Does Dynamic cut-off make sense for Lithium systems?

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thanar asked Mar 15, '23

Does Dynamic cut-off make sense for Lithium systems?

Dynamic cut-off is an option only available on systems running as ESS, i.e. the ESS assistant has been installed and configured on the inverter device, and there is a GX gateway present on the system.

AFAIK, dynamic cut-off exists so as we can set a lower cut-off voltage for when there is a large current coming from the battery, since the battery has not actually been discharged, but all that's happening is extra voltage drop due to internal resistance and such, i.e. when the load is lifted, the voltage wire rise considerably.

So, I guess dynamic cut-off makes perfect sense for lead batteries, where -for example- you could safely pull a 12V lead down to 10V when there is a 2C load on it.

However, in my opinion, dynamic cut-off doesn't make any sense for Lithium batteries. Why? Because Lithium batteries have a build-in BMS that will block discharging when the battery's minimum threshold (or a cell's) is reached. This will definitely happen when there is a hard load on the battery. So, it doesn't make any sense to adjust dynamic cut-off for a lithium battery at a lower value than the battery's BMS minimum voltage setting. That said, it doesn't make any sense to enter a higher value, either.

So, what's the point of dynamic cut-off for Lithium batteries? Please elaborate.

ESS voltage discharge level

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Modifications Space

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Answer 1 · 2023-03-15T10:35:50Z

sharpener answered · Mar 15, 2023 at 10:35 AM

With the Pylontech Force-L2 batteries the BMS does indeed cut off the battery when the on-load voltage gets down to 47.0V. But there is a lot of hysteresis so it then requires a considerable amount of charging to reverse this to normal operation so is best avoided.

It would be good to be able to use Dynamic Cut-off to keep out of this operating area in a way that takes account of the internal resistance (about 30 mohm per 3.55kWh module). However the recommeded setting of 46.0V is too low for it to work, as described in this thread. (Since then I have added more PV and a third module but it has not directly solved this issue).

So in the end I programmed my 5kVA Multi using Node Red to taper the allowable inverter power from max at 48.0V down to zero at 47.2V in 5 steps.

This works OK - but it is a pity that Dynamic Cut-off doesn't, as it would have been a lot easier to do it that way. Increasing the range over which you can set the cut-off voltage ought to be a simple change for Victron to make.

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thanar commented · Mar 15, 2023 at 03:55 PM

Excellent script idea, I will take a look the other thread. AFAIK, dynamic cut-off is completely useless; what we really need is a maximum battery discharge current setting.

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sharpener thanar commented · Mar 16, 2023 at 10:52 AM

That wouldn't be sufficient. The BMS has a fixed cutoff voltage, but because of the internal resistance the max current you want to allow depends on the cell voltages, which are strongly dependent on the SoC. Hence the stepwise approach.

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thanar sharpener commented · Mar 18, 2023 at 06:26 PM

Voltage drop depends solely on the current drawn; hence, if there was a battery discharge current limit to be implemented, we would be able to set up ESS systems with tiny batteries that would get almost 100% utilisation.

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Answer 2 · 2023-03-15T17:30:12Z

sean answered · Mar 15, 2023 at 05:30 PM

@thanar states this ... as reasoning why dynamic cut off has no place in an ESS using Li chemistry cells.

"Because Lithium batteries have a build-in BMS that will block discharging when the battery's minimum threshold (or a cell's) is reached.

But the above simply isn't correct, perhaps all Victron LI chemistry cells do have internal management systems, but there's a vast amount of ESS's built using all manner of modular cells that do not have, nor need, built in managements systems.

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thanar commented · Mar 15, 2023 at 06:42 PM

There is NO lithium battery with no BMS. I have 20kWh of DIY lithium batteries in the cellar downstairs (mainly li-ion), each and every one of them has its own BMS. Building ANY lithium battery with no BMS is a recipe for a dead LFP battery or a burned-down house, if using Li-Ion.

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Answer 3 · 2023-03-15T19:16:05Z

sean answered · Mar 15, 2023 at 07:16 PM

Perhaps I misunderstood your comment, and perhaps you have misunderstood my reply.

You said "build-in" ... which implies internal, self contained ...

I have 250kWh of ex EV cells, parallel MPs and a REC BMS, I see the dynamic cutoff feature as a useful, but seldom if ever used feature.

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thanar commented · Mar 15, 2023 at 07:24 PM

There's obviously a misunderstanding here, since I know you're not a new kid on the block here!

Afaik, a lithium BATTERY is a series of paralleled cells connected together, ALONG with a properly-sized BMS. In any form. Anything else, is NOT considered a lithium battery. A 18650 flat top cell is NOT a battery, it's just a cell. If you add to it a 1S BMS, then it is a 3.7V battery.

You're right on the "build in" part of my original statement. It actually messes up the meaning a bit. But we're on the same channel after all.

"I see the dynamic cutoff feature as a useful, but seldom if ever used feature"

So, you do agree with me; dynamic cut-off is not useful for lithium batteries. However, the ESS assistant enables it and you cannot disable it. So, I have manually adjusted all four values to my battery's cut-off voltage.

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