Lithium button and coin-shaped batteries, the CR1616, are used to power small portable electronic devices, such as wristwatches, pocket calculators, artificial cardiac pacemakers, implantable cardiac defibrillators, car key fob entry systems, and hearing aids. Button and coin-shaped batteries typically have a diameter of 5-25 millimeters and a height of 1-6 millimeters. Lithium sizes can be identified by their reference numbers. The first two digits represent the battery's diameter in mm and the second two digits give the height or thickness of the battery in tenths of mm. For example, a CR2032 is a lithium button with a diameter of 20 mm and a thickness of 3.2 mm. The CR2430 is a lithium battery, 24 mm in diameter and 3.0 mm thick. Lithium button and coin-shaped batteries have the prefix BR or CR and mainly provide 3 volts. The Renata 751 is an exception at 2 volts.