question

Hi Matthijs,

ok, then, here is the system layout on which I have recognized the two issues.

the GEL battery from Hoppecke has the following settings inside ESS Assistant (3xMultiplus II)

float: 54,0V

Absorption 57,6V

How to create SOC deadlock while in grid parallel mode

My implemeted energy management tries to use solar power when available and on the other hand keeping the batteries almost full whenever possible.

This will keep the system in FLOAT state and a slowly decreasing SOC over time! see green arrow!

Because the consumption from the battery is low, the reBULK condition is not met.

In time the configured minimum SOC value (uper blue mark) will be reached and the system goes to SUSTAIN mode.

The battery will not be further discharged, but isn't charged much as well because it will charge only until the FLOAT voltage which is 54,0V in my case. The SOC is not increased, nor decreased, maybe only a few percent range. --> DEADLOCK

To escape the situation I lowered the minimum SOC value (middle blue mark).

Afterwards the same behaviour reoccured at lower SOC.

When lowering the SOC a third time (lowes blue mark) I intenionally applied a high load to the system to trigger the reBULK condition.

After that the battery is beeing charged in BULK and ABSORPTION (57,6V in my case), see orange arrow.

The SOC jumped from about 60% to 95% which means the SOC calculation deviated about 35% from the real physical SOC.

I've created such deadlock situations several times, even with SOC jumps from 20 to 95%.

I've not yet tested what will happen in island mode when the shutdown SOC is reached. But could be interesting!

Suggestion on how to avoid / escape SOC deadlock

• Improve SOC calculation for low consumption from battery. Better matching of calculated and physical SOC will avoid the deadlock. This could be eg done by configuring a charging/discharging efficency curve.
• Implement a "deadlock safety escape mechanism": when going to SUSTAIN mode trigger BULK immediately.
• Another option is to aggressively modify the general reBULK condition, but that's not the preferred way because it will cause ABSORPTION charge too often!

DC PV excess power feed in issue

In my case 48V lead GEL battery with FLOAT 54,0V; ABSORPTION 57,6V

The system will set the target charge voltage inside the MPPT to either the above values or 0,4V higher, so 54,4V or 58,0V. See the screenshots below.

While the target voltage is raised by 0,4V the DC feed in works perfectly.

When the target voltage is not raised, the DC feed in does not work at all. The MPPT power is reduced sometimes even to a few Watts.

The changes between raised and not raised level are frequent, every few minutes or maybe even seconds. But thats more subjective because I've not tracked the level changes frequency in very detail.

The graph below shows only DC PV power. The chart was taken from an all sunny day, no clouds at all. So the curve should be nicely GAUSS like (blue), but it isn't due to the above described behaviour.

Moreover, when the target voltage is not raised, AC PV power is given priority over DC PV power to charge the battery! That means as long as excess AC PV power is available, it will be used to charge the batteries and possible DC power is lost until the target voltage is raised.

I've played a lot with settings (DVCC, SVS, STS, ...) in every possible combination, but didn't change the behaviour.

Why the target voltage changes so frequently, I don't have a clue so far.

What I subjectively can tell is, that the changes occur more frequently if there are frequent load changes as well. And the occurence is less often if there is almost no load or stable load.

I definately want to track that issue down to the bottom, but what I'm wondering now is, do I have the means on hand to do so? Please suggest! I'm prepared to gather all data and carry out all tests that you need to narrow it down! I really want to get rid of this because its a very annoying issue when you loose environmental friendly PV kWh.

However I've found a solution to escape this behaviour, but it is not a preferred solution, because I loose data visualization partially. When I disconnect the data cables (VEdirect) between the MPPT and the GX, and set the charge voltages inside the MPPT higher (0,1V is sufficent) than the settings in the MultiPlus, DC feed in always works!

The following suggestion of improvement came to me as a byproduct of the above issue:
Implement a safety voltage configuration option inside the MPPT for which an override by external control is not possible! In my configuration I have seen one time a target voltage of 61.6V by external control which is definately far too high for my batteries and I don't have a clue where it came from! I would like to set the safety voltage eg to 58V which must not be exceeded!