The Coiled Spring
The Coiled Spring is an amulet made of brass. It’s disc shaped and about an inch thick. On its front, there is a relief of a snake eating its own tail, the classic Ouroboros. The inside of the ring formed by the snake is a clear crystal panel, through which can be seen a spring like that of a watch.
That spring can store up to ten Fatigue Points which can be used to enable bursts of extra effort - most often Extra Effort in Combat (p. B357), but also other athletic feats. The stored FP can only be used for those purposes, and is not affected by negative effects that would sap the user’s own FP. In other words, it works just like the Heroic Reserves advantage from GURPS Dungeon Fantasy 20: Slayers, and almost like a power item for martial characters.
The main difference between the Coiled Spring and a normal Power Item is that it hijacks the natural recovery processes of the wearer’s body in order to “wind” itself, and cannot be recharged by the usual methods for power items. While the Spring is not at full capacity, its wearer is unable to recover their own Fatigue Points, and the amulet gains 1 FP every 10 minutes. This is a constant rate - traits which speed up FP recovery for the wearer have no effect on the amulet. The wearer regains their usual ability to rest when the amulet is fully charged, or when it’s removed (which stops the charging process).
While this does mean recharging the Coiled Spring in town is effectively free, it can make dungeon delving more complicated for martial characters, particularly if the GM enforces rules like fatigue costs for fighting a battle (p. B426 or DF Exploits p. 60). Characters who normally can’t use Fatigue Points can wear the Coiled Spring and use it to benefit from the Extra Effort rules in and out of combat, though they can’t recharge it.
Scholars who know of the Spring speculate that it was originally meant to allow golems and other constructs to surpass their limits. In an adventure, it will most likely be found worn by such a construct, or by an elite warrior who figured out it works just as well for organics. If found in a chest or hoard, though, it will always start out empty.
If the delvers try to sell the Coiled Spring, or if you decide it’s available in shops, its monetary value is twice that of a Power Item of the same capacity. That’s $3400 for the 10 FP version described here, though you could have versions with greater or smaller capacity. Note that you can’t double-dip and use the Coiled Spring as a Power Item.
Variant Springs are possible. Maybe the Flywheel of Faith can power divine magic, the Arcane Array wizardly spells, and the Goddess Gyre druidic ones. The recharging mechanics and all other restrictions are the same. They could allow a construct who can’t normally cast spells or use casting magical items due to a lack of FP to do so.