How accurate is your voltmeter? If your battery is at 12 volts with the car turned off - it is on it's last legs. A good battery should be 12.56 volts MINIMUM - 12.76-12.95 volts optimum. Just changed the battery on my wife's car - it was at 11.9 volts with the car off (car is a VW Diesel) - only found out about the battery when it got real cold overnight and the car was left outside. Went to start it in the morning and it cranked 4 times and the battery died. I charged the battery that night and let the battery sit without the charger on it overnight - the next morning the voltage was 11.9 volts - battery was toast. New battery sitting overnight has a charge of 12.93 volts on it in the morning. If your battery is truly at 12 volts - you have no reserve for your accessories with the car on and the alternator has to try and carry the full load. Have your battery load tested if it really at 12 volts - you probably have a bad cell and need a new battery. By the way - some bosch alternators will charge up to 14.7 volts with 13.9-14.2 being the norm (at about 1500-2000rpm) on all the cars I worked on. Most automatic battery chargers take the battery to 14.40 volts before shutting off or going to maintenance mode. If your alternator is charging at 15 volts at idle - your voltage regulator in the alternator is probably defective - but could be trying to compensate for a battery that is sucking the amps out of the alternator because it is unable to take a charge..