# Phantom battery current and weird electrical system behaviour

I can see how trying to figure out the electrical system can make one nuts.

I started looking at the SIMVARS you are using. It looks like there is definition problem or a calculation problem or both.

The SIMVARS should be grouped into amp/voltage pairs and measurement points. The bus measurements are: BATTERY BUS (uses power to charge, otherwise provides power when needed), GENALT BUS (provides power), and MAIN BUS (uses power). The POWER is calculated as AMPS (or LOAD) x VOLTAGE. If MAIN BUS VOLTAGE is 28.0 and MAIN BUS AMPS is -9.8 then POWER is -274.4 watts meaning the electrical system is using 274.4 watts.

There is a either a missing source or Battery Load and Amps are (incorrectly) added together. The numbers just do not add up correctly.

Iâm not sure exactly what GENALT means because there is one per engine. The C172 alternator can provide 60 amps at 28 volts.(GENALT BUS amps and voltage). Where are GENALT LOAD and TOTAL LOAD measured?

Very Confusing!

What is also wrong is that not all the items on the bus are RESISTIVE and follow the I = V/R rule.
Items like the GPS, with electronics with switch mode supplies, draw a reasonably constant LOAD over varying voltages, so their current drawn goes UP, as the voltage goes Down.

Hopefully, one day this old Dumb Down FSX modeling will be updated to be more realistic.

If PMDG & A2A can do it, so can Asobo.

Is this something already built into the game? It definitely improves on the behaviour of the system. But I saw the battery still burning all the leftover current as it used to. Charging at 30 amps? thats a bit extreme.

The new system allows you to set 3 values. âmin powerâ, âmax powerâ, âmin voltageâ
The min power value sets the power draw at that min voltage. The max isâŠ i assume it looks at the max voltage configured from the alternator and basically ramps the draw between those 2.

I havenât tested this, but i assume if you set 40W min and max, then you will get the correct behavior for screens and digital instruments etc.

How do developers like PMDG and stuff do their electrical systems? I should look into the A320NX

One thing that is seriously missing from the electrics are diodes, and basic AND, OR, IF, NOR logic.

My current workaround is to throw everything on it own separate bus with a âsample circuitâ. This sample circuit only draws 1W and is always connected to the bus. In XML I can look up the state of that circuit and toggle the bus accordingly.

Ive tried using the âbus lookup indexâ simvars, but no luck. Alot of stuff are borked.

Not suggesting for one minute that the implementation is not flawed but some of the expected operation you describe in these posts is flawed. The extra 6A draw with the ALT on will not be down to the difference in voltage as you suggested. As the voltage increases the opposite should happen. The current will reduce. The ALT current will be the sum of the equipment loads including anything that additionally draws current now the ALT is active such as itâs field and the battery charging. The battery amps will reduce to almost nothing once charged. The charging time will be dependent on the initial charge state.

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Until the batteryâs internal resistance is simulated, (ideally as a function of battery Charge), any relationship between battery Terminal voltage, charge and current drawn, will be flawed.

ie when you hit the starter, and it draws 250+ amps, the battery terminal voltage, (and that supply voltage to all the instruments) should drop.

The starter solenoid is also not simulated, so when the battery is low, the inability to start the plane, and the resulting Click click click, as the solenoid engages, only to drop out when the starter motor draws current, and then stops drawing current as the solenoid releases, cannot be correctly simulated.

Its all very dumbed down and unrealistic.

Its a shame, as it is not that difficult to simulate this correctly - 1000+ time easier than flight dynamics !!!

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Agreed. Relatively simple stuff to implement if they have someone who understands how itâs meant to behave.

Or you hire an EXPERT consultant for a few day to give you that understanding.

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This is better but itâs not entirely accurate. The ammeter in aircraft does not read starter current mainly because itâs way out of range of the indicator/sensor. In almost every light aircraft the main starter power is fed directly from the battery circuit with its own feed. In the c172 specifically the ammeter is in the line between battery and alternator. + and battery is charging, - battery is discharging. So when the alt is off and you crank the starter the ammeter may only move a small amount due to the battery voltage drop effect with the existing loads. You obviously wouldnât crank the engine normally with engine already running but if you did as in the test shown then if the ammeter read anything it would be positive due to where the ammeter is placed and itâs polarity.
I know this was only demonstrating how the sim could be improved and itâs certainly better than whatâs in MSFS right now but itâs important to note these differences just in case someone takes this as accurate to real aircraft.

30A for battery charging is feasible depending on the battery charge state although it should drop from this value after a short time as the battery charges.

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Good point â should be an easy (hack) MOD !!! (remove the Starter Motor Current from the Ammeter current reading !! )
Would be nice to add the starter solenoid, and it functionality.

• Click - Click - click with a low battery

There shouldnât be any Starter Motor Current being measured by the amp meter since the amp meter is not in the starter circuit. Since the amp meter in the C172 is between the alternator and the battery, it shouldnât show the starter motor 200 amp draw. (According to the C172N POH. Since weâre flying the C172SP, the electrical system might have been changed between models.)

Isnât the starter and starter solenoid connected to the starter switch keybinding? Basically it is either on or off. What other functionality does it have?

FWIW, I left the starter switch ON to see what happens. The battery drained quickly and when it reached 21 volts, all electrical was shut down and no external power was available. The line crew refused to do a manual start using the propeller.

Did you read the last few replies on this subject. I made that exact point a few posts back and he agreed it was wrong and made the point that it could be removed from showing on the ammeter. This is in reference the video demo made in FSX.

Yes I saw your post. Good explanation! Regarding MSFS I was looking at this as a bug. Maybe not. For me personally Iâd like more info on how IRL electrical diagrams are simulated for the aircrafts.

LOL !! Assuming you did not âburn outâ The starter motor, what SHOULD have happened is this:

(1) Individual items should have dropped out as as the battery discharged and its LOADED terminal voltage dropped
(2) Finally the starter solenoid should have dropped out, causing the starter motor to stop.
At this point the partly discharged battery terminal voltage should have risen, and some of the shut down items, come back to life.
If the battery voltage, without the starter motor load, recovered enough to allow the starter motor solenoid to engage, then the starter motor would draw the current again, reducing the battery terminal voltage and the process starting at (1) should repeat.

Is not difficult stuff âŠ and should be easy enough to simulate.

Is it needed ?

For a GAME â NO
For a Simulator â YES

Is it simulated ? NO â so MSFS looks like a ___________ ?
(Hint- begins with a âGâ )

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