So I found out one of my car's battery terminal is missing one of the battery terminal protector. One that is similiar to the one sold in Autozone. Is it really needed to get one for the missing terminal side?
That basically just looks like a spacer to help prevent the terminal resting too low down the battery post, a terminal that is resting lower down the post MAY be slightly more susceptible to corrosion but to be honest if the battery is not leaking etc and the terminals are kept clean there is no need for this spacer. If the terminals are corroded or even just look crusty.. Take them off, clean them up using a wire brush.. and coat them with petroleum jelly/vaseline to protect them.
While these are not required they do significantly reduce the maintenance needed for cleaning terminals and can prevent damage due to acid corrosion. The following type works very well as they are embedded with a gelled oil that neutralizes the acid that almost always leaks from around the battery terminal post. We install them on every battery we sell and encourage their installation after terminal repair or cleaning.