I have a '94 Toyota Camry 4cyl.
I have a voltage tester that plugs into the accessory port, it shows 6 voltages 11, 12, 12.5 labeled (Battery Test, Engine Off) and 13.5, 14 and 15 labeled (Alternator Test, Enging On). When I plugged it in with my car off, the battery was at 12 volts, which it said was low, 12.5 is labeled normal. Then I turned the engine on and drove it for a bit; the whole ride the voltage was at 13.5 which it says is low for the alternator, and sometimes dropped down ot 12.5 volts (labeled normal for battery). It rarely got up to 14 for normal alternator.
I have two amplifiers for my stereo, which I think may be why the alternator is dying if it is. I had them both turned off during the ride that I was testing the voltage. Is 13.5 volts good enough for an alternator, or should I replace it, or should I wait til it gets worse.. I don't want to get stuck on the road somewhere. Also any other suggestions would be greatly appreciated.

I also noticed the car is starting to have a bit of a rough idle when I start it, I don't know if the problem could be related.
Thanks in advance, Russ.