I have a problem with battery charging in my ambador car?

Recently I bought a new exide battery for my car, after 15 days the water level went below the low level and got it topped with the dealer. Immediately after adding the distilled water, my car did not start and had the battery charged. Again after 10 days the battery got discharged, when went to the exide dealer again , he said that there is something wrong with the alternator. Then i had my alternator serviced, still the voltmeter reading is only 12.5 volts with the engine started and the battery is not charging. please help me in this regard.