question

It depends on how the system is set up.

With a straightforward system, there needs to be a voltage drop on the battery to restart the charge cycle. This is controlled by the MPPT settings.

With more complex systems with Lithium batteries and a BMS, the BMS can be in control.

If you use the Victron connect app to connect to the MPPT, you'll see a message 'why is the charger off?' (might be slightly different wording), click on this and it will tell you why there's no output. Most likely cause is full batteries.

Which MPPT do you have? There's a software glitch on one model, which is addressed in the latest beta release.

@kevgermany

Thanks for that.

I have a Victron BlueSolar MPPT 150/35. It is set up to read the value from its rotary switch (which in my case is set to match the fact that I am using a LiFePO4 battery).

Wondering what would happen if I unplugged my AC load and turned off the inverter, I gave it a try. After some time Venus is now reporting as follows:

The 5Ws being used relate to the fact that I have a raspberry pi running venus and another raspberry pi running another application - both are connected to the distributor (i.e. powered by the solar panel / battery - not the inverter). Oddly the PV Charger is now showing 5W being transferred when before (when there was a greater load on the system) it was showing zero being transferred.

Does this relate to the bug you are referencing, or perhaps something else?

In any case this view of things is more what I expected, the solar panels are powering the load (albeit a very small one) with no drain being reported from the battery.

The 150/35 is not affected.

Low wattage values can be inaccurate.

If the battery BMS isn't connected to the RPi, and I suspect it isn't, each component will do its best. However by going into the settings for the MPPT in the Venus or using the Victron connect app, you get more control over the charging. Check that the battery settings match those from the battery maker.

Your two pictures show the battery voltage as 13.6V in the first case and 13.5V in the second case. This is an important distinction that you need to make. The float setting by default for lithium is 13.6V and absorption for charging 14.2V. Remember, these figures are quoted to only 1 decimal place. When a lithium battery has finished charging its voltage will be above the float setting on the MPPT and the battery will provide the loads until the battery falls below the MPPT voltage. There is a few % of capacity in the battery and if loads are low then it can take some time to discharge this. In my opinion this is what the first screen is showing. Only after the battery has discharged this small amount will the battery voltage drop below the MPPT voltage and only at that point can the MPPT start to supply energy to the loads. This is what the second pictures is showing, note how the battery voltage is now 13.5V which allows the MPPT to start operating. If you had a system with a larger load running at the end of absorption you would see this transition happen much faster and maybe even miss it.

@kevgermany @pwfarnell thank you both for your insights they were helpful, and @kevgermany you are correct the battery BMS isn't connected to the RPi.