battery drainage problem

i have changed the battery and charger of my dell laptop 2 months earlier and now from a few days  it was shutting down suddenly without any notification not showing the shuttting down screen just the black off screen when i turned it on it didn;t and when i plugged in the charger it acted like i have taken out the battery and plugged in again it was a reset calender and time and then it started turning off again when unplugged and battery at nearly 40 percent and now it turns off at 90 percent the battery is not draining properly and i have checked the battery working on my laptop and it says battery working normally so kindly someone help me with this