From what I have heard voltage regulators on an alternator determine when the battery has a full charge, afterwards "regulates" the charge being sent. If you don't plug that in the alternator may run at full power a all times, which with a big stereo it's good to keep the power flowing to the amps, but is not so good for the alternator. Alternator may heat up and give out right away, if not it will greatly reduce its life.
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