i use a UPS as a powersource for my mp3 car system. its internal batt is wired in parallel w/ my car's batt. but every now and then i experience a power shortage for my UPS, especially at nights, when i have to turn head lights from dim to high. im thinking of wiring a capacitor to remedy the problem. what kind/specs of the capacitor should is the best buy? my UPS is rated at 500VA