Most modern EVs do produce some heat from the battery, but this is rarely sufficient to heat the cabin. Some EVs are fitted with heat pumps, which operate in the opposite way to a refrigerator - taking cold air from outside, compressing it into a condenser which then radiates heat to warm either the batteries for efficiency or the cabin to warm the passengers. Unfortunately the Smart EQ is not available with this system, so you can only warm the cabin by using the main battery.