I finished rebuilding my alternator with kit # 11816RK-A, on my 2015 VW Touareg TDI...

Full Question: I finished rebuilding my alternator with kit # 11816RK-A, on my 2015 VW Touareg TDI.... Started the engine, it is not charging (below 12 volts). Now it only charges the battery when I accelerate the engine. I will send you a video of alternator not charging with engine at idle. Before replacement of alternator items, it didn't charge battery at all (when battery light was lit up on dash).

Answer
Hi, I am sorry for the trouble! It is not normal for a voltage regulator to be 'weak'. It is usually either operating or not. If not operating you will see over-voltage or battery voltage (12.6 Volts only what is in battery) and the battery light is on/ stays on. If multiple dash lights are on that could indicate damaged diodes. Diodes will be damaged when the alternator is working over capacity for extended times (HEAT)

The best indicator that an alternator is working over capacity is voltage loss - low voltage. This condition can/will prematurely damage a good alternator.
There are other electric issues that cause voltage loss too and can be tricky to find. We had a car that would not charge at idle. Only when the car was over 1500 rpm would the alternator charge. The customer had installed 3 alternators trying to fix it and thought they were all bad. Turned out to be a bad fan relay that was drawing more current than the alternator could be capable of at idle speed, but could compensate for at higher speed.

There is another that wouldn't charge until 'warmed up'. The car had a small key-off electrical drain on the battery and when starting up the alternator was overloaded while charging back up the battery. Once battery was in fully charged state the alternator would 'start working'... The alternator is designed to maintain a good and fully charged battery while powering the vehicles stock demands.

It is unlikely to get a faulty OEM Valeo Regulator, but anything is possible. It normally would not show anything above battery voltage if it was a defective. Unless it was stuck in full field mode and over-charging.
12.6 Volts = fully charged battery
12.2 Volts = 50% state of charge
11.8 Volts = 0% state of charge

Voltage readings under 10 indicate damaged cell in battery
First recommendation is always battery condition and then maybe ground issue. If you think a ground issue is a possibility, you can jump a ground from battery - to alternator case/mount to see if it helps losses.

Also, See here https://forums.tdiclub.com/index.php?threads/charging-woes.538126/
We are not trying to be argumentative, it is just our experience that your described symptoms 100% are something other than a regulator. It is just our experience though someone else might have different experience. We have never installed the slipring backward on one of these though it would be very easy if not marked and checked before replacement