Discharge_start_service - planning assumption

[Javert1969]

Not sure if this is the right place to ask but....

When predbat decides to do a force export/discharge and triggers the discharge_start_service, does the planning assume that the battery continuously discharges at the mentioned rate, or does it assume that the battery alters the discharge rate continuously to achieve the mentioned rate of export?

The integration on my inverter can take either approach. Does it matter that much which one I choose.

As a follow up, if the planning assumption is that the battery discharge is constant, does the system take into account the DNO export limit if PV is still being generated - with some inverters, including mine, if you trigger a force constant discharge of the battery at your DNO export limit, and then the PV generation suddenly goes up a lot, you can be exporting higher than your official limit - this because the force_discharge modbus modes in e.g. Alpha ESS inverters don't respect the overall DNO export limit at the inverter side. If I use that mode it's up to me to manage within the export limit.

[gcoan]

When predbat decides to do a force export/discharge and triggers the discharge_start_service, does the planning assume that the battery continuously discharges at the mentioned rate, or does it assume that the battery alters the discharge rate continuously to achieve the mentioned rate of export?

thanks for your question.

Predbat assumes the former, that the battery will discharge at the defined rate, anything needed for house load will be consumed first, and the remainder exported

[gcoan]

As a follow up, if the planning assumption is that the battery discharge is constant, does the system take into account the DNO export limit if PV is still being generated - with some inverters, including mine, if you trigger a force constant discharge of the battery at your DNO export limit, and then the PV generation suddenly goes up a lot, you can be exporting higher than your official limit - this because the force_discharge modbus modes in e.g. Alpha ESS inverters don't respect the overall DNO export limit at the inverter side. If I use that mode it's up to me to manage within the export limit.

Predbat leaves all delivery of DNO export limits to the inverter. It will model what those limits are and any clipping in the plan, but it doesn't control the inverter discharge to prevent exceeding the DNO limit.

What you describe sounds like a fault in the Alpha ESS inverter that would cause it to fail compliance testing. Maybe there is a tolerance allowed in the compliance standards that the inverter is allowed a period of time to ramp discharge back down if PV ramps up, I don't know.
Slightly exceeding your DNO Limit for a brief period of time if PV increases isn't going to blow anything up anyway, and I would be very surprised if the DNO even noticed. Of course if you are consistently exceeding the DNO limits and this causes high grid voltage, then they may come asking questions, but doesn't sound like this is the case

[Javert1969]

So with Alpha ESS inverters, the export limit programmed by the installer is enforced, as long as you are using the Alpha ESS app to control everything, or as long as you leave the inverter in inverter mode 0 if you are using Modbus and Home Assistant.

If you use Modbus to command other modes (there are a bunch of modes with numbers like mode 2 for example which is the most flexible), the export limit is no longer enforced and it’s up to you to only command to stay within the export limit.

I got a feedback from an AlphaESS installer on Reddit who said that all Alpha inverters in the UK work like this, and that generally if you are using Modbus it’s assumed that you are an expert user who will manage your generator contract appropriately. Maybe the compliance testing assumes that you are not using expert Modbus control modes.

Anyway - what you say seems to answer my question i.e. if I have an 8KW inverter but I have put a 4KW export limit, Predbat will command (for example) an 8KW dispatch and let the inverter take care of it, whilst modelling maximum 4KW export in the plan.

The closest I currently have to that is the Alpha “forced export” automation which commands continuous export at 4KW and continuously varies the battery discharge to achieve the 4KW export rate. If I use that mode, it means that the actual will vary from the Predbat plan as the battery will be discharging more than Predbat expects. I will test with this approach and see if it makes a lot of problems in practice.

On the other hand if I used the forced dispatch mode from the Alpha Modbus integration that I am using, and it commands an 8KW dispatch, my export limit will be exceeded continuously unless house load > 4KW.

It is of course possible to create a new automation which does a forced dispatch, capped by programming in HA to 4KW of export. Whether it’s worth the effort to do this is TBD.

I could also try to work around it by altering the maximum discharge rate configuration in Predbat to 4KW, which would avoid the issue (unless an export is planned during strong solar generation), but I am not sure whether this would have other undesired consequences.

As regards compliance testing - I don’t know - to be honest I was surprised when I found out the above info, but the installer who responded to my questions on Reddit didn’t seem surprised and I assumed that Alpha currently passes compliance testing.

Edit: FYI I also have 2 layers of voltage protection - the inverter has safety cutouts which will automatically reset the inverter and only gradually ramp it back up again if the voltage at the grid connection exceeds the legal limit. Further, I have an HA automation which also puts the inverter back to mode 0 if the voltage exceeds 252v. Unfortunatly this does happen quite a bit in the summer at my address even when I am exporting at less than my official limit so I will eventually need to contact the DNO about this. Finally the integration uses a dead man’s switch concept so that if HA crashes or goes down, after some small amount of time it will revert onto it’s own to normal mode.

[gcoan]

If you use Modbus to command other modes (there are a bunch of modes with numbers like mode 2 for example which is the most flexible), the export limit is no longer enforced and it’s up to you to only command to stay within the export limit.

I got a feedback from an AlphaESS installer on Reddit who said that all Alpha inverters in the UK work like this, and that generally if you are using Modbus it’s assumed that you are an expert user who will manage your generator contract appropriately. Maybe the compliance testing assumes that you are not using expert Modbus control modes.

As regards compliance testing - I don’t know - to be honest I was surprised when I found out the above info, but the installer who responded to my questions on Reddit didn’t seem surprised and I assumed that Alpha currently passes compliance testing.

I find that very surprising. My understanding is that there isn't supposed to be any bypasses to the configured export limit.
Anyway, it is what it is, at least you're aware of it.

Anyway - what you say seems to answer my question i.e. if I have an 8KW inverter but I have put a 4KW export limit, Predbat will command (for example) an 8KW dispatch and let the inverter take care of it, whilst modelling maximum 4KW export in the plan.

Yes

The closest I currently have to that is the Alpha “forced export” automation which commands continuous export at 4KW and continuously varies the battery discharge to achieve the 4KW export rate. If I use that mode, it means that the actual will vary from the Predbat plan as the battery will be discharging more than Predbat expects. I will test with this approach and see if it makes a lot of problems in practice.

This sounds like what Predbat is expecting. It commands a max rate discharge, your house load is satisfied and the export limit is maintained.
It should be what predbat models as well. Worst case the only real impact if predbat doesn't model it quite right is that the forecast SoC and forecast cost today will be out slightly, but Predbat will self-correct in the next 10 minute plan update.

On the other hand if I used the forced dispatch mode from the Alpha Modbus integration that I am using, and it commands an 8KW dispatch, my export limit will be exceeded continuously unless house load > 4KW.

Not desirable

It is of course possible to create a new automation which does a forced dispatch, capped by programming in HA to 4KW of export. Whether it’s worth the effort to do this is TBD.

You don't really want to have an automation continually monitoring and adjusting the actual inverter export rate. This would result in lots of inverter writes

I could also try to work around it by altering the maximum discharge rate configuration in Predbat to 4KW, which would avoid the issue (unless an export is planned during strong solar generation), but I am not sure whether this would have other undesired consequences.

Consequence of this is predbat will lock your inverter to 4kW discharge rate, so if your house load exceeds this, you'll grid import. Not desirable

[Javert1969]

Thank you this information is very helpful and is also in line with my thinking - the forced export option will be good enough for this purpose.

One comment regarding “You don't really want to have an automation continually monitoring and adjusting the actual inverter export rate. This would result in lots of inverter writes”:

According to the information from the integration designer at this site https://projects.hillviewlodge.ie/alphaess/ - the approach that I am using (and another user who provided the code template I am basing this on) should not cause inverter writes to flash memory - there are a bunch of settings that you can change via modbus in the Alpha ESS inverter - some of them write to flash memory, others are not stored in flash memory (and therefore don’t survive inverter reboots for example). The approach we are taking only writes to the latter.

We rely on Predbat to trigger charge and discharge actions using power level and soc as the drivers, with a duration timer that exceeds what Predbat will ever need. This will then get changed at the next Predbat command.

We don’t use the start time and end time functions in the Alpha Modbus control as these ones do cause writes to flash memory.

Therefore (hopefully) we are not actually doing flash memory writes (although Predbat will probably log them as such).

I have suggested to the person who made the original code that I am changing, that once we have it working well he can submit it here (and maybe also an alternate version as there are 2 different AlphaESS Modbus integrations currently available) for potentially inclusion as an Alpha ESS Modbus inverter settings for anyone who doesn’t want to rely on Alpha Cloud.

[gcoan]

once you get a working configuration, feel free to raise a PR to include it in the predbat documentation for other users. Update the inverter setup doc and add the template you created.

Cheers