Can not solve Battery charging fault on my Xzara Picasso?

The battery on my Xzara went flat and " Battery Charging Fault" message came up and the battery charging light stayed on.. The garage checked the battery changed the alternator. 2 days later the battery was flat. They bench tested the alternator, they tried a new battery. They ran a jump cable from the battery terminal to the battery and registered a charging current but there was no voltage regulation goiong on.
They are currently stumped. They have phone several Citroen contacts with zero results.
Is there some sort of voltage regulation between the battery and the alternator that may be causing this problem.
If battery is fully charged up car runs perfectly with no warning lights?????????????
Ant ideas would be welcome.