DVCC and battery cell voltage balancing - 16S Sunwoda batteries not achieving balance

[mjim (Miguel J)]

Hi everyone,

I’m looking for advice in regards to how to get my plant to balance battery cells (5x Sunwoda Atrix-5 - 16S). According to https://www.victronenergy.com/live/_media/battery_compatibility:victron_multiplus_and_mppts_setting_with_sunwoda_battery.pdf, these are to be configured as follows.

I noticed my plant, even after being active for ~5 months, doesn’t really balance the cells and stops charging at ~54.4V. According to the vendor’s official docs, the charge cut-off voltage is 55.2V.

My Charger settings are as follows, which matches the vendor’s technical documentation above.

With this configuration following the vendor’s technical documentation, I’m unable to get the plant to balance all the cells (it remained unchanged since the plant was deployed). Balance remains okay until SOC reaches 98-99%. Then it triggers and charge is cut at 54.4V.

I was advised to let the plant charge from 90% to 100% with DVCC setting a ver small charge current (2A). However the same happens.

I googled for similar posts in the forum and in case the vendor had provided a guide on how to get these to balance without success. I’m out of ideas at this point.

No alarms were triggered on the system since it was deployed and SOC never got below 12%.

Would it be possible to receive some advice on how to achieve proper balancing on the cells? Thank you.

Best regards,

[lxonline (LX)]

Cut off means the battery has gone into or will go into protection. You should never charge it to there unless you want tripping.

You also need to be slightly lower to allow the bms to bring high cells under control before hitting the cut off. The battery itself only requests 54.7v. so it also doesn’t want to get to the cut off voltage. This is normal behaviour.

You do have a really high cell there in comparison to the low one. That is probably why it is not charging higher.
The bms on the batteries are doing a terrible job of keeping the cells under balance.
This can point to a BMS that has low balance current or poorly matched cells, or bad assembly. I don’t know the batteries but this is what I have seen on other makes and models.

How long have you kept them at keep batteries charged (not at 100% that is slightly different) 1 week or 2 weeks? Longer?
That being said, they do have both voltage and current control and the one screen shot there it has requested a lower voltage so maybe the manufacturer has decided its ok as it is.

Maybe just one more check, use a meter to verify the battery is giving out the right information on its own voltages.

Set up as well sometimes has an effect as well such as daisy chained battery banks.

[mjim (Miguel J)]

Hi LX,

Thank you for your reply. Sorry I didn’t mean I wanted to reach 55.2V (cut-off) but instead the 100% SOC with cells voltage being balanced. For the past ~5 months (since it was installed), voltage didn’t reach 54.7V but instead 54.4V.

How long have you kept them at keep batteries charged (not at 100% that is slightly different) 1 week or 2 weeks? Longer?

Less unfortunately. I think the longest period I had that setting enabled for was 3 days in a row. My installer didn’t instruct me to perform a low current complete charge or anything at that time. I’m learning all of this as I progress.

That being said, they do have both voltage and current control and the one screen shot there it has requested a lower voltage so maybe the manufacturer has decided its ok as it is.

I can confirm once SOC reaches ~99% the BMS starts requesting less voltage. This is CVL during the rest of the operation of the battery. Once it reaches 100% SOC the value is magically reduced to 54.4V and sometimes 54.2V.

Setup is a bank of the 5 items with proper ventilation distances on every direction and clean air (not a dusty room or anything). Temperatures are stable all the year in that room.

As you can see below, the delta between highest and lowest voltage cells increases the closer the battery gets to 100% SOC. This is at 99% SOC and 54.2V voltage while charging at 3.5A current (super low on purpose).

Is there any suggestion from your side I could implement in order to try and have the system balance the cells? I don’t really mind having to use the grid entirely for a week or more if needed. I just want the system to be healthy/at the recommended operational parameters.

Thank you for your time and help.

[lxonline (LX)]

Highest cell: 3.44 V
Lowest cell: 3.37 V
Difference: 0.07 V (70 mV)

For most lithium-based battery packs (especially LiFePO4), a 70 mV spread is generally considered very good. Many battery management systems (BMS) only start actively balancing when differences reach 50–100 mV or more, depending on the design. So your pack appears well balanced.

Keeping then at around 3.4v per cells keeps them unstressed so i would say thats good and possibly your concerns may not be necessary.

If you really want to then check each battery with a decent volt meter to make sure the voltage it says it is it is actually that voltage is but that would be it.

[mjim (Miguel J)]

The delta is sadly that low only when SOC hasn’t reached 100%. Once it does, it grows significantly.

This is from earlier this week. 3.51 → 3.36. It’s always similar to this once it reaches 100% SOC for >2 hours. I’ve seen the highest cell at 3.53 too.

I doubt it’s the BMS measuring wrong but at this point I’ll see if I can get a second hand volt meter and check each battery individually yeah.

[lxonline (LX)]

3.36-3.51 is 150mv.
Then i see quite a voltage drop when there is small amps drawn out.

Set your absorption time to 8 hours and set keep batteries charged for as long as you can. That might help or at least highlight if there are other issues.

[mjim (Miguel J)]

Done. Would 24 hours be reasonable? If not I can leave it on Keep batteries charged until Monday or even more.

I very much appreciate your help. Thank you.

Edited: thank you Nick for your comment. I’ll leave it like this until Sunday at least and report back here if I see any progress. Once I’m done with this I’d like to publicly document the solution for the next person who faces the same problem with these batteries.

[nickdb (Nick)]

With obstinate batteries, leaving the system on keep charged for.a couple of days can be beneficial.
Sometimes it pays to do a deep discharge as well.

[mjim (Miguel J)]

So far setting absortion to 8 hour has been like this for ~5 hours. It reached absortion ~4 hours ago.

Is it expected?

[alexpescaru]

From manufacturer docs: 55.2V - 3.45V/cell for a 16S - can’t be a protection cut-off.
For sure it’s a way of saying - no more than this: this is the voltage for constant voltage, “absorption” phase.
I would set the current limit to 10A and voltage limit to 55.2V and let it keep charging up until delta is smaller than 30mV.
If possible, monitor the high cell voltage and reduce the CURRENT limit as soon as the high cell voltage is reaching more than 3.55V.
Keep reducing it up until it’s not possible to go over 3.55V per cell.
Some will say that you should reduce the voltage, but the effect will be the same - reducing the voltage will eventually make the current go down.
But you need to keep the voltage as high as possible for the balance to start and keep working.
So reducing the current, in a way, you still keeping the charging process on a constant current, voltage ramp-up way, allowing the voltage to reach the final voltage target.
During time you’ll see/learn that charging with small currents will help cells to reach more evenly the final stage and delta will be lower and lower.

[lxonline (LX)]

@alexpescaru

From manufacturer docs: 55.2V - 3.45V/cell for a 16S - can’t be a protection cut-off.

I agree this does seem quite conservative and a bit low (opinion) but it is not the first manufacturer i have seen limit it there.

@mjim
Is the high cell always the same one? If you hover your mouse over the minnand max usually the cell address appears there.

@alexpescaru

would set the current limit to 10A and voltage limit to 55.2V and let it keep charging up until delta is smaller than 30mV.

@alexpescaru

But you need to keep the voltage as high as possible for the balance to start and keep working.

This is the idea- the thing is the bms is intefering by dropping the max CVL. The only way to prevent that is remove bms control (and reset the system to continue) which means mjim will have to watch everything like a hawk.

[MSl1]

@lxonline

The only way to prevent that is remove bms control (and reset the system to continue) which means mjim will have to watch everything like a hawk.

And that is not the best solution.

Another Sunwoda user here in the forum reported also cell voltage differences of 50 mV. The current Sunwoda BMS firmware makes not the best job (compared to other manufacturers in the market). But the BMS algorithm is designed by Sunwoda according here cells used.

@mjim can ask the Sunwoda Technical Support to check his system behaviour.

In the past I have also worked with Victron gx DVCC settings to manage the behaviour of my Pytes Battery Stack. (It makes stress.)

I contacted the Pytes Support. There was a lengthy series of emails, many many logfiles, a great deal of discussion, some of it quite heated, but always solution-oriented. Since October 2025, I’ve been running firmware (V27) that fulfills all the requirements I (and other users) requested. While the CCL is reduced from 75 A to 50 A for one battery in the stack (I never saw the 75A again) in this firmware, the charging behavior is now continuous up to a state of charge (SOC) of 100%, and the balancing works as desired at ALL SOC levels. All controlled by the BMS.

Conclusion: The manufacture of the battery and the BMS must do the job. If they think that 50mV cell difference is OK, then it is their decision.

[Frisianstar]

Can you set in the BMS when cell balancing should start? Set it very low like 3.300 volts.

If you want to measure cell voltage down to the milivolt you must have a accurate 4.5 to 5 digit read-out digital multi meter, 90% of the dmm are 3.5 digit and are useless.

Very accurate dmm with 6.5 digit reading are also great for measure DC voltage drop over cables, fuses, switches.

And then accuracy of an dmm.. +/- 1% may sound pretty good it’s actually very bad: +/- 1% of 3.500 Volts could be 3.535 or 3.465 Volts a difference of 70 mV!

[mjim (Miguel J)]

First of all thank you everyone for your answers and help. I’ll try to answer everything below.

Is the high cell always the same one?

Not really. Sometimes it matches, other times it doesn’t. Currently it’s 4:16. Yesterday it was 5:15. I however have seen 5:15 repeatedly as the highest voltage cell over the past week.

This is the idea- the thing is the bms is intefering by dropping the max CVL. The only way to prevent that is remove bms control (and reset the system to continue) which means mjim will have to watch everything like a hawk.

I’d like to save that as the very last resource as that could result in permanent damage to the system. Actually I’d rather have slightly imbalanced battery cells than risking damaging the system.

can ask the Sunwoda Technical Support to check his system behaviour.

I reached out to them and even shared this forum thread with them in case they identify a solution and want to post it. However I don’t have much hope on them answering.

Can you set in the BMS when cell balancing should start? Set it very low like 3.300 volts.

To my knowledge Sunwoda offers no public software to alter the BMS whatsoever. I researched extensively and found nothing for it.

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It doesn’t really matter what voltage goal I set because this setting is controlled by the BMS and is never higher than 54.7V. Actually sometimes it lowers it to 54.2V.

would set the current limit to 10A and voltage limit to 55.2V and let it keep charging up until delta is smaller than 30mV.

I tried before and the delta remained super small until SOC got to 99-100%, then it started growing the same way I shared before.

My current candidate approach is to keep the 8 hour absortion window I set via VEConfig with the voltages the vendor recommends, then DVCC to limit charge current to 3A and maximum charge voltage at 55.2V (although as seen above, the BMS sets a lower value so this is useless unless I’m missing something). The plan is to, unless voltage grows significantly/to a risky value, keep it that way until Monday at least and monitor the min-max cells voltage delta.

I’ll report back here once I see the results.

Best regards,

[MSl1]

@mjim In your first post is the picture with all parameters to set for Victron systems. In your case the absorbtion time MUST be ALSO set. Thats why I think, the BMS is not “full programmed” (less intelligent) like other BMSes.

A good programmed BMS controls ALL relevant batterie/cell parameters by itself. It controls the voltages, currents, times, balancing and so on completly.

In other words: It says the Victron Inverters what they have to, not more. The Victron Inverters hear the “commands” from the BMS and follow this consequently.

[lxonline (LX)]

@MSl1

And that is not the best solution.

I agree.

@MSl1

Conclusion: The manufacture of the battery and the BMS must do the job. If they think that 50mV cell difference is OK, then it is their decision.

And also here. That is how they have decided to manage thr batteries.

[mjim (Miguel J)]

I know @MSl1 - but if I can’t control the BMS parameters then that’s out of my control hence not something useful to resolve this issue.

The goal here is to resolve the issue and not to just say “oh my BMS is dumb I wish I could alter foo, bar and bazz params on it”. Don’t you think?

–

Current state: 10A sustained charge for the past ~11 hours, SOC at 99%. 54.0V. BMS params say max value is 54.7V. Just 0.1V of cell voltage delta (max 3.38V, min 3.37V). Now I reduced the Charge Current Limit to 5A and will continue monitoring. Currently 02:16 is minV, 3:02 is maxV.

Edit: as soon as Voltage reaches 54.0, the delta starts growing and continues doing so. We’re at 0.05V delta already. 02:16 min, 05:15 max.

[MSl1]

@mjim

and not to just say “oh my BMS is dumb I wish I could alter foo, bar and bazz params on it”. Don’t you think?

This was NOT my intention, sure!

What I mean is the manufactures made BMS should be the first approach to control your battery. Sure, you can manually adapt via DVCC the one ore other setting. I have made this also in the past, to solve my charging problem in the high SOC states. But I solved ONE problem and got then other new (unwished) effects.

btw:

@mjim

Just 0.1V of cell voltage delta (max 3.38V, min 3.37V).

A very good value!

@mjim You habe 5 batteries in your stack. Can you post a picture of your stack, above all the DC cablings?

[mjim (Miguel J)]

It reached the point where voltage delta is the highest (>0.1V) and things get crazy.

It starts auto-discharging after reaching 54.7V (the value the BMS originally set for max voltage despite mine being 55.2V).

At the same time the BMS reduces the max voltage by its own (this is a setting I don’t control). CVL is set to 54.2 from the prior 54.7V value.

Some battery modules enter the blocked state (I assume this is because these individual items are at 100% of their SOC).

Charge Current Limit is manually set in DVCC by me to 1A so in case it takes some current to attempt a balancing, the amount is very much minimal.

MP2 state is "Absortion”. Note the value was increased from 1 hour to 8 hours by me based on earlier advice on the thread.

Behavior of the battery is: discharge a bit (54.6V), Idle, charge a bit (54.7V). Repeat.

12 hour window:

Last hour zoom (when the delta starts growing) until 54.7V is reached. Note how got a bit past it (up to 55.0V!), then discharged by its own.

No alarms were triggered during the process. Note the above graph had all the changes made by the BMS. I didn’t touch anything.

My plan right now is to let it sit like this until Monday unless one of the cells reaches 3.55V, in which case I’ll need to discharge a bit the system to keep it safe.

–

If anyone has ideas or suggestions other than ditching the batteries and getting a pack with a proper BMS, I’m open for it.

Best regards,

[mjim (Miguel J)]

Apologies as I had missed your edit asking for the pictures. Top and second batteries seem to be the ones where the LEDs turn on and off so I guess these are the ones pending to complete the full charge.

Everything:

Just the batteries:

Label on each:

Technical instructions for the specific model:

MANUAL-BAT-GREENHEISS-GH-LI-5.0-WHITE.pdf

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PS: there seems to be some movement!