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Battery voltage drops when solar/mppt connected and wont charge via grid power


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Hi everyone!

Ive recently added a single solar panel to my setup. Unfortunately thats all I can fit in my space at the moment. The small boost is all i need.

The inverter behaves very strangely when the solar panel is connected to the PV input.

Inverter: MUST Power PV1800 VPM 

Batteries: 2 x LiFePO₄WER B12-100L with balancer

The battery voltage instantly drops from 28.8V as soon as I disconnect the solar it climbs back up.


It also wont charge via the grid unless solar is disconnected. I have tried setting "Charger source priority" to Utility first and Solar and Utility but either way it will only charge via solar unless it is disconnected. I am concerned as I once turned it off the inverter and when i turned it back on I ran into an error that the battery is overcharged. I am not sure why these voltages drop. I have tried draining the battery with the solar connected and then seeing if it will charge via the grid but that does not work either.

The solar panel is directly connected to the PV input to use the inverters mppt solar charger. Is this maybe faulty?

Do I maybe need an external charge controller connected to the batteries for solar charging?

I have disconnected the solar for now to protect the batteries.

Any assistance would really be appreciated. Thanks in advance!

Screenshot 2023-05-18 at 10.38.54.png

Screenshot 2023-05-18 at 10.42.32.png

Screenshot 2023-05-18 at 10.42.58.png

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59 minutes ago, glover279 said:

Bump🥲. Any advice would really be appreciated 

Hi @glover279 I might be able to help you, but I am not too sure if the MUST is a Voltronics Axpert derivative. Can you help by telling wether your settings allow you to change the Output Source Priority to options like SUB, USB or SBU? 

And if your inverter has these modes, what is your Output Source Priority setting during those periods that you experience the 'fault'? 

Edited by BritishRacingGreen
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  • 2 weeks later...

Hi @BritishRacingGreen I apologise. I am new to PowerForum and did not see the notification when you replied.

Yes MUST is an Axpert Clone and it does have the above Output Source Priority settings.

I have tried all the options of the above setting but the battery voltage always drops and it wont charge by the grid.

Currently I have it set to Utility first and charger source priority to utility first. Is there a specific combination I should try?

Whatever I try the batteries voltage seems to be around 27 volts and charging with solar. They are actually fully charged. When solar is disconnected they show 28.8v again. I know that the batteries voltage is always higher when connected to grid. As soon as load shedding hits they drop down quite quickly. So it looks like they just arent charging from the grid at all when solar is connected?

Could these below settings have an impact on this?


Battery float charge voltage
27.0 V
Battery absorption charge voltage
28.8 V
Back to battery voltage
27.0 V
To grid battery voltage
26.0 V

Thank you so much for the assistance!



Edited by glover279
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4 hours ago, glover279 said:

Yes MUST is an Axpert Clone

In my limited experience, the Must PV1800 is only fairly loosely based on Axpert designs. I believe that this one is capable of exporting to the grid, so it's more like an Infini V II, with at least different settings numbers and some additional settings. So Axpert experience may be helpful, but not always.

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