question

t81s avatar image
t81s asked

DC coupled ESS, battery efficiency only around 70%?

Hello,

I'm wondering for quite a wile now that my battery efficiency seams to be only at around 70%.

By battery efficiency I mean, PV energy to battery vs. energy consumed from battery.

About my Installation:

  • 2x MultiPlus 2 48/3000 configured as a 2 phase system
  • MPPT 250/70
  • MPPT 150/45
  • SmartShunt
  • DIY Battery 17s EVE 280K (the BMS is not communicating to the Venus OS)

The two MultiPlus and the MPPT 250/70 are wired with 35mm2. The MPPT 150/45 is wired with 16mm2 and the Battery with 70mm2. All cables are 1.5m long. The positive and negative busbars are made of Cu-ETP.

When charging or discharging the battery absolute nothing gets hot or even warm. Not on the busbars, the cables or on the terminals of the cells.

The min. SOC is set to 15% with Battery Life disabled.

For today I exported the kWh xls from the VRM Advanced tab. With this results:

  • 2.45 kWh as "PV to battery"
  • 1.57 kWh as "Battery to consumers"

Todays charging startet at 13.9%, had it's maximum at 27.1% and discharging ended at 15% SOC.

If I account for the 1.1% SOC which was needed to bring the battery back to 15%, the usable energy which went into the battery was 2.18 kWh vs. 1.57 kWh discharged. That's an efficiency of only 72%

The Multiphase regulation in the ESS settings is set to Total of all phases. My base load is at about 250 W which means 125 W for each MultiPlus.

If we have a look her https://community.victronenergy.com/questions/56351/multiplus-485000-efficiency-curve.html

the discharge efficiency of the 3000 should be 89.7% at 100 W.

If we assume 95% charge and discharge efficiency for LiFePo4 batteries, we would multiply 0.897 (for the Multis at my base load) and 2x 0.95 (charge/discharge for the battery) resulting in 0.809, so an efficiency of almost 81%. Which I was expecting from the system.

So now where do I lose the almost 9%? Or are my assumptions / calculations wrong?

Any helpful input is appreciated.


ESS
4 comments
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Alexandra avatar image Alexandra ♦ commented ·

@T81s

Do you have any battery cable warming during charge/loads?

If so, some of the power is leaving that way.

The other thing that affects efficiency is battery balancing. So passive or active? How much does the bms work to balance the cells (usually related to cell matching etc).

0 Likes 0 ·
t81s avatar image t81s Alexandra ♦ commented ·

@Alexandra

Nope or at least I can't feel anything. I don't have a thermal camera on hand so I have to rely on touching and feeling...

At yesterdays max SOC of 27.1% no balancing is happening. Even if balancing would occur, this lost energy can't be seen by the SmartShunt which is doing the SOC calculation.

0 Likes 0 ·
Alexandra avatar image Alexandra ♦ t81s commented ·
For the balancing, extra is sent down as charge if it is is being dissipated in balancing. That is why i mentioned the type of cell balancing used can change efficiency as well as how balanced the cells are in resistance. If yoy slow down charge current does the efficiency inprove?
0 Likes 0 ·
t81s avatar image t81s Alexandra ♦ commented ·
Oh, yes you are right, the Shunt would see the balancing losses.

Yesterday we had pretty bad weather so the max. charge current was peaking to 18A and the most time it was around 6A. The cell resistance at delivery was for every cell at 0,19 mOhm and my delta cell voltage is almost over the whole SOC range below 0.015V.

0 Likes 0 ·
0 Answers