With 24+ hours per charge on a lot of these modern laptops, why do you need to carry spare batteries?
Instead of spare batteries, you should probably be looking to use equipment more recent than a Thinkpad. You get far, far more "laptop usage per watt" on a modern system than anything 5 years or older. There's just all sorts of power-saving features on a modern system (most importantly: fundamental shrinking of the transistors down to 5nm or smaller, using a small fraction of the power compared to larger 22nm or 24nm transistors)
Ex: The new "P vs E" core on Windows 11 devices. The new sleep features on DDR5. NVMe using far less power than a hard drive. Etc. etc. Modern systems use an order of magnitude less power yet offer greater compute power on the go. The drive for better transistors is strongest in the laptop form factor IMO, since smaller transistors hit the "do everything": faster, cheaper, less power usage.
On top of that are the software tweaks. I know people are pissed off about Windows 11's new GUI, but the scheduler is solid and the support for Intel's low power "E" cores, plus GPU sleeping / other power saving features, really makes a difference in terms of battery life.
> I don't have any USB-C device (not everybody lives in a first-world countries) but at least once per year I actually need at least all 4 of my good old USB2 ports. External USB hub will suck in stability of connect and ability to charge several devices at once.
USB-C is something like 4GBps.
A singular USB-C hub can support 20x USB 2.0 A devices at full speed (each around 50MBps). You have all the bandwidth you need from USB 3.0 / USB-C connectors. You just need a hub so that the port can be "split" into more physical ports.
> External USB hub will suck in stability of connect and ability to charge several devices at once.
So right now, you carry a bunch of extra laptop batteries around and USB 2.0 ports that you only use ~once per year (or so).
When instead you could be carrying around a Li-ion charging station, using more efficient laptops (to get 24+ hours per charge, negating the need for spare batteries), and using a $30 USB-C port+hub on the rare occasions you need a large number of simultaneous devices on your computer?
It sounds like what you really need is a portable Li-ion battery.