So the Army Cache artifact, rare, says get an army cache of 80%. 80% of what?

100% = the # of armies you would get if you used a Free Cache power

Using an FC gives you armies, resources & money.

I don't even recommend that artifact, but since you upgraded to rare, maybe if you have enough rare artifacts sacrifice it for epic artifact

Maybe that's a better answer - I don't recommend the Army Resource Cache artifact.

The # of armies/resources/money an FC provides is an unknown formula + a random factor, but based on your army/money/resource income. But in all cases, none of these active Cache artifacts return enough to be worth choose over some of the other more useful artifacts.

Yep. For me, SAC+ACB+BMB artifact is the best combination but choose your own lol

I think, both the FC power and the active cache artifacts give you whatever you would produce in some time frame. But I could be wrong. Here 100% of an artifact equals the FC power and whatever time frame is used to calculate this. then, each other percentage can be calculated based on this. If I had to guess, 100% could be something like 1h or 2h or so. But I never bothered to figure this out. If you have other artifacts, I agree with everyone so far and would advise against this artifact in favor of (almost) any other artifact. Only in the very early days I had and used an active resource cache artifact because it literally was the only useful one I had. If you have them, SAC and TW are probably the only active artifacts that are generally recommended, with some others being useful from time to time, like TS/QS. But there are a lot of active artifacts that wouldn't even be useful if they only cooled down for an hour, like Fog Buster for example. And the caches are in a similar ball park.

Also if you use SAC powers or artifact, your current production is higher and therefore you get more armies from FC also resulting in a higher boost of your artifact.