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.