I found the following procedure, in this document G1000 Nav III Line Maintenance Manual Rev. L 190-00352-00
2.7.6 Main Battery Ammeter Calibration Procedure
Perform the following to calibrate the main battery current indication (ammeter) after the 0563.06 software is loaded:
1. Ensure all aircraft switches are in the off position and all circuit breakers are IN.
2. Disconnect the J2 connector at the J-Box.
3. Using a power supply with an ammeter apply 28.5V to any pin on the J2 connector (to apply power to the current shunt inside J-Box). Reference the power supply to aircraft ground (GN002).
4. While pressing and holding the enter (ENT) button on the PFD, apply power to the essential bus by turning the S BATT switch to ARM. Release the ENT button after the words “INITIALIZING SYSTEM” appear on the PFD. This will put the PFD in configuration mode.
5. Keep all other switches OFF.
6. On the PFD navigate to the “CAL” page group using the FMS outer knob. Navigate to the “HSCM CALIBRATION” page, shown in Figure 2-32, using the FMS inner knob.
Figure 2-32. HSCM Calibration Page
7. The HSCM ENABLED GEA I/O A CHANNEL needs to stay on 1A.
8. Press softkey 2 (second from the left), 3, 4, 5 in sequence on the PFD.
9. Verify that the power supply ammeter shows no current flow. During this calibration procedure 28.5V needs to be applied to the shunt, but no current can flow through it.
10. Press the STORE softkey on the PFD.
11. Select YES and press the ENT button when the “Calibrate HSCM offset?” message appears.
12. Wait for the GEA to configure. When the GEA configuration is COMPLETE press the ENT button. The CURRENT CORRECTED VALUE should go to approximately zero. The CURRENT CORRECTED VALUE corresponds to what the user sees on the G1000 ammeter when powered up in normal mode.
13. Turn S BATT switch to OFF and return aircraft to normal configuration.
14. Charge the standby battery.
15. Perform an extended ground run to see if the main battery ammeter indication is fixed. If the indication is fixed we will get the official approval put together via Engineering Order (EO) to allow flight of that
16. Load the original software (0563.03) back on the airplane. The instructions for loading the 0563.03 software are included in SB07-34-02 – Garmin G1000 System Software Upgrade to Version 0563.03
BUT, i don’t know if this can solve my problem.
Digging into the G1000 user manual, I also learned that ammeter would show amber, only if current drawing is below -1,5A.
That’s why on my photo, the negative value (above -1,5A) is white, in other words, not interpreted as threatening by the system.
Now, I have to find a power supply with an ammeter…. 
Perhaps take it to a good avionics tech and get them to sort it? If not familiar be careful playing around in g1000 config menus. It is a notoriously fragile system to upgrade or configure.
Dear Jason,
You are my good father’s voice, advising not turning minor thing into big disaster… thank you :-)
However, what I learned over few years, is that I cannot pop up at my avionics shop with my “problem”, just asking them to sort it out.
Because they may take most (all ?) of my monthly wage before I can understand anything.
My objective is to define the trouble as accurately as possible. If it appears my problem is just about offset setting, I may well live with it for a while.
And I guess I will know that quickly, if the battery don’t charge during the flights.
Then, being “sure” enough, i will ask the shop for their help, pointing toward the possible solution.
I know there are very experimented professionals here, and I am sure some of them may hate customers asking for help, trying to dictate “their solution”, and calling for low cost intervention.
I try hard not to be one of them, and at the same time, to work upstream what I can.
An accurate question, as well as an accurate description of the trouble, can’t harm I guess :-)
That s why all input are appreciated.
And if even if it doesn’t solve my case, this will improve the global shared knowledge of the community.
I found another manual dedicated to configuration of G1000 set-up in Cessna 350/400.
As you know those aircrafts are equipped with two batteries and two alternators, with ammeter reading for each for them.
So there’s a calibration procedure for each canal (ammeter inputs of GEA71), which allow to bring to zero the readings, with a the electrical system properly configured.
It is very well detailed, with many screenshots.
You can find it here. Interesting stuff is to be read from page 2-116 to 2-121.
Interesting. If you can’t do it yourself, it sounds like you want a freelance avionics guy who knows this kit. But you aren’t likely to get any of the config files outside of the dealer scene, so make sure you don’t do anything which wipes any of them out. Years ago I tried to get my hands on a KFC225 configuration diskette and totally failed – they are very closely guarded so only dealers can work on these systems.
What puzzles me most is how the G1000 could – apparently – just lose the ammeter calibration. A typical shunt produces 75mV (well, that’s the standard shunt voltage for commercial ones, including ones used in cars) so if the full scale was 75A that is a 0.5mV offset error to produce a 0.5A zero error at the indicator. Now, in the 20th century, and I’ve been doing analog design since c. 1975, an A-D converter with a zero stable to say 1/10 of that would have been easy to design. In the G1000 era, say year 2000, even more so. There should simply be no need for calibrating such a thing, especially as it is a bit involved to do in the field.
Were there any errors displayed anywhere e.g. EEPROM checksum?
Peter wrote:
If you can’t do it yourself, it sounds like you want a freelance avionics guy who knows this kit.
To do it myselft, I would first need the ground power supply.
I read somewhere that a firm in Shoreham build very nice one, with IP67 sealing :-D
About the freelance guy… Not sure he does exist.
There is a firm at my base, with a guy well educated to G1000 set-up, so when I will have determined (more or less) the problem, may ask him.
Peter wrote:
But you aren’t likely to get any of the config files outside of the dealer scene
From what I understand, I don’t need any config file.
HSCM (standing for High Side Current Monitor) seems to be a module inside the GEA71 (Engine Airframe Interface Unit) and can be calibrated from the config mode without any extra stuff. It doesn’t need a PC, or anything else, just an action over the J-Box to get the system ready for calibration.
Peter wrote:
What puzzles me most is how the G1000 could – apparently – just lose the ammeter calibration.
Me too
The problem is that I have NO (zero) background in electronics / electrical system. So what I observed is that with the new battery, the offset did appear. My deduction is that the Concorde sealed battery, does not behave as the Gill flooded one…. But can’t translate that in technical terms. Does it make any sense ?
Peter wrote:
A typical shunt produces 75mV (well, that’s the standard shunt voltage for commercial ones, including ones used in cars) so if the full scale was 75A that is a 0.5mV offset error to produce a 0.5A zero error at the indicator.
This looks like dark magic for me :-)
All the more you are juggling with Volts and Amperes. From what I understand, this is an Ampere offset we can calibrate from the config page.
What I could do, to begin, is entering the G1000 config mode, to take note of the actual OFFSET ADJUSTMENT (as there must have been an adjustment when new). This will be a basis to discuss this hypothesis.
Peter wrote:
Were there any errors displayed anywhere e.g. EEPROM checksum?
Peter, what is EEPROM checksum ?
What I can say is that there were no CAS alert 
PS: googling with “g1000 battery discharge”, I found several reported cases similar to mine. Between -1A and -0,2A is what other have observed… But you guess, problem have been exposed, not the solution :-D
I read somewhere that a firm in Shoreham build very nice one, with IP67 sealing :-D
You could borrow mine but shipping would be very expensive. The ammeter is however accurate to 0.1A or better.
OTOH you can check for zero current just with a meter across the shunt in the C182, whose location should be easy to find. A ground power unit with an ammeter is probably of little use because it measures the total current drawn by the aircraft via the external power socket, and this is not what the G1000 is showing.
About the freelance guy… Not sure he does exist.
They have to be very discreet because they are probably risking their job at the dealer. I had a funny situation a few years ago. I used one avionics guy in a company. He then left that company, but said to me that he will be available for freelance work. Then he got a job at another firm. I emailed him a few times about some freelance work. He didn’t really reply, but next I heard from the owner of his new company telling me to stop canvasing his employees!
The problem is that I have NO (zero) background in electronics / electrical system. So what I observed is that with the new battery, the offset did appear. My deduction is that the Concorde sealed battery, does not behave as the Gill flooded one…. But can’t translate that in technical terms. Does it make any sense ?
It would be really funny if you reinstalled the Gill and the system read 0.0A That would be truly weird.
I don’t think it is the battery. I think something has happened to the G1000. It looks like a corrupted config of the current sensor.
What I could do, to begin, is entering the G1000 config mode, to take note of the actual OFFSET ADJUSTMENT (as there must have been an adjustment when new). This will be a basis to discuss this hypothesis.
If you find that value to be the factory default then it probably has self-reset, which is good news.
Peter, what is EEPROM checksum
Only a comment that if the config (probably in an EEPROM) got corrupted by a spike, the checksum on the EEPROM should fail. But I have no idea how config storage and error detection of it are implemented in the G1000.
I inquired Concorde to know if they had had such cases for one part, and if 28v could be less than enough to allow full charging. Here is their answer.
No. The 24V battery normal open circuit voltage fully charged is approximately 26 V depending on the battery temperature.
__
To charge the battery the voltage MUST be higher than the battery voltage which is normally 28 to 28.5V
__
If the generator / alternator system is indicating the normal system voltage of 28 or more volts and the battery ammeter is showing that it is discharging, then the battery is off of the normal DC Buss and is hot wired to something to cause it to drain or to provide power to it
I will check connectors, just in case.
As well as config page and will report.
I also inquired Cessna support to get their view on this. No answer yet.
Your speaking on mV’s in a situation with many high signals, either RF and possible inductance of high loads. Is anything changed since? Is the black box with battery relays, current sensor etc opened, to do the testing of the overvoltage etc (this would be regular maintenance) while the battery was changed?
I don’t think Peter’s input on damaged data plausable, especially due to the very small offset you are experiancing.
Your speaking on mV’s in a situation with many high signals, either RF and possible inductance of high loads. Is anything changed since? Is the black box with battery relays, current sensor etc opened, to do the testing of the overvoltage etc (this would be regular maintenance) while the battery was changed?
I don’t understand the above.
I don’t think Peter’s input on damaged data plausable, especially due to the very small offset you are experiancing.
It would be plausible in a scenario where the G1000 maintained config parameters in groups according to the subsystem, and upon encountering corrupted / out of range values in one group it reset that group to factory defaults.
That is fairly normal in electronics, although doing it silently is obviously not a great idea. But then avionics design is full of strange stuff and bizzare “features”. Look at some King autopilot bugs, and many of their “engineers” left for Garmin in the 1990s.
