Most e-cigarettes are powered by re-chargeable lithium ion batteries. Conventional lithium ion batteries are based on a cathode made from lithium cobalt oxide and produce a voltage output that tends to decline as they discharge. The change in power supplied by the battery to a heater, from being fully charged to being nearly discharged, can therefore significantly impact the amount of liquid vaporised, and hence inhaled by a user. The present disclosure relates to a control unit for an electronic vapour system. The control unit for an electronic vapour provision system includes a battery for providing electrical power to a heater which is used to produce vapour and a circuit board for controlling the electronic vapour provision system. The control unit also includes a sensor unit to detect an inhalation on mouthpiece. A tip end of the control unit may be provided with an electrical contact, the electrical contact being mounted on a coil spring. The battery is a lithium iron phosphate battery. The battery provides an output voltage which remains at an approximately constant voltage level as the battery is discharged. In an embodiment, the output voltage of the battery when 80% discharged by successive puffs of the electronic vapour provision system is no more than 0.25V below the output voltage of the battery when fully charged, said battery having a rated capacity in the range 250-600 mA hours to support at least 100 puffs of the electronic vapour provision system, each puff drawing a current of at least 2.5A from the battery
