Lithium as a substance, and indeed lithium batteries, until around the mid-1980’s was classified for dangerous goods regulations under Class 4.3 (Substances which, in contact with water, emit flammable gases). Experts and regulators were persuaded that smaller batteries present reduced hazards compared to their larger counterparts through the supply chain. Therefore the criteria for classification was redesigned around the weight and power outage of the cell or battery. Consequently, lithium ion batteries are classified by the equivalent energy content in watt-hours (Wh), while lithium metal/alloy batteries are classified by the weight of lithium content in grams.
