Cell balancing in a Pylontech battery

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peter12381 asked Oct 22, '21

Cell balancing in a Pylontech battery

Hello Victron friends
I am installing a third Pylontech US3000 battery to boost the storage capacity of my off-grid system, and am once again getting high voltage alarms. The battery arrived with SOC = 39% and I am charging it by itself using solar panels via the EasySolar II. The high voltage alarms started at 73% SOC & 50.6V and so I used the option in Remote Console DVCC settings to limit the charge voltage to the max possible without triggering alarms, ie 50.5V. I have included a screenshot from BatteryView.exe which shows what I presume is the problem: Cell 3 is at 3.51V, much higher than the others which are 3.3 - 3.4 V. The situation has changed little from yesterday to today, and I have some questions to ask of anyone who might know more about the internal cell balancing processes than I do.

- Is an external current input necessary to charge the low voltage cells, or do the cells redistribute charge among themselves? If they can redistribute internally, does it also happen when the battery is disconnected but switched on?

- I understand that cell balancing uses quite small currents. Am I going to have to wait for all of the other cells to catch up with cell 3? That would be a lot of amp hours and presumably will take a long time.

- Would the cell balancing be adversely affected if I connected the three batteries together? Running off three batteries rather than one would be more convenient as it would give me enough capacity to last through the nights and I wouldn't have to juggle the batteries.

Thanks!

BMS Pylontech alarm

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Answer 1 · 2021-10-22T03:50:57Z

Matthias Lange - DE answered · Oct 22, 2021 at 03:50 AM

Leave the voltage at 52V but reduce the current and let it charge, it can take some hours to balance the cells.

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peter12381 commented · Oct 22, 2021 at 05:29 AM

Thanks for your response Matthias.

That is a different approach to what is in the Victron/Pylontech setup document, which says to limit charge voltage rather than current. Fundamentally, I don't want to do anything that could affect my warranty on the new battery. However I see that you are employed by Victron, so maybe I can take this as official advice?

I am sure that if I raise the charge voltage it will precipitate a cascade of high voltage alarms, even at low charge current. Are you saying that I should ignore that?

Thanks

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Matthias Lange - DE ♦ peter12381 commented · Oct 22, 2021 at 09:27 AM

Voltage rises because of the charging current.

With a lower current the voltage will rise slower and the BMS has more time to do the balancing.
With a very low current you will never see a high voltage alarm because the BMS has enough time for balancing.
You can also combine a lower voltage with a lower current.
The very first charging always should be done very gently.

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heleb Matthias Lange - DE ♦ commented · Jul 19, 2022 at 01:54 PM

<<The very first charging always should be done very gently. >>

How many Amps do you suggest?

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Answer 2 · 2021-10-22T07:01:36Z

seb71 answered · Oct 22, 2021 at 07:01 AM

Cell3 and Cell4 might be bad/lower capacity than the other cells. Especially for Cell3, the difference in voltage (compared with the voltage of the other cells) is very big.

Since you are off-grid, your options are limited. Ideally you would want to not use that battery (just keep it charging) until all cells gets balanced/equal voltage at the top (assuming they would eventually get to that point).

Contact Pylontech (or your seller) and see what they have to say regarding this big voltage difference between cells.

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Answer 3 · 2021-10-22T08:42:07Z

CristiMV answered · Oct 22, 2021 at 08:42 AM

This is normal, and it will happen every time you add a new battery to the system. In 1-2 days this will disappear. Still, why in the BatteryView you see only one battery, if you have now 3 units connected?

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Answer 4 · 2021-10-23T22:18:56Z

raymiller answered · Oct 23, 2021 at 10:18 PM

I've just been through the cell balancing exercise with two Pylonthech UP2500 (24V) battery modules. A couple of variables to keep in mind.

- it would seem either the time between manufacture and commissioning or since the last charge has a large effect on the time it takes the cells to be in balance.

- The most difficult battery to balance took many days to balance.

Procedure

Charge the new battery at a low current like 1A with the voltage set to 3.55V per cell. Using Pylontech software BatteryView is useful but not necessary, using Cerbo GX data can be used to monitor max and min battery cell voltage (it at least worked for my battery be it in a label reversed state).

- If and when the battery then gets to the Alarm state when one cell has reached 3.6V it will be necessary to disconnect the charger and let the battery sit for say 30 minutes to allow the 3.6V of the highest cell voltage to settle lower below the alarm threshold.

- then start charging at a reduce the charge current to 100-200ma (experiment on the highest current you can get away with without reaching the 3.6V). I used a separate constant current and constant voltage power supply. Set the voltage to 3.6Volts per cell and adjust the current to 100-200ma to prevent the highest cell from reaching 3.6V while the weak cells catch up. The catchup time can be days.... The below graph is my process of cell balancing, as you can see an initial cell balancing occurred around event 53, then event 105 the battery entered into an alarm state and nothing much happened the weak cells were charged very slowly.

At Event 170 I reduced the charge current (100-200ma) sufficient to drop the maximum cell voltage below 3.6V, then the BMS seemed to go into action balancing the weak cells more rapidly than before. You know when the full balancing has been achieved when the LED indicators on the Pylonthech battery goes into idle (single flashing top green LED) after maintaining 3.55V per cell for about an hour.

The Pylonthech BMS seems to be advanced but the software is poor at balancing the cells if they are unbalanced beyond a certain state, resulting in a poor customer experience. If I was commissioning a set of batteries it may be very prudent to bench charge (as it seems it can take days) all the batteries before going to the site eliminating time-consuming commissioning.

I hope this helps anyone with this cell imbalance problem.

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raymiller commented · Oct 23, 2021 at 10:36 PM

I should say the charger voltage should be set to 3.55V per cell ideally, 3.6V per cell also works.

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seb71 commented · Oct 24, 2021 at 07:19 AM

Set the voltage to 3.6Volts per cell and adjust the current to 100-200ma to prevent the highest cell from reaching 3.6V while the weak cells catch up.

It does not work like that.

You can't set a cell voltage. You can only set a battery charging voltage. Then the balancing circuit will attempt to discharge the high cells when they get close to the top, while the other cells continue to charge. So if you have cells with great differences in voltage (like in this case), you have to reduce the battery voltage, too. Not just the charging current.

A low charging current will help because it gives the balancing circuit a chance to discharge that high cell at the same pace or faster. All cells will rise in voltage at a lower pace. Including the "high" cells. So the balancing circuit has more time to do its job (to discharge the cells with higher voltage near the top).

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mvas seb71 commented · Jul 15, 2023 at 01:02 AM

Actually, we can get to 3.6 volts / cell

54 volts = 15 cells x 3.6 volts / cell

Yes, it does work like that.

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seb71 mvas commented · Jul 20, 2023 at 07:32 PM

54V battery voltage does not necessarily mean that each cell has 3.6V, as I explained above.

Identical cells and identical charging states are not a given.

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Answer 5 · 2021-10-23T22:44:12Z

raymiller answered · Oct 23, 2021 at 10:44 PM

Leave the voltage at 3.55V per cell (53.25V) otherwise all the cells will not fully balance. Setting the charger to a constant current (100-200ma), constant voltage (3.55V per cell) is key to achieving the cell balancing in the shortest time.

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tiego commented · Jul 28, 2023 at 09:17 AM

Do you have a link of a suitable power supply vwhich is available on Amazon?

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Answer 6 · 2021-10-26T01:19:20Z

peter12381 answered · Oct 26, 2021 at 01:19 AM

I'm giving it the gentlest charge I can using the Easysolar II. Charge current is set to a max of 1A, and the charge voltage as high as it will go without creating high voltage alarms from the recalcitrant cell 3. Currently I'm at 50.9 V, and am increasing the voltage in 0.1 V increments when the charge current falls close to zero. I'll give this a go, and if I don't get anywhere soon I'll take it up with the supplier and Pylontech. I'll keep everyone posted. Thanks for the support so far.

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Answer 7 · 2023-07-14T19:09:59Z

tiego answered · Jul 14, 2023 at 07:09 PM

Good day,

I would like to join the question here to understand more precisely how the cell balancing works with the Pylontech.

The equalization takes place according to my information only if:

-> At least one cell has over 3.450V &

-> The cell difference within the battery is over 0,030V &

-> The battery is being charged &

-> The battery is not in error &

The balancer discharges the cells with 50mA.

So, my questions:

If the charge current of the battery is only 100mA, this is split between all 15 cells and thus all cells receive only 6.66mA. Does the balancer still discharge the cells that have a difference greater than 30mV with 50mA? Or only with 6.66mA
Is the charge current within the battery always distributed equal between the 15 cells?
When the battery is almost full I have 3.527V at the cell with the highest voltage and 3.378V at the lowest in the battery, so a difference of 0.149V. Is this value OK?
When the battery is almost empty (SOC 20%) I have 3.235V for the cell with the highest voltage and 3.120V for the lowest in the battery, so a difference of 0.115V. When the battery is then charged again 100% it goes into cell overvoltage fault.
If the battery is discharged only to 30% or even higher I have no problems and the system runs perfectly.

What is your assessment? Does this have to do with an unbalanced battery or does the battery have an internal cell fault.

I am trying to balance the battery with 1A each. See if it brings something.

Thanks

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sharpener commented · Jul 20, 2023 at 03:13 PM

@tiego wrote <If the charge current of the battery is only 100mA, this is split between all 15 cells and thus all cells receive only 6.66mA.>

I think this is incorrect, the cells are in series so the 100mA is available to all cells equally. In which case then if you are right about the 50mA balancing current it would make a big difference to it but it would not reduce it to zero.

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Answer 8 · 2023-07-14T20:58:20Z

dayandnight answered · Jul 14, 2023 at 08:58 PM

Hello @tiego ,

according to my experience I would use a bench power supply and select voltage with 53,2V and max current 2A. When the battery is going to reach SOC 100% it will reduce the current on it's own by the BMS. BMS might even cause that it suspend charging and switch the battery to standby. Don't worry! Be happy! Keep it on 100% SOC for many hours.

It is important that the battery is reaching regularly 100% SOC. In this state it will do the balancing of the battery cells and it might take really very, very long. Days! When some of the cells causes HV alarms it is on you to decide how to go on. I had already some Pylontech which claimed for some time HV alarms and after several time to charge it to 100% SOC it had perfect balanced cells and no longer any alarms. My understanding is let the BMS do it's job.

Nevertheless it might be that some of the cells are faulty, than there will be no way to balance them. Without knowing details and the history of the battery it is only guessing. Ask your supplier or Pylontech for help!

Servus - DayAndNight

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Answer 9 · 2023-07-20T19:24:39Z

kolli answered · Jul 20, 2023 at 07:24 PM

Tiego, compared to the charging behaviour of my Pylons your cell values sound unbalanced. With current ESS mode your Pylons should get charged by the MP2 up to 52,8 V (formerly it was 52,4 V) and get balanced at that Voltage. My cells don't differ more than 50 mV at any charging state, at 52,8 V they differ around 30 mV, not much more or also less.

I would recommend you balance your Pylons fully charged at 52,8 V for some hours.

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Answer 10 · 2023-07-28T09:10:19Z

tiego answered · Jul 28, 2023 at 09:10 AM

thank you all for the answers,

can I use a normal lab power supply (Amazon like) for charging or is there a charger from Victron over which I can charge the batteries from 230V AC without the risk of something going up in flames?

So I charge the batteries with the inverter to SOC 100%, then I remove them and charge them with 52.8 V or 53.2V with 1-2A with the lab power supply for a few hours?

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