Test your voltage regulator:
Step 1 Start your car, make sure your vehicle is in park with the parking brake applied. Open the hood and find the battery.
Step 2 Carefully place your black meter lead on the negative battery terminal, and the red meter lead on the positive battery terminal. Now turn your meter on, placing the selector switch to the VDC position 20 Volt range.
Step 3 You should be reading around 14 Volts on the display, if it is much lower, than your battery is dying or dead, and the alternator/regulator is not functioning properly, causing the battery not to charge. If the reading is higher than 14 Volts, than your voltage regulator is bad, and will either need replaced or adjusted. Again, on most newer cars, the voltage regulator is part of the main alternator.
Regulator adjustmentAdjusting an external voltage regulator
If you find that your voltage regulator is out of adjustment, then there are methods to adjust on some models, sometimes this is a viable option. Check your vehicles repair manual, it may have listed procedures for your type regulator. If you do not find adjustment procedures in your repair manual, look for an adjustment screw/bolt under the cover of the regulator, and with the voltmeter connected, adjust it until it reaches the desired 14 Volt range. Note: Not all models can be adjusted, be sure you research your particular model.
Also, another couple of reasons to have low voltage at the battery while charging are loose or corroded grounds. Look really closely at all the grounds for tightness and corrosion, as this will cause high resistance in the charging circuit, and the alternator may not have enough juice to overcome it.