15 R2, Power Issues

Hi everyone, hope you are all having a great new year so far,

I have a fantastic Alienware 15 R2 which has had power issues for a far too long time now. What is occurring is as follows:

Occasionally when I unplug the power supply, the laptop will perform a hard and immediate shut-down, even if the Battery is showing 100%. Sometimes, I can unplug it and it will stay on like normal, running off the battery power with no issue. It seems as though it stays on when under light load, but when playing a high-demand application such as a game, it will turn off immediately.

On a side not, if I turn the laptop on while unplugged it will always turn off after about 2 seconds. It will never turn on under battery only power.

Now obviously it sounds like a battery problem, so I changed the battery to a new original dell replacement, as well as the power supply to an original Dell replacement incase it was overcharging and damaging the battery, but after these 2 changes it still isn't working correct and had no change in the condition.

Any help or guidance would be greatly appreciated!