When people gave connected an Orion direct to an alternator they have reported voltage swings as the alternator voltage regulator interacts with the voltage control on the Orion input. Both controllers are trying to control the same voltage by varying the same current. A battery decouples supply and demand current and buffers the voltage / current response. Not saying this will definitely happen but it is seen frequently. Alternator to battery chargers have different input voltage / current control to avoid this.
Can not help on the capacitors size.