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dognose avatar image
dognose asked

DESS: Totally unexpected Grid-To-Battery

I'm using DESS since a few weeks now - and occasionally it does some total unexpected Grid-To-Battery-Charging:

Example Today:
- Solar Forecast is 66 kWh
- Battery is around 83% SoC already
- Energy prices are "high", with a definitie LOW later at 2pm

Yet, DESS decided that it has to charge 2.4 kWh from grid at 10am...

Any Idea what could be a trigger for this strange behaviour?

I mean, even if there wasn't sufficent solar expected - shouldn't the grid-to-battery have been scheduled for 2pm, when energy prices are basically halfed?

This has nothing todo with accuracy of the forecasted data id say - even with the forecasted data, this is strange.

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dognose avatar image
dognose answered ·

Again today at 10...


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dutchsolarfreak avatar image
dutchsolarfreak answered ·

In general DESS behaves like this:

As far as I know (and observed in my systems) if target SOC cannot be reached at the end of the hour (due to missing sun for example) DESS tries to reach target SOC by charging the battery from grid.

Sometimes even when DESS charge battery from grid is disabled!!!

My observations are that if DESS is enabled the ESS settings are not always respected.

For example the ESS max charge/discharge current limits, especially the MAX charge current limits are exceeded.

From my point of view this is not correct. As I said it just my option.

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dognose avatar image dognose commented ·
Yeah, it looks like it calculates a target soc per hour, but if there is less Solar than expected, it pulls from grid.


That would make sence, if you manually setup target SoCs to be reached, for instance because you're in a ship and will sail away at X.

However for a Home System, this is pretty strange. Especially when there is enough solar and/or cheaper grid prices later the day...


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dognose avatar image dognose commented ·

Yeah, most likely that's the issue here, from the docs:


Also keep in mind that, when doing a consumption forecast, the system does not known when during the hour that consumption will be done. If the forecast does not match the reality, Dynamic ESS does keep the planned battery usage stable while punishing the forecast inaccuracies on the grid (selling the excess to grid, buy the additional need from the grid). This can be less ideal, but it is an issue of the forecasts being inaccurate, nothing that the Dynamic ESS decided.

I have a total of upto 18kW "sinks" which I orcestrate to consume solar overhead, while having a battery reservation.


(Ev, electric water heating, pool heater, some micro consumers)


So, guess these are kind of impossible to predict for the forecast as they are dynamically turned on, when excess solar is available.


I will reconfigure DESS so the "maximum charge power" matches the reservation I've setup.

Maybe that reduces the offset and grid compensation, because DESS does no longer expect to be able to charge the battery at 14kW. (Using a reservation of 3000, this is sufficent for summer time)


---

But anyhow: offloading the "missing soc level" to grid-pull should at least undergo another recalculation of soc levels, taking estimated solar and grid prices into account.

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marcmanusch answered ·

I have exactly the same issue! The system load the battery at 10:00 from grid. The SOC is around 80% and many solar is left on this day. The energy price is not low at the moment.

The system buy energy and waste my money.


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It need to be fixed, I want to use DESS, but this is not a feature, it’s a BUG.


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dognose avatar image dognose commented ·

It need to be fixed, I want to use DESS, but this is not a feature, it’s a BUG.

From a Software-Developer Point-Of-View: I just can agree.

Same for me, I have a dynamic tarif, so i'd love to use DESS - but if it buys energy during "sunny days" just because "it sticks to a plan" - it's none-sence, unusable.

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marcmanusch avatar image
marcmanusch answered ·

@Dirk-Jan Faber (Victron Energy) please fix, it makes no sence.

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wb2 answered ·

Hi All, also from my end unwanted en unexpected behavior since some days. It loads when prices are high and unloads when prices are low. Yesterday evening target soc was at 27% which is good, then this morning the target soc was at 46% and it was buying at highest prices.

Thanks for some update on this oNe. I now switch off DESS to avoid surprises. I did work perfectly untill last week. Thanks



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