really low rpm's

i just got done installing a 300 watt amp and a subwoofer. i also installed a capacitor to eliminate any lights dimming. i took it for i ride since i had the battery disconnected and i noticed that when i come to a stop the rpm needle was around 200 rpm which is really low it is usually around 800 rpm. what could be causing this? do i need a bigger alternator or a new battery or was it just the computer resetting itself?
any help would be appreciated.