How do I interpret electrical system indications in a G1000 C-182?

Question

I recently switched to a G1000 equipped aircraft C182 and trying to figure out the specifics of the G1000 and how it works in the 182.

One that I can't really figure out with the manual and the POH is the Electrical System

I have the following Indication (in flight around 30 min after takeoff)

M        BUS      E
28.0     VOLTS    28.0

M        BATT     S
+1.5     AMPS     +0.3

Does this mean that the main and the backup battery delivery 28 Volts?

The + Amps mean that the alternator is charging the batteries ?

I noticed that the BATT Amps (M) never goes to 0 is that normal? I remember from the old analogue gauges that at some point they stay in the middle/0.

What are normal/acceptable values for BUS/BATT in a C182T?

Answer 1

You can find this in the C182T Operating Handbook. Section 3 has information about what values should be considered a malfunction:

HIGH VOLTS ANNUNCIATOR COMES ON OR M BAT AMPS MORE THAN 40

It also says that the M bus should be at least 20 volts in normal operation:

The Main Battery supplies electrical power to the Main and Essential Buses until M BUS VOLTS decreases below 20 volts. When M BUS VOLTS falls below 20 volts, the Standby Battery System will automatically supply electrical power to the Essential Bus for at least 30 minutes.

Section 4 has the expected values when starting the engine:

6. BUS E Volts
   • CHECK (Verify 24 VOLTS minimum shown)
7. M BUS Volts
   • CHECK (Verify 1.5 VOLTS or less shown)
8. BATT S Amps
   • CHECK (Verify discharge shown (negative)

Section 7-54 describes the electrical system in detail, including wiring diagrams. It says:

Normal bus voltages with the alternator operating shall be about 28 volts.

And:

Indicated voltages between 24.5 and 28 volts may occur during low engine RPM conditions

As for the ammeter:

A positive current value (shown in white) indicates that the battery is charging. A negative current value (shown in amber) indicates that the battery is discharging. In the event the alternator is not functioning or the electrical load exceeds the output of the alternator, the main battery ammeter indicates the main battery discharge rate. In the event that Standby battery discharge is required, normal steady state discharge should be less than 4 amps.

So long story short, all the information you need is in the POH and the best way to get all the details is to read it carefully. Your POH may be slightly different from the one I've quoted here, so if there are any differences then you should follow it, not this answer.

Answer 2

This all very much depends on the specific aircraft and how its set up but in general..

+Amps means that the system is charging (pushing current onto the batteries/across the ammeter reading the current in the system). An ammeter in a system is mainly to know if you are drawing from the batteries or pushing to them in other words indicating an alternator fault. I would say anything in the + range is fine. If you really want to test it out, next time you are in the plane (and able to) try running all the lights and electronics at the same time. If your current stays in the + or 0 then your alternator is supplying enough current to run everything at once. I would not worry about +.30 Amps and frankly I would look to see what the acceptable accuracy of that meter is.

For voltage it sounds like your plane has a 24V system. Generally alternators run at a higher voltage so you can charge the battery. Again this reading is more to know of a failure. Your alternator most likely has a voltage regulator on it so you get a single (or close to it) voltage across the RMP range. Alternators will generate varying voltage across the RMP range so you need to smooth that out as you can damage or under power your electronics. If you start to see the voltage vary as you spin up the engine and slow it down you may have a bad alternator or voltage regulator.

Answer 3

So in summary, quick answers AFAIK to your questions on what the displays mean:

Indications (in flight around 30 min after takeoff):

M        BUS      E
28.0     VOLTS    28.0

Your Main (M) Bus voltage is 28.0 V. Your Essential (E) Bus voltage is 28.0 V.

M        BATT     S
+1.5     AMPS     +0.3

Your Main (M) battery is slightly charging at 1.5 A. Your Standby (S) battery is slightly charging at 0.3 A. Yes, +Amps means the alternator is charging the battery.

Note 1): -Amps i.e. a negative value, would mean the battery is discharging, which could occur if you have various switches/equipment turned on and if for example (a) the engine is running at low/idle RPM such that the current generated from the alternator is less than the total current drawn, or (b) the alternator is not running at all such as when either the engine is stopped or the alternator drive belt has broken.

Note 2): The Main battery and Standby battery are both (nominal) 24 V batteries connected to a (nominal) 28 V system so I don't believe it is abnormal to have a slight charging current 30 minutes into a flight, but you could mention your observation to your aircraft mechanic to check whether those values are within the normal range.

Note 3): Those relatively low currents may not have caused a noticeable deflection in an old analog gauge. For example if that gauge had a range -40 A to +40 A then 1.5 A would have been less than 2% of the full-scale deflection so it could have appeared as if the needle is sitting "in the middle" at "0" even though there is 0.3 A or 1.5 A charging current.

That's just AFAIK but if the POH or a licensed A&P mechanic tells you differently then obviously go according to the higher authority!