Opinion on knock-off batteries for laptop...?

I have been retired since 2004. I am now on my 2nd lap top, I always bought the 3 party batteries and they outlasted the OEM battery and the laptop itself. When my current laptop battery started to fail after about 10 years I was ready to buy another one but I removed the battery for awhile then followed some HP prompts to prolong battery life. That was about 3 months ago and battery has lost capacity but is still working. You don't always get what you pay for. I don't know if this is just coincidental but I had an old Oral B with a Li battery that would no longer charge and the company was nice enough to send me a new one. When the new one died also I put the 20 year old one back on the charger about 5 years ago and it still works today. So when one gets weak I just alternate between the two. They won't fully charge when low they have to be completely discharged to come back to full strength. Anyone know why this is?