DESS will buy during the very cheap hours for selling. So, it will keep the battery empty enough for that purpose. This allows you to feedin / sell your PV on high prices, without adding a battery round trip that causes efficiency losses.
Or simplified: buying in the almost 0 cent hours is more benefitial than using solar and MISS that purchase opportunity due your battery was filled with solar energy instead of beeing sold right away.
Your feedin price now is about 10 cent, you can buy and sell today for 0 and 30 cents.
That means:
Every kWh of solar that goes to the battery NOW, would make you miss:
10 ct. Immediate profit.
30 ct trade profit “on another kWh”
Every kWh of solar that goes to the battery NOW could be seen as
Thanks for the explanation @dognose, this approach makes sense to me indeed. Looks like the system is running good now. Still need to check the BMS/battery config as I cannot see the fill capacity of the battery being used, but this is for later.
Just to add one more consideration here:
Beside an imprecise SoC Report of the BMS, this effect could also be caused by configuring the battery capacity in dess “to large”.
If the battery is smaller than configured, it’s Soc will change faster than intendet and it will reach target soc to early as well.
@dognose Still a discussion item on returning solar to the grid instead of the battery.
Yesterday (Sunday 28 April) we had negative energy prices in NL (between 12:00 and 15:00). At 15:31 the battery was not full and the energy price was around 8 cents (see also screenshots). Instead of charging the battery with as much as possible power (from solar) the solar generated power was returned to the grid. I know this was according to the schedule but it seems to be less efficient money wise. Putting the free solar into the battery so that can be sold back to the grid at (in this case) at 20:00 seems to me it makes more money compared to returning it to the grid at 15:32 for 8 cents. Is my view correct? Is this something wrong in my config? Ot is it the algorithm that works like this?
And how can I overrule the algorithm so that all solar power goes directly into the battery? Changing the config to “Charger only” didn’t make a change, the solar was continued to be returned to the grid!
I’m experiencing the same issue. My setup is equivalent to Rene’s. The sun is shining, and my PV system is feeding power into the grid. Unfortunately, my battery is discharging.
It appears that the prediction logic only charges the battery up to 80%. As a result, I’ll have to buy electricity from the grid tonight at 23:00 — which is more expensive than the lowest tariff plus the effective battery cost.
Same here, energy price is 9 cent at the moment. Not really worth returning it to the grid. I would charge the battery with the solar and for sure not discharging the battery. I’m still struggling to understand the algorithm behind this. Also realising that the algorithm is probably complex. @dognose are you cable to check and explain why the battery is discharged at this low energy price?
@RGroothuis If DESS charges your battery now (instead of returning to the grid) your battery will be full around 14:00. At that time prices are negative. So you need to pay for your solar => grid.
To me your DESS strategy doesn’t look that bad (to sell at 9 cents).
I think that is pretty obvious: you can buy cheaper from grid 13-15, than your feedin the hours before is. So, it’s preferable to feedin right away and buy later.
Youre effectively getting money for buying 13-14, pay nothing 14-15 AND have the earnings from feedin.
Charging Solar to battery instead would make you loose that feedin-earnings and reduce the amount you could buy for free.
Thanks for the feedback. i believe I understand what you are saying. I will keep monitoring this and I want to see and understand it in practise as well. Anyway, I will keep monitoring.
Some additional information @dognose which needs to be taken into account for your explanation as well as on the explanation of @DutchSolarFreak . The motivation to sell the solar to the grid at that low price is valid under the assumption the battery can be fully charged at that lower (or minus) energy price. It is currently 15:27 the battery is not fully charged yet (at 85%) and in the next hour it will buy from the grid at 13 cent. While it was celling at 9 cents 11:00 hour. In this situation it looks to me it is more efficient to load the battery with solar instead of returning it to the grid. I’m now (almost) charging the battery with a higher price compared to what I’ve sold if for. I hope you understand my explanation and the situation.
And now even the battery charging stops! Continue charging the battery (with the available solar power) and selling it later at a higher price would make more sense to me.
To be able to judge whats going on it would be nice if you could upload also the graph with the SOC line.
Be aware that there is an issue in the official firmware release that just 10-15 minutes before the full hour DESS sometimes does “strange things” in case the solar production or consumption is higher/lower than expected. There seems to be a fix for this in the latest beta release.
Thanks Martijn (@DutchSolarFreak) will provide those graphs next time as well.
Regarding the firmware version, I’m running v3.60~66 right now (not planning to upgrade to v3.60~68 as their seems to be some issues) since 30 April. Before this version I was running v3.60~59 for some time. Which (official) version are you referring to?
