Testing your alternator with a voltmeter is the easiest and most effective way to find out if it has enough power to recharge the battery. You don't have to disconnect the battery cables to test the alternator; in fact, disconnecting the battery, while the engine is running, is dangerous. Not only do you risk shocking yourself, you risk damaging the alternator and other important engine components. Instead, let the battery be the window to the alternator's condition.
Raise the hood and locate the battery, removing its cover if there is one. Identify the positive and negative posts on the battery. The positive post has a red connector and a "+" sign beside it; the negative post has a black connector and a "-" sign beside it.
Attach the positive wire of the voltmeter to the positive battery post and the negative wire to the negative battery post. Always connect the positives first then the negatives to avoid electric shock.
Start your vehicle's engine. Look at the reading on the screen of the voltmeter. It should be between 13.6 and 14.3 volts if the alternator is strong. If the reading is below 13.6 volts, the alternator is too weak. Take it to an auto parts retailer for further diagnosis. You may have to replace the alternator if nothing can be done to recharge it.
