The standard for years has been the Balmar MC-614. There are lesser Balmar models and some competing brands that do about the same thing.
Top of the class right now is the Wakespeed W500. The main difference is that the Wakespeed actually observes the charge going into the battery (as well as voltage) and regulates based on that. The Balmar and all the others only guess at the charge, inferred using voltage as a proxy for current which can have large errors. The downside of the Wakespeed is cost.