The three main power sources in a watch are either the tension caused by a powerful spring stored inside a barrel that systematically releases its tension to power the watch movement, a conventional ‘button’ battery, or a capacitor, which is very similar to a battery but can be recharged by a variety of different mechanisms, such as solar power or, as with an automatic watch, the movement of the wearers wrist.
Many shop assistants – and thus their misinformed customers – often confuse automatic watches with watches containing a capacitor. Watches that are powered by a capacitor but charged by the movement of the wearer’s wrist are called kinetic watches. There is a myth surrounding these kinetic watches, frequently peddled by fundamentally false marketing campaigns that you will never need to change your ‘battery’ again. In a sense it is true, because they don’t even have a battery in them. They have a capacitor, which although does recharge, loses the ability to do so effectively in about 8 years, meaning, somewhat ironically, that a kinetic watch lasts about the same length of time without attention as an efficient digital, or a low-load analogue such as the Swatch Skin watches.
So how does the movement of the wearer’s wrist translate into power? It is achieved by an oscillating weight, which is attached to the movement and rotates around a fixed point when the watch is being worn.
There are a few different types of weight, such as a uni-directional weight that rotates fully but winds in one direction only, a bi-directional weight that winds no matter which way the weight is moving, a limited movement weight that bounces back and forth through about 300 degrees, an eccentric oscillating weight, which could be either uni- or bi-directional but is offset in its placement, and more archaic forms of winding weight such as the bidenator (kind of a back and forth motion like a pedometer) and the rarely used pawl lever system (good luck finding one of those).
Effectively, an automatic watch is a mechanical (wind up) watch, that doesn’t need to be hand wound if it is worn frequently, i.e. within the duration of its power reserve, which is usually at least 36 hours, and often much, much more thanks to new technology that has enabled the use of multiple barrels and mainsprings to power the watch.
So why is an automatic better than a pure mechanical?
Here’s the answer: during constant use, the mainspring always stays fully, or close to fully wound, meaning the supply of power to the escapement (that’s the part of the watch that releases the spring’s stored energy) is constant and sufficient to maintain high and steady amplitude (an advantage of storing your automatics on a watch winder).
As a watch runs down, amplitude falls. A fall in amplitude will show isochronous error, curb pin error and exaggerate any poising errors, so the maintenance of higher amplitude, as achieved by the constant winding of an automatic watch, is preferable.