almost free rust removal using battery charger and washing soda?

Muriatic acid is a mild concentration of hydrochloric acid. It is sold for cleaning bricks (i.e., fireplace cleaner) and for adjusting the pH in pools but it also attacks iron oxide which why it works for removing rust. Hydrochloric acid is widely used in industry for pickling steel to remove rust.

Phosphoric acid is also used for removing rust. It is also the main ingredient in the metal preps that are sold for promoting paint adhesion. All the "rust removers", such as, Navel Jelly, you can commercially purchase have phosphoric acid as the active ingrediant. You can quite often purchase phoshhoric acid at the hardware store but the concentrations available make it less agressive than the concentrations of muriatic acid that are available.

Mix either with baking soda will nuetralize it making safe to dump down the drain.