Old laptop battery capacity suddenly increased, and now decreased again.

Hello. As title says, I have an old Sony Vaio laptop I've been on daily, using it non-stop for years now. It is on a second, non-original battery. As usual with old tech, its battery is dying, and reached under the 1% of its original capacity (replacements are expensive, so I have to hold on to it until it completely dies). So I'm using it plugged, and occasionally discharging it multiple times. (Laptop can run without battery, but I don't want to take the chance just yet, voltage can get volatile here.)

Then suddenly, this happened:

And while I was enjoying this unexpected "gift", it went down under 2000 mWh last night, and keeps at that level so far. What could be the reason of this sudden spike, and why did I lose it again in an instant moment aswell?