Laptop power cuts in and out when using heavy applications, just looking for opinions

[jester74864]

I have an old computer, and I know the battery isnt running like it used to. Before I go buying a new battery Id like other users opinions on the symptoms I describe below, just in case my adapter might me on the way out as well.
When running applications that work the computer (Anything that gets the fans going nowadays) the power displays power on/power off as if I were unplugging/re-plugging the laptop.
Most of my apps/games/programs I need the battery to be 100% charged,else the battery discharges even with the adapter plugged in. It also takes a long time to get from 99% to 100%
I used to be under the illusion that when plugged into power a laptop didnt necessarily need a battery. Im far from knowledgeable with hardware, so the way ubuntu displays as if power on/off when the computers under a bit of load makes me wonder whether there might be an issue with the adapter as well?
Im running a ~8 yr old alienware m15 r2 if that helps.
Thanks

[Autodave]

I have no experience with an Alienware machine, but normally, there is a processor inside of the battery itself that determines the state of charge, charging rate, etc. My first thought is that that processor has or is failing and the battery needs replaced.

I am assuming that the battery is an easy remove/replace. If so, try one thing before you plunk down money for a new battery: remove the battery from the laptop. Remove EVERY cable from laptop. Let it sit like that for 10-15 minutes. Reinstall battery and then plug in charger ONLY. Power up and see if that helped.

[jester74864]

Thanks for the reply! Ill give that a try