They make capacitors for car stereo systems. It's been debated if they work or not. Most people who do car audio would never be caught with one in their car. If you do it right you don't need anything to help. Just adding another battery won't help because you need more amperage from your alternator. You may just need a stronger battery though. Most people don't need a new alternator with their systems. There is a formula to figure out how much current your amps draw. That, plus having to charge the battery is what you need to have for amperage from your alternator.