Here is the last post-annual surprise I wanted to share.
My last Gill battery (G-243) last just 2 years, and this time I decided to go with a sealed Concorde (RG-15M) regardless of cost (x1,5+)
The battery shows stronger voltage before start-up than the previous Gill (25V against 23,5V).
The surprise comes from the electrical readings after stabilization (let’s say 30 minutes after take-off), showing a discharge of the main battery.
BEFORE (for the last 5 years) IT WAS THIS:
NOW I SEE THIS:
This is as if the alternator could not supply enough power to support the load, but I don’t think it is the case.
I should add that the result was not modified when I increased electrical load (lights, pitot heater…)
After start-up and turning on the ALT, the ammeter shows positive value, hence battery is able to charge at this moment.
I contacted Concorde, no answer yet, and I have no clue of what’s going on.
I will report of course when (if) troubleshooting goes forward…
There is no way the battery is being discharged with 28,1 V on the bus…
Is the ammeter reading wrong ?
Is the voltage regulator faulty ?
Would be a coincidence as it was Ok up to the battery change.
Concorde answered I should increase the alternator output to 28.2 + \ – 0.25V
Not sure this is the solution, and have no idea how to do that.
There was recently a very similar discussion on the CAAC (Cessna Advanced Aircraft Club) forum. On the C400 there is an indication of alternator current, and battery current. The former should be positive (powering all consumers and charging the battery), the latter should read 0 if the battery is fully charged, >0 if it is charging or <0 if the battery is discharging.
On this particular C400/G1000 the battery current showed -4 A although everything else looked normal and the bus voltage was 28,7 V on both buses. It was mentioned that there is a way to calibrate the G1000 amp reading.
I sent the OP a PM and will keep you posted if he comes up with anything.
I have just done a load test on the Concorde RG24-15. Before that I fully charged it with a 28.000V power supply. It finished up drawing about 20mA (0.02A) eventually. There is no way the battery could be discharging at 28V.
But without knowing the circuit diagram of the G1000, one can’t say what those readings mean. There may even be a fault somewhere…
Note that in these simple constant voltage charging systems, the battery will never be simply “fully charged”. In the TB20GT for example, the current indicator measures the current going in or out of the battery. So before you start the engine it reads say 10-15A. During starting, it shows no higher because the starter motor current doesn’t pass through the meter 
Then, once the engine is running and you have pressed the ALT (alternator field current connect) switch, the voltage regulator wakes up and the bus voltage rises to 28.0V, and then you see the battery charging. For a few seconds it might show 30A, which quickly drops, and during a flight it is usually showing about 0.2A, which is a net positive current going into the battery. If you flew for many hours this would fall to less than 0.1A – as per my 1st para above.
The matter is more complicated by a small temperature dependence of the cell voltage, but these simple charging circuits don’t allow for that.
Maybe somebody knows how the G1000 is wired. I have a collection of G1000 installation manuals – example – but it would take hours to go through them. If anybody is willing to do that, let me know…
I would also check the bus voltage with engine running and make sure it really is 28.0V and not say 27.5V.
Aviathor wrote:
On the C400 there is an indication of alternator current, and battery current. The former should be positive (powering all consumers and charging the battery), the latter should read 0 if the battery is fully charged, >0 if it is charging or <0 if the battery is discharging.
In my case, there is no way to read the current output of the alternator which would give an information on the electrical load.
I only have the second indication, which let me know if current is going IN or OUT of the battery as you stated.
Aviathor wrote:
It was mentioned that there is a way to calibrate the G1000 amp reading.
I really would be glad to get some information on this. Never heard of that, but that’s not a standard practice…
I just start wondering if I should have stayed with Gill, even with its high infant mortality rate… No, no, no, that was a good idea…
Peter wrote:
There is no way the battery could be discharging at 28V.
We all agree with that, that’s why the tech answer from Concorde (setting 28.2V output to the alternator) may not improve anything…
For the sequence, I agree and observe the same:
1) First you draw current from the battery with basic ON (the engine and alternator are OFF)
2) After start-up, when turning on the alternator, it shows net positive current (charging, never higher than 8-9A). This is still the case with the Concorde.
3) Quickly, the current is decreasing (tens of minutes):
Peter wrote:
There may even be a fault somewhere…
Peter, you really don’t want me to sleep in peace….
Peter wrote:
Maybe somebody knows how the G1000 is wired.
I am not sure if it is strictly about G1000 wiring, or electrical diagram of the C182.
If fault is in the regulator stage, the G1000 has nothing to do with it.
Peter, do you think it could be a calibration problem, as Aviathor suggested ?
Foot note: the flight was just 1:15 (LFCK-LFMA).
I should try with a longer flight to see what happens.
PetitCessnaVoyageur wrote:
I really would be glad to get some information on [ammeter calibration]. Never heard of that, but that’s not a standard practice…
It’s somewhere in the configuration pages which you can’t access without special procedures. I saw it flash past when my avionics shop configured our G1000 after a software upgrade.
there is no way to read the current output of the alternator which would give an information on the electrical load.
No, but if it is wired the way I think it is wired, you can establish the aircraft’s current draw by switching on various bits on the ground and seeing what the battery current is. Then, if say everything=on is 25A, and in flight the battery current is nearly zero (as it should be) you know the alternator is outputting 25A.
Peter, you really don’t want me to sleep in peace
It seems unlikely you would get the -0.5A indication. But maybe there is a config / zero setting option, which got corrupted when the battery was changed?
Somewhere in the aircraft is a big shunt across which a small voltage (50-100mV) develops and this is used to measure the aforesaid current. If you can find this shunt, you can connect a digital meter across it, start the engine and… quickly you will have your answer.
There are several Garmin dealers who are on here.
If the battery constantly discharges during a long flight, you would have a perpetual motion machine… You can sell it for good money!
Peter wrote:
It seems unlikely you would get the -0.5A indication. But maybe there is a config / zero setting option, which got corrupted when the battery was changed?
Maybe Aviathor will bring light to this.
Peter wrote:
There are several Garmin dealers who are on here.
Maybe they pop up !
