Why is roomba not charging?

My Roomba 625 is not charging. I already cleaned the contact points and replaced the battery (my friend had the Roomba stored for a couple of years before gifting it to me). The dock works. When I put the Roomba on the dock, the clean button lights up green and the battery light lights up yellow/orange. It also makes a beeping sound, like a little song. It does this and then goes dead. Then 10 seconds later does it again. I’ve reseted it. It won’t let me run a diagnostic. I’m out of ideas. Help!