Can anyone confirm how the charging system works on a 2013 Versa SL 1.6? Does it have a variable rate? Charges upon start up then drops. Thanks.

ALL automotive charging systems charge variably, they have to, the engine never stays at the same rpm.

It can be normal to charge slightly more right after a start, the alt putting back in what the start just used up in power. Past a certain point though it should not drop. The alt output should never really approach the same voltage as the battery by itself, it should stay somewhat higher.