High-energy cells, as follows: (a) ‘Primary cells’ having an ‘energy density’ exceeding 550 Wh/kg at 20 °C; (b) ‘Secondary cells’ having an ‘energy density’ exceeding 350 Wh/kg at 20 °C; Technical notes: 1. For the purposes of high-energy devices, ‘energy density’ (Wh/kg) is calculated from the nominal voltage multiplied by the nominal capacity in ampere-hours (Ah) divided by the mass in kilograms. If the nominal capacity is not stated, energy density is calculated from the nominal voltage squared, then multiplied by the discharge duration in hours divided by the discharge load in Ohms and the mass in kilograms. 2. For the purposes of high-energy devices, a ‘cell’ is defined as an electrochemical device, which has positive and negative electrodes and an electrolyte, and is a source of electrical energy. It is the basic building block of a battery. 3. For the purposes of high-energy devices, a ‘primary cell’ is a ‘cell’ that is not designed to be charged by any other source. 4. For the purposes of high-energy devices, a ‘secondary cell’ is a ‘cell’ that is designed to be charged by an external electrical source. Note : High-energy devices do not apply to batteries, including single-cell batteries.