my daily driver is the Thinkpad T400, the most powerful laptop that you can libreboot to remove completely the proprietary Lenovo BIOS in favor of a fully open source BIOS. The distro on it its Parabola GNU/Linux-libre: a 100% free distribution.\
I have done a series of mods on this machine, and more are to come. What follows is a list of some useful resources on the T400 itself and the mods compatible with this device.
The Libreboot project provides free, open source (libre) boot firmware based on coreboot, replacing proprietary BIOS/UEFI firmware. I made a guide on how to do that. It's [here](https://bytemeifyoucan.lol/p/my-t400-libreboot-guide/).\
After librebooting the T400 any hardware that requires proprietary blobs will need to be removed and changed. In the case of this model you will most likely only need to replace the wifi card. One of the most suggested is the Atheros AR5BXB112 AR9380. It is a 3x3 450mbps WiFi card which works perfectly with free drivers and firmware, it should cost between 10 and 15 euros on Ebay or similar sites.\
It is possible to install a series of quad-core processors on the T400, taking advantage of the fact that they have the same socket of the dual-core Core Duos that come with stock machine.\
The original post can be found [here](https://thinkpad-forum.de/threads/core2-quad-mit-coreboot-libreboot-auf-t500-wahrsch-auch-t400-benutzen-beta.199129/). (in german)
Cooling mods really do exists out there, even tho I have never tried one on my T400s, considering i have heard mixed opinions about the actual improvements when it comes to the CPU degrees. Mileage may vary.\
All guides follow more or less the same principle: remove the metal sheeting from the plastic bottom casing and drill holes in the case.\
Examples of the procedures are [this](https://thonkpeasant.xyz/guides/other/cool.html) and [this](https://www.instructables.com/Fix-a-Thinkpad-T400s-Thermal-Issues-Once-and-For-/).
The bluetooth of the T400 is 2.1, you can easily upgrade the bluetooth 4.0 by swapping the card located behind the LCD front bezel. Most recommended model is **Bluetooth 4.0 Genuine Lenovo FRU 60Y3305 60Y3303**.\
More info [here](https://www.thinkwiki.org/wiki/Bluetooth_Daughter_Card_slot).
The most commond usage for the ultra bay, when a cd reader is not needed, is to use an Hard Drive caddy module to have a second 2.5 inch drive. Parts are easily available for about ten euros.\
Another rare but worth mentioning accessory is the [Serial Ultrabay Slim Battery](https://www.thinkwiki.org/wiki/Serial_Ultrabay_Slim_Battery): a battery pack that slides into a Serial Ultrabay Slim. It has three 3.6V lithium-ion cells, for up to 2.3 hours of battery life.\
I am currently working on a variation of a mod I have seen floating around the world wide web. It's [this one](https://hackaday.com/tag/thinkpad-ultrabay/). In the article the laptop used is the T420 so I am changing the 3D printed part to fit the T400 ultrabay.\
The T400 Express Card slot allows you to slot in any compatible Express Card 54 and by extension any Express Card 34. One common example is using a card with one or two (depending on the model) USB 3.0 plugs, with up to (i think) 5Gbps transfer speed.\
Other types of Express Cards can be found scattered around the web, from the classic card reader, to different port extensions, to SD readers, to even modules for eGPUs with awful performances.
The LED model has a way better screen and is quite rare at this point, few are left on the marked. If you have the change, always go for the LED model.
It is possible to replace the CCFL screen with a LED one, like [this one](https://www.panelook.com/LTN141BT04-002_Samsung_14.1_LCM_overview_24142.html). Remember that aside from the screen you will need a new ribbon cable and a new inverter. Search for **LCD Cable for LED (PN: 93P4592) and LED inverter (42W7949).**
Battery life on these old devices is not ideal, OEM batteries cost alot and garantee only a short period of charge, while off-brand batteries range from being unusable after 25 cycles to being a straight up scam.\
T400 batteries come in two different shapes and sizes: a 6 cell battery and a 9 cell battery. There are people experimenting with building their own Thinkpad batteries. It is not impossible but requires a good amount of knowledge and experience on the matter.\
An example can be Alexander Parent which built his own T420 battery and made a guide on how he did it [here](https://beta.aceparent.me/#/battery). The repository with the pbc files is [here](https://github.com/iam4722202468/ThinkpadBattery).\
Another example is the Youtuber polymatt who made [this video](https://www.youtube.com/watch?v=9PaTKBc88CI) on how he cloned a ThinkPad 701 battery.\
A reddit user posted a comment saying he succesfully did a re-celling of a x230 battery [here](https://www.reddit.com/r/thinkpad/comments/8ue5fz/comment/e1fxtec/).
In summary, it is theoretically possible to disassemble a T400 battery and change the cells with high quality ones and have a functioning battery. It's gonna be tough.
The jellow barrel jack port used to charge the device can be changed in favor of more modern USB C one. This is a common mod in most old T and X series devices.\
Projects for most models can be found with ease while surfing the web, [here](https://www.reddit.com/r/thinkpad/comments/l5g9cj/convert_a_lenovo_laptop_with_square_power_input/) is a mod that removes the square power input, granted it is not the T400 input, but the procedure is very similar.\
Parts needed to do this modification can be found at [this shop](https://www.tindie.com/stores/mikepdiy/).
The distro i use on my librebooted T400 is Parabola GNU/Linux-libre, a community-driven, "labour-of-love" effort to maintain a 100% free (as in: freedom) operating system distribution that is lean, clean, and hackable.\
I made a guide on the manual installation of Parabola with an encrypted setup and Open-RC [here](https://bytemeifyoucan.lol/p/parabola-installation-guide/).
