You are better taking it to a specialty shop, they can perform all the necesary tests for around $15-25
Heres a quick test:
With a d. c. voltmeter, you can do a quick test to give you a clue. When idling, with accessories off, most cars will charge the battery from 13.2 to 15 volts. You can turn on some accessories, (or headlights) and the battery voltage should stay in this range. If the car has many accessories, you may need to rev up the engine a bit for the alternator to keep up with the load. If the output is low, it could be a bad alternator, bad regulator (often part of the alternator) or bad connections. If the output is high, it is probably the result of a bad regulator, or connection.
Battery voltage and charging voltage:
While you are at it, if you are having trouble starting your car, you can measure your battery voltage (remember to set the range up on manual meters to around 20 volts). The - terminal on the meter goes to the negative battery terminal, and + to positive. A fully charged battery will measure around 12.6 - 13.2 volts with the car off. With the car running, you can measure the battery voltage at idle with lights, and accessories off. Most cars will charge from around 13.2 to 15 volts. If it is lower, check your connections, belt tension on the alternator, and the alternator. If the voltage is high, you may have a defective regulator. Alternators can fail gradually, with a bad diode, for example. In this case the alternator may still work, however not be able to keep up with accessories such as lights and the heater / air conditioner. When this happens, the car battery may slowly run down
Rob Hernandez, AFM co-Founder.