hi ,i have bought a good condition dell latitude 7480 laptop and after using it for some days it came up with an issue that after i charge my laptop fully 100 then i unplug it and use it in intervals of 10 to 20 mins for two days but when i shut down my laptop on 40 t0 50 percent on the end of second day and use it on the third day with that remaining battery it instantly drops from 40 or 50 or how much battery was remaining,direct to 6 percent plz help me out that what to do or is that really an issue