If you were to use a 20A battery in a regulated mod that needed a 30A battery the chip in the mod would protect itself by telling you "high temperature warning" and refuse to fire if the board started getting hot.
However, the mod has no idea if the battery is being stressed too hard or not. And a battery being stressed too hard will have a shortened life and, if stressed hard enough, can vent or worse.
Some mods might monitor the battery voltage drop but that doesn't tell you how badly the battery is being overstressed as different batteries have different voltage drops. Some mods can monitor the temperature of the circuit board but that doesn't tell you the temperature of the battery, no matter how close you mount the board to the battery.
You must still select a battery that can handle the current the mod draws without overheating.
That was sick. Great explanation.
Though I have to ask, is the Subox Mini capable of any of these functions? Meaning let's say I'm using the Subox Mini and I make the mistake of overdrawing on the battery, what happens?