Start the car and use a voltage tester across the battery terminals. If the voltage reads 12.5V- 14V your alternater is fine. Means your charging. Is this your factory voltage meter? If so, it could be a loose wire at the meter or other end. It may also be a cracked wire which with normal road and engine vibration is intermittently opening and closing the circuit. If you can, run 2 wires to battery with a voltage meter. Install a temporary switch on positive wire to turn on/off voltage meter. Watch the new voltage meter, if it reads continous voltage above 12.5Volts then it is your other voltage meter or wires to it or possibly the voltage regulator.