Stackpole has extended its CSRF2817 range of surface-mount foil-on-ceramic resistors up to 100mΩ – it continues to reach down to ... while 41 – 60mΩ parts can be operated at up to 3W – lower values go ...

Just put a load across the terminals, maybe an incandescent bulb or a beefy power resistor, and wait ... a fairly simple PCB, four ceramic power resistors, a holder for a single 18650 cell ...