Originally posted by TheMark:
What 2X95SE said.
Check the voltage across the battery with a multimeter, should read in excess of 12 volts if all is well, with the motor runnning.
12 volts is not enough. If you only read 12 volts you will most certainly have a problem.
The proper process was already described. Check the battery voltage with the engine off. It should be about 12.5 to 12.8 depending on the state of charge and condition of battery. With the engine running, preferrably at about 2000 RPM, the voltage should be between 1/2 volt and 2 volts above the reading that you had on the battery with the engine off, depending on the state of charge of the battery. A low battery will have a lower voltage reading and a well charged battery will have a higher reading. 14 volts running is a reasonable rule of thumb.
If the alternator is not charging, make sure that the fuse in line to the alternator hot lead (B+, or battery plus, or main battery lead to the alternator) is not blown. This can be confirmed by checking voltage directly at the alternator B+ connection. The voltage there should be the same as the battery voltage.
