Like everything else political, I think whether or not one views a candidate, a person as being charismatic is a personal opinion. For my purpose it was pitting candidate against candidate. How one's charismatic personality can attract voters to him that otherwise may not have voted for him.
The top four according to Larry Sabato was FDR, JFK, Reagan and Bill Clinton as having dynamic charismatic personalities. I'd have to add Obama to that list, while in my opinion, Obama had less charisma that those four. I was also talking about attracting voters from America as a whole, not just one's supporters.
Supporters of certain candidates are usually entranced by that candidate. But how that candidates fairs with those who aren't his avid supporters is where charisma comes into play, especially with independents, the non-affiliated voter.
I do think Sanders had more charisma than Biden, but we're talking about the Democratic primary voters. Less than a third of all voters nationwide. Sanders didn't have a chance to test those waters and to see if he could or couldn't attract the independent voter. He was tested only by Democrats for the most part although some independents did vote in the open primary states. The primaries are more about name recognition and political stances, ideology, party loyalist.
You haven't yet got into the battle for the less political, the less to non-partisan and non-affiliated voter. They haven't had a chance to weigh in yet.
There's no doubt in my mind that Biden isn't charismatic in the sense of a Bill Clinton, Ronald Reagan, JFK or even an Obama. But not being Trump may be enough. Hillary certainly wasn't, her personality wasn't the type to attract independents. Neither was Kerry or Gore, two statues. McCain didn't have it either nor Dole or G.H.W Bush. If you don't have it, you better hope you get matched up against someone with less charisma than you which happened in 1988, Bush vs. Dukakis, 1968 Nixon vs. Humphrey, 1976 Carter vs. Ford and so on.
Perhaps the most important thing is how a candidate and as president later on can connect with the people. Once again the best at this were the four Larry Sabato pointed out. FDR, JFK, Reagan and Bill Clinton. then in fifth place, I'd put Obama.