Time until full keeps increasing on charge.

I've just replaced my dying Macbook battery with a 3rd party Anker battery. It came half charged and seems to work fine but when I started charging it, the charge% goes up slowly but the Time until Full increases as well. So before it was at 50-something% and "1:30 until full". Now it's at 86% and "2:01" until full.

Do I need to worry? Calibrate? Is the battery a dud?

Thanks!