2004 touareg alternator output?

Hi all, Im after some technical help, i have a 2004 touareg 3l v6 tdi bks, im having issues with slow cranking, i replaced the battery a few weeks ago, and then it started happening again, ive checked charging voltage on a new battery and im getting around 13.9v however its the current output that concerns me, with an amp meter im charging at 13 amps at peak, which slowly drops to 2amps even though the battery needs charge i.e. its cca should be 750 and it was 650 before starting. Makes no difference on load/off load, idle or 3k rpm, nothing changes. Im suspecting the alternator however the dfm signal is what im unsure about, does the alternator provide an output to the ecu from the dfm pin or does the ecu tell the alternator how much current is required? Ive tried voltage drop tests, all my wiring is fine. Any advice would be greatly appreciated

What country are you in? Something doesn't sound right with the brush/regulator assembly or perhaps the clutched pulley on your alternator is slipping.

How many miles on your Treq? On my N/A Jetta, I polished the slip rings and replaced the brush/regulator assembly at 185 K miles. The brushes were not to the point of arcing yet so I didn't have to replace the slip rings, just polished the grooves out of them.
The bearings were fine and the clutched pulley with about 100K miles on it was fine as well.

In the uk, i wasnt sure if it had a clutched pulley, its a 190a water cooled, so im reluctant to repair it myself, new one quoted at £400 which isnt horrendous however if it turns out the engine ecu isnt telling it to output enough amps its a waste of money, thats my concern, im not sure what controls the load output