Long after I got my P14s gen 5 AMD, I always found one thing wrong with it. The tiny 52.5Wh battery. Absolutely hated it and wanted a bigger battery. I had two options
Option A: get the 57Wh from lenovo for the T14 gen 6 (fits and works) which was useless for the upgrade size)
Option B: mod it into an entirely new laptop
(FOR THE P14S GEN 5 AMD/T14 GEN 5 AMD USERS: Numpad section of the keyboard is not functional however everything else works including the glass touchpad, this is because the EC lacks support for the numpad) (Tested on all thinkpads listed)
(THIS is NOT the case for T14 gen 5 Intel users or T14 gen 5/6 AMD users + P14s gen 6 AMD users, everything is functional)
(THIS GUIDE IS NOT FOR P14S INTEL USERS)
The motherboard layout used in the T16 gen 3 and 4 is the same as the T14 gen 5/6 AMD + intel and P14s gen 5/6 AMD only machines.
Benefits of this upgrade is:
Vastly bigger 86Wh battery that runs for much longer. Numpad for those who need it and yes it can have a glass trackpad from the X1E/P1 gen 5/6 !!!, I can confirm it works just fine !. Also bigger screen at 16 inches which is what I personally prefer.
What you will need is almost every part apart from the motherboard or heatsink for your model. Yes its quite an expensive upgrade but it does work and it was worth it. You would wanna have this mod because you get a T16 built for cheaper than what's commercially available. Most parts I got was a mix from the aliexpress store and the lenovo store (total cost will be a lot less if you get from aliexpress but since I needed a functional laptop fast I got from lenovo)
You would need
Entire display assembly including hinges (140 total since I used mix parts from lenovo and Ali)
Palmrest (29gbp from aliexpress)
Trackpad glass one from X1E/P1 gen 5/6 (33gbp from aliexpress) (VERY VERY VERY much worth it over mylar)
Keyboard (29gbp from aliexpress)
Bottom cover (25gbo from aliexpress)
Trackpad cable (25gbp from lenovo)
Right side IO cable + Board (65gbp in total, yes ouch, from lenovo since I couldn't find these on aliexpress)
Battery 86Wh (highly recommended to buy official lenovo one its at 81gbp but aliexpesss has 3rd party ones at 35-45)
What you can carry forward is the:
Motherboard
Heatsink
Power button.
You can carry over previous antennas if you're careful and properly fix them in the lid
(Since the motherboard is the main part that costs near 500-700 depending on what you have)
The mod is very much worth it since T16s go for 1200-1500 depending on where you live, so a T14 gen 5 here is 550, parts all in all + shipping 380 ish so you can build one for a lot less)
I built one on AMD but I needed the numpad so I switched my board out for Intel.
Hope this helps anyone get into the modding rabbit hole of thinkpads and wants extra battery life + all that.
Any questions please ask away !.