Auto shut down

My dell inspiron 15 i5559 series just stopped working. I no longer have an original battery since it failed working months ago so i just bought a replacement but it only works for like 1-2 hours so what I did I just used my charger/power adaptor and tookthe battery out so when theres a power outage I still have a back up power supply for my laptop. So 2 days ago it just automatically shut down so I used my battery and it worked fine for like 1.5 hours and since it was already lowbat I use again the cord but get was it just opened for a min or 2 then it turns off automatically. Now I’m not sure what’s wrong it cause they tries to test the charger and said it was ok but it’s not charging the battery there will be like an orange color that’s blinking on the bottom side of the laptop where it will indicate that it’s on ir charging. Now I need some help on who knows what might be the problem? If it’s already the mother board that has a problem on it? I tried to open it with the battery it is still turning on and says low battery need charging. Really need help. Not sure why it works on a battery not with the power adaptor.