You're definitely going to have to upgrade. I have a 70 watt amp for my kicker panel and rear speakers, and I had a 150 rms amp for my sub and it killed my new battery. I took my sub amp off and it still killed my batter. So I got a 150 amp alternator off of Ebay and it's doing the trick, and been checking my voltage level on my battery and it's still showing GREAT levels. Eagleautoelectric is the guy on Ebay I bought the alternator from.
Here's a little trick for alternators, a little smaller pulley on the alternator will make it spin faster and be able to put out maximum power at a lower rpm, you may have to make the belt tighter as there will be less pulley surface to belt contact. Good Luck.
A 500 watt amp requires less than 42 amps. The lighting and ignition only needs 13 on a stock Mustang if your alternator puts out more than 55 amps you should be fine. Upgrading is always OK but a 72 amp alternater puts out over 900 watts.
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