ESS going into Sustain too soon

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question

Tom Ranson asked Oct 22, '22

ESS going into Sustain too soon

Hi all,

Grid parallel ESS

Cerbo GX with ESS with DVCC

MultiPlus ii

SmartShunt

2x MPPT 100/30

24V battery bank (8x 6V FLA in series/parallel)

My ESS is going into Sustain far too early, when the batteries are only at c. 90% SoC. I have it configured as Optimised with Battery Life.

I believe that the reason for this is the values configured for the Dynamic Cut Off, however I am using the ESS default values for Gel/AGM batteries, as I believe this is the closest battery type to my FLA. I am seeing the system enter sustain mode at 23.99V, and therefore my batteries are not discharging significantly (minimum SoC set to 50%). I believe I should be able to take the batteries down to 23.00V without any issue or risk of damage.

Could anyone guide me further on this or help me optimise my settings please? I'm not sure that my dynamic cut off values are appropriate for FLA batteries, but can't find any documentation or guidance on this anywhere.

ESS battery

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Answer 1 · 2022-10-22T18:49:39Z

gfillary answered · Oct 22, 2022 at 06:49 PM

Im having the same issue. I'm running a 48v system with a multiplus II with 8 x 170Ah power safe batteries after starting with 100% fully charged, it goes into idle mode after on 4.5Kwh has been drawn. However my smart shunt is still saying I have 50.20v's

Im missing as setting somewhere....

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Answer 2 · 2022-10-22T19:28:43Z

nickdb answered · Oct 22, 2022 at 07:28 PM

People, SOC is a derived metric. If the measuring instrument is flawed or setup incorrectly then it is meaningless. Voltage is king. You can only hit sustain if you have setup ESS wrong or the voltage dropped long enough. Read the manuals, setup your systems properly and it will all work out. These are bleak posts that just suggest you need a professional to help.

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Alexandra ♦ commented · Oct 22, 2022 at 07:38 PM

@Tom Ranson

@gfillary

https://www.victronenergy.com/media/pg/Energy_Storage_System/en/controlling-depth-of-discharge.html#UUID-6b8f380a-3e19-c040-9dad-673577d775d5

Lower your dynamic cut offs, default does not work for everything.

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Tom Ranson Alexandra ♦ commented · Oct 22, 2022 at 08:33 PM

Hi @Alexandra thank you. Yes, I have read and re-read the documentation and still can't fathom what the correct and safe dynamic cut-off values would be in my situation with FLA batteries. There is no help in the documentation for this battery scenario, and as far as I can tell, no useful information on the forums.

If you could assist me with understand what would be appropriate values in my specific use case, I would much appreciate it please.

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Alexandra ♦ Tom Ranson commented · Oct 22, 2022 at 08:42 PM

@Tom Ranson

It is hard to say, as they are also affected by age, bank size, battery manufacturer recommendations.

For the most part as long as they are not below your low DC cut off, you will be fine. Even setting them all the same to eliminate that setting as the problem.

You can still program your inverterter to shut off at a certain SOC (depends on how you want to use them). FLA does have quite a voltage collapse under higher load.

All I can say is you need to experiment. Use the VRM or GX to see where the voltage drops under each c rating load and set it from there.

The discharge curve graph information from the battery manufacturer will help as well as it usually shows how the battery acts or is supposed to act under different loads.

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Alexandra ♦ Alexandra ♦ commented · Oct 22, 2022 at 08:43 PM

Most batteries do not have a default setting on the Victron by the way, so you are not in your own boat there.

Are you running your batteries as a keep charged system?

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Tom Ranson Alexandra ♦ commented · Oct 23, 2022 at 09:12 AM

Hi @Alexandra as optimised with battery life. The problem is that Leoch don't seem to publish much useful information for my DT126 batteries. I believe they are essentially a Trojan T-105 knock-off though.

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Tom Ranson Alexandra ♦ commented · Oct 23, 2022 at 10:09 AM

Hi @Alexandra

So my low DC cutoff (for sustain) is currently 23.00V. This I understand is a safe lower voltage and represents 50% SoC as far as battery voltage goes for these batteries. I do not wish to discharge my FLA's below 50% DoD.

My MPii inverter is also configured to shut off at 50% SoC, however at the moment we're not getting anywhere near that before it goes into Sustain, due to hitting the dynamic cut offs.

So are you saying that it might be appropriate for me to set all four dynamic cut off values to 23.00V?

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Alexandra ♦ Tom Ranson commented · Oct 23, 2022 at 11:03 AM

@Tom Ranson

Yes that is what I am saying to do.

We are now troubleshooting and then we can work out frommthere what to do.

See how the system runs at each c discharge after that and if sustain is triggered again.

What size multiplus do you have? Is it the 3kva?

Looking at the discharge curve 6v per cell at 75A is where it is rated new.

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Tom Ranson Alexandra ♦ commented · Oct 23, 2022 at 11:19 AM

@Alexandra ok, I will set all 4 to 23.00V for testing and monitor it. I don't see how any harm can come from this as if anything the inverter will still go into sustain too early due to the voltage collapse at higher currents.

Sorry for being thick, but how should I go about testing how it performs for each c discharge? I am by no means an expert and am trying to learn more every day.

Yes it is the 3kva MPii.

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Tom Ranson Tom Ranson commented · Oct 23, 2022 at 11:35 AM

@Alexandra actually, here are the default values for the Gel/AGM battery type. So setting them all to 23.00V would not be appropriate I don't think? I look forward to your advice.

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screenshot-2022-10-23-123233.jpg (131.1 KiB)

Alexandra ♦ Tom Ranson commented · Oct 23, 2022 at 12:20 PM

What size inverter do you have?

And what load is on the system when sustain is triggered.

Bow often does the system get to a full charge (by definition this is all the way through a full absorption)

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Tom Ranson Alexandra ♦ commented · Oct 23, 2022 at 12:58 PM

@Alexandra It's a 3kva inverter, however it's currently significant limited to only 250W (another problem, another story which I am discussing with Victron).

The load last night when it hit sustain was only 250W, so approximately 10A.

The batteries get a full charge every night on an ESS scheduled charge (cheap rate electricity).

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Alexandra ♦ Tom Ranson commented · Oct 23, 2022 at 02:14 PM

Then change the top two c ratings to ths same value as the third one. You are wanting to cut off at 24v so the inverter will switch off (in Theory) before the ess values are met, so should not trigger sustain.

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Tom Ranson Alexandra ♦ commented · Oct 23, 2022 at 04:53 PM

@Alexandra so by changing the 0.005 and 0.25 C values to 22.80V, the inverter will remain operational down to 22.80V (and even lower for the higher 2C discharge rate)? I would only ever want to enter sustain if the battery was going to hit and remain in a very low state of charge, but given they are recharging every day, this should not be a concern? I just want to make sure that I fully understand the change you are recommending. N.b. all my loads are on AC-In (AC-Out-1 and 2 aren't used). When the system enters sustain, the inverter stops, so it's currently stopping when the battery still has energy to give (down to 23.00V which would be approx 50% DoD). Is this to be expected?

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Alexandra ♦ Tom Ranson commented · Oct 23, 2022 at 05:18 PM

@Tom Ranson I 100% understand what I am asking you to do. To eliminate the ESS dynamic voltages as the cause of the sustain, you need to do this. [EDIT: or you need to troubleshoot your battery bank itself]

You have your shutdown on 50% SOC. (is it possible your SOC is actually inaccurate maybe?).

once you have seen how the system behaves on the different loads, you can revise them upwards. but first you need the data from the VRM to see what is happening and get some data as to where to set it.

At this point my suspicion is your batteries are not well as they have an increased internal resistance which causes voltage sag. ROI on lead acid is terrible in cyclic use.

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Alexandra ♦ Tom Ranson commented · Oct 23, 2022 at 05:41 PM

What have you been charging them up to as absorption/bulk? have you ever equalized them.

How old are they? what kind of maintenance has been done on the bank?

are all your battery terminals tight still?

Have you tried taking them out the system, letting them rest and seeing of there is a difference between all the 6v cells? voltage wise?

Have you ever made the system shutdown on low battery voltage?

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Tom Ranson commented · Oct 22, 2022 at 08:31 PM

Hi @nickdb thank you for your response. I totally agree and understand that SoC is only a calculated metric, however I am still struggling to know what is the best practice for the dynamic cut off values when using FLA batteries (as there is no corresponding battery type default in ESS), which isn't documented or as far as I can find, discussed on these forums.

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