Comment by avar

    > How do you even drain a 3.7V lithium
    > ion battery below 3.3V?

Connect the + and - terminals with an appropriately sized resistor, it'll drain all the way to 0V.

    > My devices that use 18650s will
    > not let them go below that.

Because you're not using the + and - terminals, you're using the + and - supply of a BMS, which is connected to those terminals. For this sort of testing you need to bypass the BMS, which'll have its own voltage cutoffs.