Freezes on battery power, even after replacing battery/hard drive??

Hi, I am a user of a HP Probook 4420s laptop. Everything works perfectly fine except that when I unplug the laptop from the charger, after about 5 minutes the screen freezes and my wifi light turns off. I have to hold down the power button to restart it.

Thinking it might be a problem with the battery or hard drive, I replaced both of them. But the problem still occurs. What else could it possibly be? I'm not expecting an exact diagnosis, more just suggestions as to where the problem could stem from/what course of action I could take to figure this out.

Thanks so much!