Acer aspire 5334 powering off

hi.
My acer battery was getting bad and my computer suggested I change it. Now with my charger plugged in(with or without battery) my laptop will power down after 15-20mins. I have ordered a new battery and will try it but I would have thought my laptop should run fine without one plugged in? Any idea the cause of this? My laptop would run for several hours on safe mode but even now safe mode is being powered down