I was not able to find firm answer on the question whether those inverters (MPII) are capable of running without battery and solar charge controller, t.e. without DC.
In my system the battery will most probably appear some time after the Multi. I am planning to do the installation incl. cabling before that. Together with this to have the MPII installed and connected to the AC mains, so I can update/check options/configure/etc.in advance. Is this possible to run it only on AC in?
If yes, is it also possible to have the AC out if AC in is connected, so the Multi can supply power through the transfer switch (like a pass through mode, do not know if it is correct to call it this way)?
On a later stage, will be installed the PV panels and Solar charge controller and the battery.
The system needs the battery at least. In the config file tou would set the battery low cutt-off voltage and without the battery the unit will switch of and give an low battery alarm.
It is never advisable to do any firmware updates without a reliable dc power supply… I normally use a Lab power supply to update/configure units in my office. 4 small 7 ah batteries in series is also more than ample to do what you want to.
Hi @JacoDeJongh and thanks for your answers.
I have a cheap adjustable dc power supply (like a lab power source), capable of 60V/5A, hope should do the work.
However it is strange to me what happens to a working system with Battery and AC in connected, when you should disconnect the battery for some reason or when the BMS triggers high discharge disconnect or disconnects the battery for some other reason. Does it mean that the whole system will stop no matter that you have AC in (which in theory can supply the loads through the transfer switch?
If I get it right the DC is needed for the Multi internal electronics (the brain needs DC), so it cannot operate in AC pass-through mode without DC?
That is correct… The system will always need a battery… Except for one condition that I have witnessed… If the BMS stays on, and the load is less than the Incoming PV, and you open the battery fuses, the Multi will remain on… This is due to the MPPT keeping the DC-bus voltage above the cut-off voltage…
This is not true for when the battery switches off and the Multi looses connection to the BMs, or when there is no PV to maintain the DC-bus voltage…
Most hybrid inverter brands applies the same rule and disconnecting the load when no battery is detected is a great way to protect your batteries/system…
I suppose that in such situation you are disabling any charging (either from the charge controller or Multi).
Is this the default setting, so the lab power supply can be safely connected to the dc bus?
I am not sure what will happen if the multi try to supply power to the lab dc power source.
The solar charge controller should be sitting idle if no PV modules are connected, so it should be OK.
It doesnt have any other power to supply to the dc power supply… You would only use it to update/configure the system. Dont try and run it like this permanently.
Yesterday I was doing the AC cabling part to and from the Multi to the PDB and after finishing I measured the connections with my ohmmeter verifying that all is connected right.
I noticed that there is connection between the L and N terminals of the AC-out 1 output of the Multi. The unit was powered off of course.
So, the terminals that show to be bridged (unit is turned off) are:
N terminal of the AC-out 1
L terminal of the AC-out 1
N terminal of the AC-out 2
What is wrong and whether it is wrong?
I am still working on the DC part before having the unit switched on, but I will not connect the AC out wiring before understanding the above.
The wiring is following the diagram I shared before in this thread.
I think you probably just discovered that the secondary side of the transformer in the unit has a low resistance.
A quick test this side on the 250W 12V phoenix measures 22Ω across the output. It will be lower on the 3kVA, so low that you might mistake it for a short.
Also not surprised that AC-out-2 neutral is connected to AC-out-1 neutral That’s completely normal.
In short: Yes, live and neutral on the output is connected together… through a really long wire wrapped around a toroidal core
Thanks for the explanation @plonkster. I feel a little ashamed of not counting the transformer.
The N terminals are OK of course, just mentioned them.
Just measured the resistance with the multimeter (not accurate enough), on the terminal screws (even more inaccurate) it showed around 0.9Ohms.
As mentioned I have the wiring scheme as shared in the diagram (just have separate cables for the N and PE coming from the PDB, but they are connected in the PDB, they are laid for future purposes if needed), having TN-C type of network and wiring. I also do not have RCD of course. Should I disable the ground relay in the Multi settings?
If you are going to run an ESS system, you will have to select the relevant grid code anyway, and NRS097 will not allow you to disable the bonding relay anyway.
I am unsure if running TN-C like this is legal in South Africa? Of course it is perfectly legal up to the kiosk, but I think once you bonded it once (as you did in the main DB, where the neutral and earth bar is bonded), from there on you are supposed to have separate earth cabling.
