DESS: PV returned to grid instead of battery charging

Q&A and troubleshooting DESS
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41

Then your expectation is not ideal here:

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

  • bought for 10 cents (opportunity costs)
  • sold later for 30 cents.
  • gross profit, 20 cents, so half of approach 1.
1 Like
42

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.

43

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.