altenator without a tester

Do not do this, you run the chance of doing serious damage to the ECU as disconnecting the alternator with the engine running can induce a voltage spike that can destroy delicate electrical equipment, only use correct testing equipment.

Just to add to this i agree buy your self a multimeter you can get them for 10 or less.Have the battery tested first if its bad replace it then make sure your battery cables are clean and tight.Then start the car up see what the voltage at the battery is then turn on the headlights ac etc and see what the voltage is let us know what you find.