Volkswagen Repair: Starting Problems, cold winter mornings, volkwagen golf

Question
1989 Volkwagen Golf MK1

In the mornings, particularly cold winter mornings, the car would totally not start.  I observed that the battery is flat.  Bought a new battery and started it.

The following morning, the car wouldn't start again with the new battery.  The battery was totally flat.

Would you be of assistance in this regard in terms of what causes the battery to flat out within an evening.

Answer
Arthur, thank you for your question.   This is a brain teaser.  There are many causes for a battery to go dead overnight. Sources of battery drain are: the clock in the radio, the onboard computer, a short in the electrical system (somewhere).  A failing alternator, voltage regulator, solenoid on the starter and the starter itself.  Any of these can cause the battery to go dead.  The most probable is the voltage regulator (sometimes internal to the alternator).   The last time this happened to me, it turned out to be a bad alternator. The time before that it was an aftermarket clock that was installed.
Hope this helps