I am confused. I have a Thinkpad X220 I bought used. The original battery is still usable for my purposes. (I don't use it for travel so I haven't tried using it for more than a few hours at a time.) I just looked on Ebay and the most expensive batteries listed for the X220 are less than $50, with some less than $20. Am I missing something?

As a rule of thumb, any laptop battery for less than $50 is recycled crap.

There are companies who mix-and-match recycled cells and combine them into Ebay batteries out there. Beyond the explosion risk innate to Li-Ion, you're going to get bad battery life with these recycled cells in the very best case.

The cheapest "legitimate" laptop batteries I've found come in the ~$80 mark.

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A good and proper 18650 cell costs $7.50 alone, and you'll need six of them to build a laptop battery.

https://www.amazon.com/Samsung-INR18650-25R-Rechargeable-Bat...

That's $45 for 6x proper cells straight from the manufacturer. Plus labor, plus all the other parts of the laptop battery (chassis, protection circuit, etc. etc.). Anyone selling for less than $50 is straight up recycled mix-and-match junk. Its the only way you get to $20.

And before you go look up cheaper cells: Ultrafire cells are recycled junk. So ignore all of those listings. Its best to stick with Panasonic / Samsung / other name brands.

Man, I was thinking about taking on this challenge next. I have 2 of the extended batteries that work excellent but the regular battery wont hold a charge for more than a few hours.

So the main issue was not damaging the case? Did you have any references or guides?

The cases aren't meant to be opened. I followed some guides on YouTube. Most require using a saw and really sharp knife.