T480 can hold 2 batteries. A swappable and an internal one. At least under FreeBSD the swappable alone gives about 10-11 hours on low brightness. The internal one carries the rest of the way.

I did replace the screen with a low power screen, which easily had the biggest impact on battery life.

I haven't tested the T480 under OpenBSD in a while, but my guess is that if I ran it with "apm -L", it would get close to the same numbers.

Oh wow, thanks! And a low power screen is what? I thought they’re mostly the same, with probably exception of the most recent oled ones.

It's the Innolux IGZO Low-Power FHD Panel. There are similar ones or same on AliEpxress for example. Compared to the default FHD screen of the T480, The low power screen cuts the power consumption of the display by about 40%. While also having better contrast and brightness. Biggest power-saving upgrade for the T480.