Battery charge suddenly drops from about 25% to 6%

Sorry to say it, but I don't think there's another answer to be had; what you are experiencing is usual for a battery that needs replacing.

Their expected life-span is only about 1 year (according to the manufacturers).

The reason the readings act weird is because when the battery becomes old and faulty it can't properly gauge the total capacity or current charge, so the gauge readings are based on inaccurate numbers.

There is some problem with the Lenovo Battery/UEFI Firmware. The Battery Manager on Window or tp_sampi module under linux is usually configured to extend the batteries life by only charging it to 85%.

But discharging the battery from 85% (eventually somehow related to hibernate) can cause the batteries charge counter to drift off more and more over time. The result is a sudden drop of battery life at some point (in my case, at around 45% it suddenly drops tp 6%). The battery reports 85% way too early.

People reported that setting the thresholds to 100% mostly resolved the problem after a few full charge cycles.

I know this is an old issue, but I've had the problem myself and the solution was to charge it fully, then leave it plugged in at 100% for about 2-3 days. After that, it discharged normally without any sudden % drops.

The issue I was having with Arch linux using TLP's battery re-calibration: the battery percentage would suddenly drop from around 60% down to 6% during the re-calibration itself. I didn't try discharging it normally, previously. The problem occurred after an extended period of leaving the laptop plugged in at around 50% capacity.

I've reset the charge thresholds on my Thinkpad now, and I'm going to observe how it works. If the problem re-appears, I can simply re-calibrate it the same way again.