to the electric motors. The full charge of the battery is replenished only by loading it on the electrical grid. While in this series mode at higher speeds and loads, (typically above at light to moderate loads) the gasoline engine can engage mechanically to the output from the transmission and assist both electric motors in driving the wheels, in which case the Volt operates as a power-split or series-parallel hybrid. After its all-electric range has been depleted, at speeds between , the Volt is programmed to select the most efficient drive mode, which improves performance and boosts high-speed efficiency by 10% to 15%. While operating modes are switched automatically the Volt allows the driver to choose from three drive modes: normal, sport and mountain. The mountain mode, which is expected to be required only under unusual power demand conditions, increases minimum battery state of charge (SOC) to around 45%, thus maintaining performance on steep and long grades. The driver hears more engine noise due to the higher rate of power generation required to maintain this mode. The sport mode causes the engine to rev higher, and the response to the throttle pedal is quicker. The Ampera has an additional option, the "City Mode" or "battery hold", allowing the driver to save the energy currently stored in the battery for use when traveling in urban areas or restricted zones. The 2013 model year Volt includes a "Hold" option to provide the same choice. Battery The 2011 Volt's lithium-ion battery (Li-ion) battery pack weighs and "consists of 288 individual cells arranged into nine modules. Plastic frames hold pairs of lithium-ion cells that sandwich an aluminum cooling fin. The design and construction of that aluminum plate was critical to ensuring an even temperature distribution with no hot or cool spots across the flat, rectangular cell. The battery pack has its own cooling circuit that is similar to, but independent from, the engine cooling system." For the 2011/2012 model years, the battery pack stores 16kWh of energy but it is controlled or buffered via the energy management system to use only 10.3kWh of this capacity to maximize the life of the pack. For this reason the battery pack never fully charges or depletes, as the software only allows the battery to operate within a state of charge (SoC) window of 65%, after which the engine kicks in and maintains the charge near the lower