Reverse Power Assist or battery setpoint to supplement existing grid setpoint

Q&A and troubleshooting DESS
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1

With DC coupled PV, feed in excess, the battery will be charged first. In charging time, any given grid setpoint is maintained. If battery SOC reached 100%, remaining power will override the grid setpoint to feed into the grid. If there is more power from the MPPT than inverter (or grid) capacity, the power will be dropped by the MPPT as it cannot go anywhere.

Therefore I am searching anything like a battery setpoint. The goal is to keep the battery SOC at a certain level below 100%. For cloudy weather forecast, this could be a battery setpoint of 80% SOC while sunny sky requires a battery setpoint of only 50% SOC.

Step1)
ESS should charge battery if actual SOC ist below battery setpoint. If battery setpoint reached, any power from the MPPT goes to grid instead charging the battery up to 100%.

Step2)
During the day, MPPT power increases until a point where it exceeds the inverter/grid capacity. This time, the battery setpoint is overriden to charge the excess DC power into battery. This is true until 100% SOC reached and the MPPT has to drop additional power as it cannot go anywhere.

Step3)
If sun disappears in the evening, maximum available grid feed in will be maintained from battery for an extended time until battery setpoint SOC reached. After this, only the (feed in) grid setpoint is maintained to avoid power purchase from grid. Depending from the night length, this causes a battery SOC setpoint what might be smaller than the given value. When sun rises again, next cycle begins with Step 1 again.

To avoid any power drop by the MPPT, the operator now needs to adjust the battery setupoint instead several adjustments of the grid setpoint over the day. The success then depends only from the battery size and accuracy of the weather forecast. If battery capacity is high enough, the battery setpoint doesnt need any more frequently adjustments over the complete year.

I am not sure, if this operation is possible to achieve with existing ESS (not DESS). Maybe any user already wrote any script to realize the idea?

2

You would need to write the logic yourself using nodered or similar and export via a negative setpoint.
Conventional ESS exporting requires a full battery as the process is driven by overvoltage, which can’t happen below full SOC.
Some members are doing this as suggested, but default ESS cannot.

3

Hello @nickdb ,

I don’t find the thread about the node red configuration to do so, could you, please, share the link ?

Thanks a lot,

best regards,
Patrick.

4

Hi, do a few searches on the old modifications section archive at communityarchive.victronenergy.com

There should be a few topics on using negative grid setpoints and various forms of logic around that.

Essentially you would using inject nodes to run at the specific time or times that you need the setpoint balanced, wrapped with some basic conditional logic that says if SOC is >=< X then make the ESS setpoint Y.

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