Long power reserve
Watches with a long power reserve can continue to operate for longer than 2 days without additional winding.
Historically, the mainspring in a watch movement could keep it running for about 40 hours, or just under 2 days. This was a result of many factors:
- Mainsprings must deliver consistent power
- Barrels must be small enough to fit inside a wrist-mounted movement
- Winding tension must be low enough for hand or automatic winding mechanisms
As early as 1913, companies sporadically produced watches with longer power reserve. Some features massive barrels containing very long springs, others used two barrels or more, and some focused on increased movement efficiency. Many of these feature a power reserve indicator to show how much runtime remains. Note that some double barrel movements use the second barrel for other functions rather than longer power reserve.
Long power reserve watches are typically named for the number of days they can run without winding. Hebdomas ("Holy Week") watches feature a 7-day runtime, while others are simply named 5-day, 7-day, 8-day, and so on.
Jaeger-LeCoultre introduced their first double barrel 8-day power reserve pocket watch in 1919 with Calibre 144. This was followed in 1928 with Calibre 134, an 8-day alarm movement, and an 8-day minute repeater the next year. In 1931, Jaeger-LeCoultre introduced the most famous classic 8 day movement, Calibre 124, which was used by many companies for decades. This basic design served as the template for Calibre 879, introduced in 1997 in the Reverso Septantième and later movements.
In 2000, IWC revived their in-house movement capabilities with Cal. 5000, boasting 8-day power from a single mainspring, thanks to the high-torque Pellaton winding system. But this proved challenging to customers, since the varying power of this spring caused the watch to run fast at first, then slow as it wound down. IWC switched to a double-barrel system for the replacement Cal. 52000 series.
Some watches can control the varying force of the mainspring using a a special regulation system, as with the Lange 31 or the DeWitt Academia Tourbillon Force Constante, by which the driving force of the mainspring is held continuously equal during the entire running time.
The advent of silicon components has seen a revival of longer-reserve single-spring movements. By reducing internal friction in the escapement and other components, along with reducing the weight of the balance, silicon movements have quickly advanced. For example, the 2018 Baume & Mercier Cal. BM12-1975A boasts 5 days of power reserve.