Main Battery Low voltage

Let’s take one issue at a time.

Once the engine is up and running on “alternator A”, both lanes are being powered by “alternator A” and will typically show the same voltage, yours look good at 13.8 volts.  The airframe and battery are supplied by “alternator B” in normal operation, and the battery voltage should be higher than what your are seeing, typically over 13.5 volts.  Before we go further, we need to know how your amp meter is set up. What does the amp meter read with the engine stopped and some electrical loads in the aircraft turned on?  Does it go to zero, or does it read a negative number?  If zero, your amp meter reads the total current provided by the alternator.  If a negative number, it’s measuring the current either charging or discharging the battery.  

Regarding the fuel pressure, I suspect the fuel pressure gauge installed is not a differential type.  The 912iS regulates its fuel pressure in relation to manifold pressure, so if your sender references ambient pressure you will get low readings at low engine speeds because the manifold pressure is lower than ambient.  The fuel pressure reading will increase as you advance the throttle and manifold pressure approaches ambient pressure. The fuel pressure is likely correct in relation to manifold pressure, your gauge is just wrong.  There two fixes for this.  The best is a differential fuel pressure gauge which has an air sample tube running up to the airbox.  However, I believe the Dynon EFIS has a calibration setting that takes a standard gauge sender input and calculates a proper reading based on ambient pressure, altitude and manifold pressure.  Most manufacturers are getting this right now days, but back in 2017 this problem was common.  If you search this site you will find a lot on this, and I believe the proper Dynon setup was even posted.  

Hi Rod,

  not sure i see a problem here. Both voltages are the same 13.8 V which is enough to charge a lead acid battery. Battery voltage is 12.3 so a little low but there is 9-10 A going into the battery so it is being charged.

Have a look after a longish flight the battery Voltage should be closer to the Lanes and the Amps should drop to almost zero meaning the battery is now fully charged.

The low fuel pressure reading is common and annoying this is because the fuel pressure is referenced to the Airbox an at low rpms and low power the airbox pressure in very low well below atmospheric so although the FP is 3 Bar higher than Airbox and very safe the absolute pressure referenced to atmospheric can be low as you saw... at WOT comes right. There was a long discussion on here about that so search for a better explanation. There is a "properfix" which involves installing another fuel pressure sensor the alternate is to get used to it or lower the error bar a little in the dynon settings.

Finally check the Battery as I remember it Rotax reccomends at least a 16 AH battery for the IS engine and some manufacturers put smaller ones in at the factory. perhaps you are still on the original battery so after 7 years it will be getting near end of life ?

I have seen 11 AH battery with duel G3X which draw 7A each so after a cold start the Battery is almost drained. I recommend a LFP battery around 20AH they will charge very quickly after start whereas due to surface effects a Lead acid can take an hour to get back to fully charged.

Hi Glenn,

The way I read this is that the two meters showing 13.8 volts represent the two individual “lane voltages”, rather than alternator A/B voltages (see the photo Rod posted). The lane voltages are measured at the ECU, and are both powered by alternator-A. So essentially they are both indications of alternator-A output voltage.  

The only indication we have of alternator B voltage is the battery voltage at 12.3. Depending how the amp meter is configured, that reading of 8 amps could be the total output of the alternator rather than the battery charging current. If that’s true, the total airframe load is likely more than 8 amps, with the battery contributing the remainder.  That would account for the declining battery voltage. As such, I think there may be a problem with alternator B, or regulator B, or wiring between. 

Hi Jeff, thank for your reply. The amp reading is always zero when the engine is off.

Good explanation of the fuel pressure system, I’ll take a closer look at what I have.

Hi Glen, Thanks for your reply. Looking at the logbooks the main battery was changed 2-years ago to an HC20, which has a 20Ah capacity. I’ve never had a problem starting, even when the voltage is down to 11.7V. (having the 2 screen on, with the engine off, really drains the battery quickly).