Just adding to what everyone else has said:

With ignition off, your battery output should be a nominal 12V or just a bit over. That is the voltage output of the battery, period. Anything less indicates a weak battery or one that has been drained and not sufficiently recharged.

With the engine running at idle and all accessories off, you should be seeing 13.8-14.V volts. Only the alternator will be providing that type of voltage, not the battery. If you are seeing anything less, then the alternator is not providing enough juice to drive the accessories and simultaneously recharge your battery.

If you have confirm belt tightness, then most commonly, either the voltage regulator has failed or the brushes in the alternator have worn out.

You have mentioned that you are running high watt amps...these require a pretty healthy appetite of current, and the stock 80-amp unit may not be up to the task. Alternators are the first to go when someone brings in a car with a mega-watt sound system that has charging problems.

You might consider upgrading to the high-performance alternators, which are designed for heavy-load accessories. They start at 100-amp units up to 250 amps, and I have seen setups in SPL sound-offs that run several in tandem!

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car