@Kiev Thanks very much for the low voltage power supply section.
The 12V output of the DCDC to the aux battery during a charge when the fluctuations are happening remains stable around 14.2 /14.3V.
Do you know where the AC coil relay 5V supply comes from? Which 5V regulator? Where is the transistor that drives the 5V for the coil? My eyes are not good anymore even if magnifier lents...
For your info, at the moment I put an external power supply at 5V that supplies the coil and it is driven by a transistor which base is driven by the original 5V for the coil. In this way I do not charge of the 70 mA necessary to the coil from the original circuit.
Monitoring the original 5V over time the fluctuations are much less in amplitude (the drop of voltage was down to 4.6V instead of 2.6V).
In the second charging session (on the way just while writing) such original 5V looks even more stable (not less than 5.06V).
Of course this is not a cool solution and generally I want to go to the origin of the problem.
But I want to avoid to remove the lower board (which would be great for deep checking specially where the IC702 and IC317 are located and relative components around them).
Also because the car has almost 120K km and the pack has many cells very weak, limiting the Range at 70 km only , in this season.
Another (not so cool, I agree
) solution that I already prepared (but not tested yet) was to I put an external power supply at 5V that supplies the coil and it is driven by a transistor which base is driven by an RC circuit with 2-3 seconds of delay. In this way the AC input relay closes its contacts (and keep them closed) when there is power via EVSE, apart 2-3 seconds at starting point necessary for the precharging the big 3 caps.
Do you or sameone else know if that AC input relay works just for the precharging circuit?