...since it's been left on and plugged in pretty much 24/7 for the past year-and-a-half. The thing is, I want to buy a new battery, but I don't want to ruin it the way I ruined this one. So, I thought I could use my old battery when I leave my laptop on the charger, and switch it out for the new battery when I need to take my laptop on the road (which I would then let die and then charge it back up fully whenever I use it), so I get the best of both worlds. My question is, would this plan have any unforeseen negative effects? Like, hurting my laptop or something.
you have just answered your own question just dont use new battery when plugged in all the time also on my sons vaio he has an option to keep his battery depleated or to charge up 1-100%
If the laptop is made by HP, you might be able to get a new battery for free thanks to one of their many recalls.