There are several articles online about how hard it is to store Li-Ion batteries, eg: http://batteryuniversity.com/learn/article/how_to_store_batt...
Although the actual cause might be different (I'm no expert) I've become cautious of super cheap Chinese 9-cell batteries.
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.
So the main issue was not damaging the case? Did you have any references or guides?
I’ve been wanting to order some but obviously don’t want to buy them from some rando off of eBay because I don’t trust that they will
1. actually be new, and
2. that even if they are new they won’t explode in my face
I don't know how easy it is to get them to ship to you, nor how easy it is for individuals to buy from them.
Because the batteries are knackered and new ones aren’t available for old models. Tho’ this does vary by vendor. Chinese knock-off batteries are usually available but carry their own risks.