Tears of the Kingdom – Battery Vials?

One thing that the gameplay footage did not get into were the vials on Link's hip, which have taken the slot of the Sheikah Slate. We can see in the footage that Link only has one in the beginning of the game:

In all the previous material, like the artworks and trailers, he always had four of these green vials with him. But since this device has eight slots, it already looked like the number of vials gets gradually expended from the start.

Aonuma doesn't explain what the vials do and they clearly don't limit the usage of your abilities, like Ascend or the Master Hand. Still, the gameplay might have given us the answer what these vials are used for:

They are the battery for the Zonai devices, like the fans. You can always see the icon next to Link and not to the devices. And no matter how many devices you use at once, there is only one battery icon displayed. It makes sense for Link's battery belt to be the power source for all of this, where he also needs to stay near the activated devices in order for them to function.

Aonuma's self-made hover raft wasn't able to stay up in the air for too long, mainly because it has significantly drained the battery. This indicates that the battery is something that can be upgraded, because after all we saw Link flying with a similar construction high up in the air already:

Maybe this is what he can do with four vials already... Maybe it needs more. But it makes complete sense that this is something that will be upgraded over the course of the game, since it's essentially a second stamina bar. You shouldn't be able to just fly up to any sky islands with your self-made vehicles right from the beginning of the game and this prevents it.

But it also looks like you might be able to find additional power sources, like the big green canister on the DIY car we also saw in the last trailer. At the time it wasn't really clear whether this was an object for transportation or a power source, because we didn't see a similar power source with the flying machine. But it all makes sense when Link has a small power source of his own, but can also hook up additional power sources. My guess is that these canisters won't refill automatically, unlike the vials on Link's belt, because again there needs to be some limit.

PS: There is also an excellent video about this by Monster Maze, who had the exact same theory. To be honest, it came out around the same time I was going to write this, which stole a bit of my thunder, but I wanted to finish this post nevertheless.