Earlier quoted context omitted.
I've tried rebuilding the battery for an x220 twice and both times the battery case ended up too damaged to be used. I'm about to fork over $100+ for my forth battery for this machine.
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?
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.