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bjorn-n avatar image
bjorn-n asked

MultiPlus-II discharging to grid slows down and even stops (before minumum SoC is reached)

I am discharging to the grid at a rate of up to 35A@48V. But as the voltage of the battery goes down, the discharge rate goes down and eventually stops completely. Here are a few screenshots of the readings on the GX device.

The battery voltage can be between 36.0V (0% SoC) and 50.4V (100% SoC). Battery is approx. 10 kWh @48V (220 Ah).

The minimum SOC (unless grid fails) is set to 10%.

The grid set point is set to -500 W.

screenshot-at-2021-03-03-17-50-19.png

screenshot-at-2021-03-03-18-12-48.png

As you can see, the battery voltage is 40.5V, so there is energy left in the battery...

I have set dynamic cut-off values to 39.0V (0.005C), 37.5V (0.25C), 36.9V (0.7C) and 36.0V (2C).

Sustain voltage is set to 36.0V (during the first 24 hours) and 39.0V (after 24 hours).

My question has two parts (that are probably related):
1. why does it slow down so much (only 2.4A discharge rate @ 40.5V)
2. why does it stop discharging?

Multiplus-IIESS
screenshot-at-2021-03-03-17-50-19.png (21.7 KiB)
screenshot-at-2021-03-03-18-12-48.png (21.2 KiB)
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seb71 avatar image seb71 commented ·

Going by those voltages, you have lead-acid batteries.

Discharging them to almost empty is a quick way to destroy them.

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bjorn-n avatar image bjorn-n seb71 commented ·

If they were Lead-Acid, they would have been dead already :-)

These are Lithium Ion batteries.

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seb71 avatar image seb71 bjorn-n commented ·

What kind of lithium ion?

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bjorn-n avatar image bjorn-n seb71 commented ·

12S74P=888 cells of Panasonic NCR18650B

a.k.a. 2 Tesla battery modules in series

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bjorn-n avatar image bjorn-n commented ·

Currently testing again. Still doing fine at 41.7V. So far the discharge current appears to not being reduced.screenshot-at-2021-03-04-11-19-37.png

screenshot-at-2021-03-04-16-48-44.png

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bjorn-n avatar image bjorn-n bjorn-n commented ·

Similar "behavior" as before. The screenshots below have been taken seconds apart. Discharge current fluctuates between 2-5A.

screenshot-at-2021-03-04-18-38-13.png

screenshot-at-2021-03-04-18-38-22.png

In stead of taking screenshots manually, I'll setup logging to fully understand the behaviour.

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screenshot-at-2021-03-04-18-38-13.png (22.6 KiB)
screenshot-at-2021-03-04-18-38-22.png (21.3 KiB)
1 Answer
Meine_Energiewende avatar image
Meine_Energiewende answered ·

Sounds like Tesla packs.

My experience with Tesla and Victron is: They will not discharge lower than 40.5 (40.8) volts if grid connected. I you use them off-grid it will discharge to 39 Volts. But this don't matter because you should not discharge any further than 40,5 for lifetime reasons. The Tesla packs are made for about a 1000 cycles and you may double this live time with only discharging to that values.

Attached you will find a discharge curve of that Tesla packs. As you may notice there is not much juice left in it at a voltage of 40.5 volts. Voltages will drop very quickly after this point. So do your batteries a favor and stay at this voltages.

Hope this are really Tesla (or 12s LiIon) packs.

Jens

tesla-module-charge-curve-850x478.png


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bjorn-n avatar image bjorn-n commented ·

These are indeed Tesla modules: 2 modules in series to form a 12S74P battery.

I do not know if the individual cells are identical to, but it's being said they are close to the Panasonic NCR1650B cells.

According to the spec sheet: nominal voltage of the Panasonic NCR18650B cells of 3.6V (not 3.7V) or 43.2V for the setup. Each cell has a minimum capacity of 3.2 Ah and a typical capacity of 3.35 Ah. So the total setup is somewhere between 237 Ah and 248 Ah.


Let's make that 20.25V threshold per module (40.5V threshold for the two modules) 20.30V for ease of reference in the graph. At that voltage the batteries have 10-15% energy left in them according to the graph:

20210304-tesla-module-discharge-curve.png

Source: https://turtleherding.com/2018/06/mod-tesla-powered-tiffin/


According to another source, at that same threshold of 40.5V for the two modules or 3.375V per cell, I have 0,50 Ah of the total 3.25 Ah left. Which comes down to a SoC of 15%.

Follow the red or green line, as the maximum power I can discharge at is 0.15C (35A/237Ah) or less. The maximum discharge rate of 0.15C is equivalent to 0.5A (green line) in the chart below.

20210304-panasonic-ncr18650b-3400mah-green-capacit.png

Source: https://lygte-info.dk/review/batteries2012/Panasonic%20NCR18650B%203400mAh%20(Green)%20UK.html


Yes, I know that deep discharging hurts the battery. At this point, I am testing the setup. I also want to determine the round-trip efficiency etc. So I'm willing to slightly hurt the battery in order to know what the limits are.

Once I'm done testing, I intend to keep the SoC between 20% and 80% to keep the batteries (and my wallet) happy. :-)

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20210304-tesla-module-discharge-curve.png (616.4 KiB)
20210304-panasonic-ncr18650b-3400mah-green-capacit.png (27.3 KiB)