Honda Cars India Ltd. — Battery charger problems

Address:	Panchkula, Haryana, 134102
Website:	Honda cars India

I have honda city iVtec 2009. I am facing problem of battery voltage variations during running my car. Because of this at night lights starts dim for 3to 5 seconds and again become brighter. Then I check the battery voltage with my instrument and observ that when the car is in idle and stands in stop position then voltage at battery terminals is 14.6 volts, and when I move my car after some time voltage become 12.5 volt for few seconds and again become 14.6 that is why my cars light britness varies and AC fan speed also vary . So it means alternator of my car stops charging intermittently when the car is in motion and when the car is stop position and engine running idle then the charging is ok at 14.6 volt. Can any body tell me the reason of this charging problem at my no [protected]