Ampere-hour (Ah) efficiency: The quantity of electricity measured in Ampere-hours which may be delivered by a cell or battery under specified conditions. Ampere-hour capacity: The total number of Ampere-hours or watt-hours that can be withdrawn from a fully charged cell, indicated by Ah or mAh. Battery: Two or more electrochemical cells connected together electrically in series, parallel, or both, to provide the required operating voltage and current levels. Crate: Charge or discharge current, in Ampere, expressed in multiples of the rated capacity. For example, C/10 charge current for a cell rated at 20 Ah is: 20 Ah/10 = 2 A. Capacity: See Ampere-hour capacity. Cell: The smallest electrochemical unit of a battery used to generate or store electrical energy. Coulombic efficiency: See Ampere-hour efficiency. Cutoff voltage: The cell voltage at which the discharge process is terminated (it is generally a function of discharge rate). Cycle life: The number of times a cell can be discharged and recharged until the cell capacity drops to a specified minimum value usually 80 % of rated capacity. Depth of discharge: The quantity of electricity (Ampere-hours) removed from a fully charged cell, expressed as a percentage of its rated Ampere-hour capacity. Energy density: The ratio of the energy available from a cell to its volume (Wh/L) or mass (Wh/kg). xxi Internal resistance: Expressed in ohms, the total DC resistance to the flow of current through internal components (grids, active materials, separators, electrolyte, straps, and terminal) of a cell. Module: The smallest modular unit, consisting of a number of individual cells connected together electrically in series, parallel, or both. Nominal voltage: The average voltage of the cell. The operating voltage of the system may go above or below this value. Open circuit voltage (OCV): The difference in potential between the terminals of a cell when no load is applied. Pack: Two or more modules connected in series, parallel or both. Power density: The ratio of the available power from a cell to its volume (W / L). Round-trip efficiency: The ratio of energy put in (in MWh) to energy retrieved from storage (in MWh). Self-discharge: The loss of useful capacity of a cell on storage due to internal chemical action (local action) and parasitic currents. State-of-charge (SoC): The present cell capacity in relation to maximum capacity. Terminal voltage: The difference in potential between the terminals of a cell when a load is applied.