Normally anything between 14 and 15V is a good charging rate. Above or below this and you have problems. It's quick to find out if you have a battery drain... undo the negative terminal on the battery, place and Ammeter between the negative post on the battery and the negative terminal, check the reading. Above 0.5A and you have a problem.
Also try and drive the car a bit more... leaving your battery in a slightly discharged state for long periods of time causes the cells in the battery to go hard and therefore the battery will not charge properly because the chemical reaction taking place inside the cells will not occur properly. Meaning you will have to buy a newer battery sooner... = $$$ =