Outgassing is a phenomenon that occurs when a battery is short-circuited or the power demand on the battery is too high. The battery is burnt from the inside and releases a large quantity of gas (outgassing = release of gas), which must be evacuated to avoid an explosion. This phenomenon can be very dangerous, so it's vital to keep your batteries in perfect condition, to transport them in protective cases and to respect their power rating when using a mechanical mod without electronic safety.
