First off, I think you’re completely right in that laptop batteries are definitely a non-ideal solution. And, I’m really not an expert in this, so take my words with a grain of salt.
You could mitigate a bit of the dangers by doing some of the following (I only did the first):
- Reducing the max charge level to 50% of the capacity.
- Monitor your batteries health to alert for any discrepancies.
- Switch out your batteries every couple of years (which is super easy without downtime on the aformentioned old thinkpads).
If you are an under $100 budget, there seems to be an argument that maybe you are willing to risk a little bit for that extra power reliability.
From experience, older thinkpads usually sell for cheap, come with an inbuilt monitor, and are built sturdy. Highly recommend.