11 volts is too low. A properly working charging system must be 13.5 to 15 volts when engine is running above 1200 rpm or os.

Now the problem is usually one of two things:
* Alternator (less often)
* Regulator (more often)
Don't pay money for a new alternator when it is more often just the regulator.

Less like it could be bad wiring. Check the voltage at the alternator, not at the battery. If it is 14V there, but only 11V at the battery, you have bad wiring.