If you've replaced the stator & regulator & voltage drops & it charges erratically, then you most likely have a short of some kind.
Your thread heading states you have a 2013 RSX 850i.

  1. Is your battery original? Determine if you need a new battery first by testing it. If battery is fine...
  2. & all electrical parts are preforming as expected, than check main harness & secondary wiring for frayed outer casings & resistance. Electrical parts generally aren't returnable, eliminate all other possibilities before buying them.
  3. The wiring inside maybe corroded, possible after 7 years. Unplug wiring & install temporary harness to check resistance & corrosion theory.

This is one of those take it apart 1 piece at a time & determine what is faulty projects. Remember it maybe multiple pieces of wiring that are bad.