This might not help a lot but here is my FW13 story.

I got it somewhere around 2024/2025 and the battery life was very bad. Like a couple of hours tops.

I was sad, but lived with it for a while.

Then when I started using NixOS (love it) the battery life improved dramatically.

I think it is because I configured some Framework-hardware-specific configuration thingy that was made for the device.

So making sure you have the right firmware or whatever really does change a lot.

I don’t think it’s even close to 18 hours, but it is way better than before.

Also, setting the screen to 60hz instead of 144hz might save you some battery.

Good luck!