The cap takes the strain off the battery. Heres a example for a car.

Battery - Cap - Amp - Speakers

That's how most people do it. Now for my setup when I had the car running and before my carputer was stolen.

Battery - cap - carputer - Amp - speakers

The battery charges the cap and from the cap it goes to what ever device you connect it to that uses high amps and wattage.
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