In the Phenom100 POH the descriptions for the "BATT DISCHARGE" CAS message is "During a normal system operation, at least one battery is discharging."

I'm a bit confused as to what exactly does normal operation mean.

  • Do I get the message as soon as I turn on both batteries ?
  • Does it show during the starting sequence if the GPU or other StarterGenerator is connected ?

And most important where can I find this information? I have not been able to find it in the POH or any other manual

I've also found this video on youtube but it seams to be from an older version and while it is showing the message during the first engine start it doesn't show it for the second when BATT1 should be discharging for sure because it's used to power the DC buses. Moreover this video contradicts what I know for sure from the EV and the simulator.

I've also checked the K-codes for this particular CAS message and it has none

  • 1
    $\begingroup$ If it's like any other battery system, it should mean that the battery charger isn't working and the battery is discharging. It may soon be dead. $\endgroup$
    – Ron Beyer
    Commented Jun 14, 2017 at 11:46
  • 5
    $\begingroup$ I don't work the Phenom but I have worked numerous aircraft that are capable of providing a BATT DISCHARGE message. In every case that I've seen, it comes because the battery is in use, but not being re-charged. For example, if a GPU is connected, and the batteries are on, the battery may be in use, but it is not DISCHARGING because it's being continuously charged by the GPU. $\endgroup$
    – Frank
    Commented Jun 14, 2017 at 11:50
  • $\begingroup$ @Frank I've updated my question with an video from youtube the problem is that during the start of the second engine the CAS message is not showing $\endgroup$ Commented Jun 14, 2017 at 12:13
  • 4
    $\begingroup$ I worked on the software for the GCUs on the Phenom 100/300 (around 2008) but can't for the life of me remember when that unit might send a signal to the CAS that would generate that message. Unless the design has changed since then, there isn't a separate battery charger. The starter/generators are 28 VDC and the GCU uses contactors to control the connection to the batteries. $\endgroup$ Commented Jun 14, 2017 at 14:25
  • 1
    $\begingroup$ @toby It looks to me like this would be normal system ops. It's kind of like starting your car from the battery. The initial engine start pulls directly from the battery (and discharges it - not entirely but partly). Once it's running, the alternator is providing a constant 13-14 volts to the vehicle (and topping off the battery). This scenario applies to the engine start sequence as well. The first start pulls from the battery. Now you have an engine generator providing a source of power, presumably to a TR, which can support the next engine start on the DC/Battery Bus. $\endgroup$
    – Frank
    Commented Jun 16, 2017 at 12:20

1 Answer 1


I am not into the Phenom 100E nor the avionics.

However, me and many others think that there is not much space for interpretation on this message.

In the G1000 handbook of the Phenom 100, on page 129 in the section "CAS Messages > Caution Messages", it says:

BATT DISCHARGE Battery discharging under normal operation


Electrical Status page on G1000

Source (Page 122)

This is the electrical status page as shown on the G1000 of the Phenom 100E. I marked the batteries yellow and all parts actively generating electricity and affecting load of the batteries in red.

When will BATT DISCHARGE pop up?

BATT DISCHARGE will pop up, if the load on the batteries is higher than the charge. This applies in following situations:

  • Master is on and systems are consuming electricity from the batteries but no engine and generator is on and no GPU is connected to the aircraft.
  • If the message pops up during flight, in which GEN 1 and GEN 2 usually charge the batteries, there is a problem with the generators. Additionally there can be displayed an ELEC EMERGENCY warning on the CAS.

What we see in the video

  • Start of engine Nr. 2 When he starts the first engine, we can see three CAS-Messages: BATT DISCHARGE, SHED BUS OFF (definition of SHED BUS below) and GPU CONNECTED. The engines on a Phenom are cranked by electricity, not by pressure air, so the BATT DISCHARGE makes sense first. However, the GPU is connected. Now, many things could happen here. Maybe an engine start is using lots of power, even more than the GPU can deliver.

  • After start of Nr. 2 After the start (~ 1:32), we can see on the status page, that the GPU was disconnected. The Generator of ENG 2 is already running. On the CAS there is still the BATT DISCHARGE and GPU CONNECTED to see. They disappear at the same time. This is probably because of some delay in the CAS-System. That means the battery is discharging, even though the GPU is connected (~ 0:55). It could be because of a weak GPU.

  • Start of engine Nr. 1 (~ 1:58) When the second engine gets cranked up, we can see, that the GPU was removed shortly after start of engine Nr. 2. However, there is no BATT DISCHARGE caution message anymore. Now engine Nr. 2 charges the battery. Maybe the generator is stronger than the GPU before. While starting Nr. 1, the SHED BUS is off again.

Unfortunately the video doesn't show, when engine Nr. 1 has started and is stabilized. It would be interesting.

Personal conclusion

As already said, I am not a pilot nor an engineer and only have the knowledge I have self-taught. On this knowledge-basis, I would say following looks most logic for me:

The GPU delivers only a small amount of electricity to the plane. It can't balance the load of the battery. I don't know if this is possible, but looks logic for me. After engine start, the generators of engine Nr. 2 provide the aircraft with lots of power, enough to charge the battery and to start the other engine.

The theory of a GPU, providing only a small amount of electricity would be supported by how the SHED BUS works, below. The EDL disconnects the SHED BUS because the batteries are loosing power. You can see this at ~1:08 in the video in the ELEC section of the MFD. It shows: BATT 1 - 23.5V and BATT 2 - 25.8V. This is well below the normal voltage of 28V. A bit later at 1:28 we see that both batteries have dropped to ~24V and are still discharging. At ~1:30 we see the generators providing electricity and the batteries are charging again fastly. Thats also when the SHED BUS is coming on again. At 1:58 the SHED BUS is off again and the batteries are down at 22V and 25V.

So, when I look at the facts, it looks like a weak GPU for me. However, there is still one question: Why is there no BATT DISCHARGE when starting Nr. 1? My theory is: It looks like there is some sort of delay in the CAS-System. As we have seen in the video, the GPU is already indicated to be disonnected on the MFD, but the CAS still says it is connected. Maybe the message BATT DISCHARGING hasn't appeared because of the delay. We lack the last seconds of the video, so we don't know if the message maybe appears or the generators provide electricity early enough, before appearing.

(I think this question needs more attention. It's a nice puzzle question that deserves much more upvotes.)


  • I have now found GPU values I can't confirm to be true:

GPU Limits

27V to charge

26-29V for operation

800-1000 Amps


  • The SHED BUS

The shed bus powers non-essential aircraft systems. In the case of the phenom 100 this is things like the air conditioning, passenger power sockets and entertainment systems, the toilet and some lighting.

The shed bus is the first thing to lose power (be shed) should the aircraft not be generating enough electricity through, for example, a generator dropping offline.

From my question about the shed bus by sp1.

  • $\begingroup$ Thanks for the answer my problem is that actually if you look at the video in my answer at 1:08 we have the BAT DISCHARGE CAS message during the first engine start, but at 1:58 during the second engine start we don't and BATT1 is used to power the DC buses through EBC1 and QSC during the start sequence do you have any idea why there's no CAS message during the start of the SG2. $\endgroup$ Commented Jun 26, 2017 at 7:13
  • $\begingroup$ @toby I updated the answer. I will also continue researching and edit further if I find something new. $\endgroup$ Commented Jun 26, 2017 at 8:07

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