Long! Ford Explorer! Related DD help.

My go to site for my explorer seems to be down. Having a starting issue! Get in hit the key and clicks once. Checked all cables and connections and cleaned posts. The battery that was in it I believe is bad. Ive tried another Known good battery and experieced same issue. I can tap on the starter several times and it will fire up with the good battery wont with the weak one. Thinking I have 2 issues a battery that's going bad and starter that is bad. I noticed the other day i had to get a jump start and it wouldnt start till i tapped with a hammer till i got the rapid click. Then jump it. Any help or advice?