my car battery goes flat weekly, even though my alternator output is constant 14.3 Volts. The battery voltage does not drop during the night, however, it decreased significantly by afternoon, as if commuting would have drained it. Did you ever experience a similar issue, or do you have a suggestion as to what could be causing this? Parasitic draw is 50 milliamps. Battery is couple of years old, but in good condition.
First off, take your battery and have it tested. It's quite easy for even a relatively new battery to develop a problem where it's not able to output its full rated current or voltage when needed.
If the battery checks out, then your next likely culprit is the alternator/voltage regulator. In most vehicles today, these two are integrated.
There are also shops that will test your alternator, make use of one of them.