2013 Ford Fiesta SE 5 speed. The battery has been in my car since I bought the Fiesta new. It is almost 6 years old. No problems. Beginning to get a little corrosion on the negative post. How do I assess the health of the Battery (Motocraft BXT-96R-500) How can I tell when the battery needs to be replaced short of it refusing to work. That exact battery is no longer available. Back in the day I learned to disregard paying for many year batteries, and just buy the heaviest one of the same physical size (lead acid ones). Is that still the way to go, or has technology of lead acid batteries changed quite a bit. There is a peep hole on the top of the battery, but I don't know how to interpret what it displays. Nothing in the owners manual about it.