Good find Peter !!
From the dedicated equipment list (from new), I see that my aircraft has the S3100-344 (MC01-3A I.C. 10)
From your information letter, I should have the CS-3200, am I right ?
So not the hall effect sensor 
Back to the beginning 
So after today’s flight, few new informations:
- the J-Box / MCU is well a MC01-3A IC.10
- the HSCM, in the config mode, shows that NO OFFSET has been set… Or erased during the battery change ?
- I noticed that the Main (battery) ammeter has 0.5A steps… So (minus) -0.5A is the first step under 0.0A. (different from the stand-by battery, which has 0.1A steps)
- finally, after a short flight (30 min), the ammeter showed in the end… 0.0A, and didn’t go anywhere under !!
Here, a video from the G1000 config mode (HSCM calibration). Notice what happens when changing power source (main or stand-by):
Here is a short video of the sequence, from battery draining to alternator charging, after start-up:
Nice work
I do not like the 0.5A increments in the current display. They deprive one of the opportunity to detect some small fault which is gradually developing.
Also it probably means the error was not 0.5A anyway; perhaps it was 0.250001A or some such
Thanks for the video (which I know takes time to do) and for the superbly informative posts which I am sure will help many others.
Here is the answer from Cessna support:
Good Day,
I highly recommend that the High Side Current Monitor Calibration be done on the airplane. This should correct the issue you are having with the indication. Thank you.
When I inquired them, I hadn’t accessed yet the config mode, so they answered without those elements.
Last report, after 1h30 flight, is the ammeter doesn’t go below 0.0 where it settles as previously.
I definitely think the problem was linked with the HSCM calibration we discussed above.
There must be an offset (even before the battery change), and with the new battery, the condition could have changed slightly…
For now it is normal, maybe the battery break in period ;-)
Or maybe I am somewhere between 0.0 and -0.5, and it will again show negative one of these day.
HSCM calibration would be my next step in that case.
