ThinkPad

not much to it, undervolt CPU/GPU/cache in small increments (like, -75 mV at a time) until it starts to freeze or reboot, back off a bit, test for a day or two then set and forget. usually -150 mV is achievable and that makes a significant difference for the battery, coupled with turning turbo off for intel_pstate driver. if it doesn't work, bad luck with the silicon lottery and also them models are what 8 years old at this point, so there's also wear and tear in the form of electron migration.

https://wiki.archlinux.org/title/Undervolting_CPU

I haven't played with undervolting on Linux yet, but I do recall that there was the Plundervolt vulnerability a while back, to which OEMs responded with BIOS updates that disabled undervolting.