Dashborad showing battery light?

during driving my battery light came on checked it with battery tester it shows it's working with alternater, the car starts up but the light stays on checked the electrolyte it was low filled it up taken the battery terminal cleaned them still same battery lights up when you starts the battery light dimmming then when hit the accelarater it brightens do i need a new battery had the car for 4 years battery not changed