Charlie Sheen has an upcoming autobiography and a Netflix documentary about his life … and while his ups and down will be well-documented, TMZ has learned something he’s never talked about … he’s been one of the most charitable celebs in Hollywood.

The actor’s going through some lean years financially, but sources with direct knowledge tell us when Charlie was rolling in money — in the “Two and a Half Men” days — he donated between $26 million and $30 million to various charities.

We’re told Charlie’s massive donations were spread around a few noble causes … including Autism Speaks, cancer research and projects aimed at helping the homeless.