I have a capacitor on my system. Recently my battery died and the guy who came to fix it said it was because of the capacitor and thinks it drains energy from the battery. I told him that wasn't what capacitors do but can they drain the battery?
No the cap didn't drain the battery. Your alternator is prolly not up to the task of powering all your equipment. What is the RMS value of your entire system and what size alternator do you have?