte slows dramatically at low T.
SEI layer conductivity drops.
At −20°C, R_int can be 3–5× higher than at 25°C.
Same cell, much less power available.

BMS consequence SOP = V²/(4·R_int) — power limit falls with R.
BMS derates P_max_chg and P_max_dsg in cold.
Fast charging blocked below ~5°C (plating risk).
Heating pack to 15°C before charge is optimal.

SOC effect R_int is highest at very low SOC (empty).
Lowest around 50–70% SOC (ideal operating zone).
Full charge (100%) also slightly higher R due to
phase stress in cathode.

Aging effect Each cycle adds SEI thickness → R_int grows.
At EoL (80% SOH), R_int typically 2× new value.
Power fade (SOP↓) often noticed before capacity fade.
Cold + aged = severely restricted power.