What causes a "BATT DISCHARGE" CAS message on the Phenom 100E?
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 &quot;CAS Messages &gt; Caution Messages&quot;, it says:

BATT DISCHARGE Battery discharging under normal operation

Source

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.)

UPDATES

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

Source

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.