Issues balancing with Batrium BMS

[hummmingbear]

I have a 4-battery packs with cells in 16S (EVE 280K) and am using the WatchMon-CORE kit with a CellMate-K9 for each pack. I have had issues with a few cells not staying balanced in different packs. What is confusing to me is the voltages batrium is reporting...if my pack voltage is 55.20 that would mean I should either be at 3.45 at each cell OR some cells should be above 3.45 and some cells should be below 3.45. In the screenshot below, you can see none of the cells are above 3.45 but there are 3-4 outliers below with the lowest at 3.38

I don't understand how that can be, and I'm a bit frustrated I can't get my cells in balance. I want to avoid having to open my pack and manually balance them. My charge controller is set to bring the pack up to 55.2V (3.45/cell) for 1-hour, and then go to float at 53.6. My batrium BMS is set to start balancing at 3.45, so any cell above 3.45 it will start using it's resistors to bring the cell(s) down.

I can't get these few cells back into balance for the life of me. What am I missing here? How can the pack be at 55.20 when not one cell is above 3.45 but multiple cells are below?

Any help/input appreciated, thank you.

 

[sunshine_eggo]

The Batrium tracks to X.XXX volts, but it reports in X.XX and rounds. There are measurement errors as well. I see the same behavior. Total voltage doesn't influence balancing nearly as much as properly configuring the balancer.

Balancing should be set to 3.40V

enable autolevel:

Hardware
Cellmon
EDIT
Select Auto Level
More

Enter the following parameters:



The Batrium will engage balancing when cells are over 3.40V and 0.02V higher than others. It will only work on the top 0.01V of cells.

Set float to 54.4V to allow for more balance time.

Once you achieve 0.02V balance in the 3.40-3.45V range, you can likely disable autolevel and simply use the bypass voltage setting.

EDIT:

My Batrium brought this into balance using Autolevel... took like 56Ah from the highest cell to finally get it balanced:

NMC Bank, Bus Bars and Batrium Love

It has been years in the making, but I finally have the NMC battery poised for deployment... (okay, only half, but still). Summary: 7X Ford Fusion/C-Max Energi PHEV batteries, 7.6kWh rated per pack. Each battery: 84S 25Ah 3.7V NMC cells. Four subpacks of 21S in series Packs have tested...

diysolarforum.com

IMHO 3.38-3.45V means you're not horribly imbalanced.

 

[hummmingbear]

The Batrium tracks to X.XXX volts, but it reports in X.XX and rounds. There are measurement errors as well. I see the same behavior. Total voltage doesn't influence balancing nearly as much as properly configuring the balancer.

Balancing should be set to 3.40V

enable autolevel:

The Batrium will engage balancing when cells are over 3.40V and 0.02V higher than others. It will only work on the top 0.01V of cells.

Set float to 54.4V to allow for more balance time.

Once you achieve 0.02V balance in the 3.40-3.45V range, you can likely disable autolevel and simply use the bypass voltage setting.

EDIT:

My Batrium brought this into balance using Autolevel... took like 56Ah from the highest cell to finally get it balanced:

NMC Bank, Bus Bars and Batrium Love

It has been years in the making, but I finally have the NMC battery poised for deployment... (okay, only half, but still). Summary: 7X Ford Fusion/C-Max Energi PHEV batteries, 7.6kWh rated per pack. Each battery: 84S 25Ah 3.7V NMC cells. Four subpacks of 21S in series Packs have tested...

diysolarforum.com

IMHO 3.38-3.45V means you're not horribly imbalanced.

Thank you for your reply!! I will try those settings...
With Autolevel on does Bypass Voltage setting even matter? My 4-batteries total 57,344wH, so I'm wondering if my settings below make sense for my size?
I really appreciate your help, you've replied to some of my older posts in the past too, thank you! I should get a full charge today so I'm curious to see what happens.

I know they are not far out of balance...but I still want to be able to get them close.

 

[sunshine_eggo]

Thank you for your reply!! I will try those settings...
With Autolevel on does Bypass Voltage setting even matter?

No. Auto level overrides the bypass voltage value, or at least that's been my observations.

My 4-batteries total 57,344wH, so I'm wondering if my settings below make sense for my size?

Size isn't much of a factor. Mine's 23.3kWh. It's just how fast the passive balancers can burn off the extra capacity on the high cells.

I really appreciate your help, you've replied to some of my older posts in the past too, thank you! I should get a full charge today so I'm curious to see what happens.

You just need to keep the high cells above 3.40V, and they should burn off. If you find your balancers aren't overheating, you can change bypass banding to 0.02V to potentially activate more balancing.

I know they are not far out of balance...but I still want to be able to get them close.

Agree. Settings you posted look good.

 

[hummmingbear]

Do you know why batrium states:

Extra Bypass Mode should not be used with LiFePO4 batteries and should not be left on permanently. This may shorten the life of your cell monitors.

If they are only above your threshold for 30min to 1hr a day is it really going to burn the resistors out?

Their config has always been very confusing to me. I don't quite understand all the fields even when I look at their wiki. Glad you think my settings are good for now. We'll see how it balances today, just getting to 3.40v right now

 

[sunshine_eggo]

Do you know why batrium states:

If they are only above your threshold for 30min to 1hr a day is it really going to burn the resistors out?

Their config has always been very confusing to me. I don't quite understand all the fields even when I look at their wiki. Glad you think my settings are good for now. We'll see how it balances today, just getting to 3.40v right now

I leave it on, but only due to catch voltage anomalies. Sometimes, the high cell voltage will jump 0.1V higher. I leave autolevel on set for 0.07V as it immediately corrects those conditions. You can see it in the thread I linked.

My low current active balancers really do maintain them adequately. I'll occasionally enable auto level out of a sense of OCD every couple months or so.

 

[hummmingbear]

So I'm getting little to no progress with balancing here. For instance cell 48 was initially at 3.40. But it and the other low cells drop to 3.38 (edit, now 3.37).



I upped the charging voltage to 55.45 (measured at the shunt), as the voltage for each pack is closer to 55.2 which is my actual target (3.45/cell), see the screenshot below. I'm calculating the pack voltage by summing each cell in the pack:


I feel like this has been the same scenario for days and these cells never balance. Can't tell if it's a cell issue or balancing power on batrium is too low. I upped the absorption time to 2 hours but happy to go longer to give it time.

 

[sunshine_eggo]

I'm seeing 23 cells balancing.

As you can see from the thread I linked, it took 4 days for me to balance ~56Ah of discrepancies between cells on a 450Ah battery.

I've seen 280Ah EVE LFP cells in the as-received condition varying by about 12% SoC meaning 34Ah that needs to be burned off.

Your balancers look like they're getting hot. They shut down at 75°C. Do you have a fan/fans blowing on the heat sinks?

On shunt menu, Edit, Advanced, disable re-calibrate in bypass.

open Telemetry, select Cell Info, sort by Bypass mAh, and screen cap it:



This will tell you how how much is being bled off.

Get some cooling on your balancers and change bypass banding to 0.05V.

 

[hummmingbear]

I'm seeing 23 cells balancing.

As you can see from the thread I linked, it took 4 days for me to balance ~56Ah of discrepancies between cells on a 450Ah battery.

I've seen 280Ah EVE LFP cells in the as-received condition varying by about 12% SoC meaning 34Ah that needs to be burned off.

Your balancers look like they're getting hot. They shut down at 75°C. Do you have a fan/fans blowing on the heat sinks?

On shunt menu, Edit, Advanced, disable re-calibrate in bypass.

open Telemetry, select Cell Info, sort by Bypass mAh, and screen cap it:

View attachment 132547

This will tell you how how much is being bled off.

Get some cooling on your balancers and change bypass banding to 0.05V.

Thank you for taking the time to respond here. Much appreciated. The low cells keep going lower... 3.36 currently. Here is a screencap of what you asked (and I turned off recalibrate as well)



Current state screenshot below. FWIW It doesn't reflect all cells being balanced as it alternates between them:

 

[sunshine_eggo]

When the unit is balancing, it affects voltage of balanced cells and adjacent cells. The balance wires, which measure the voltage are also now being used to draw current, so voltage is influenced. Each cell measurement is based on the prior cell, so those are affected as well. Don't be concerned that a cell is dropping. The last cell in a line is often a little wonky. #48 is low, and that's the 16th cell. There are some bugs in the balancing algorithm, and voltage readings aren't always reliable. This is per Batrium in response to my observed voltage 0.1V random jump. I also have similar 0.1V drops.

Raise your Bypass voltage to 3.50. I can't remember the criteria for resetting the Bypass mAh counts in the cell info telemetry, but I'm pretty sure that's what's happened, and you're not seeing the actual cumulative.

You may also be overrunning the auto level limits with charge current:




You may also want to allow a higher discharge limit.

This was roughly after 3+ days, so I was getting about 10+ Ah/day on most cells:

You should be getting higher numbers as the K9 has a higher balance current.

 

[hummmingbear]

Raise your Bypass voltage to 3.50. I can't remember the criteria for resetting the Bypass mAh counts in the cell info telemetry, but I'm pretty sure that's what's happened, and you're not seeing the actual cumulative.

Doesn't this mean that bypass won't start until 3.50? I don't totally follow that one.

I can't remember the criteria for resetting the Bypass mAh counts in the cell info telemetry, but I'm pretty sure that's what's happened, and you're not seeing the actual cumulative.

Interesting. I'll see what it says tomorrow.

You may also be overrunning the auto level limits with charge current.

I don't totally follow this. When you say to set it above my max charge current, do you mean set the amperage to the max current I push through? I make around +5kw of charge on a sunny day, so at It's somewhere near 100A...so set it to 120? What does this do? And as for discharge limit, not sure I follow here either.

You should be getting higher numbers as the K9 has a higher balance current.

The max I've seen is .72A. They state here that is has 500mAH of balancing power, which I don't get as I'm getting 720mAH per cell? https://wiki.batrium.com/product/cell-monitor/cellmate-k9 (under balancing)


And again, thank you for the troubleshooting help. I know these aren't way out of balance, I'm just worried that either 1) They will continue to drift and I wont be able to keep them in balance with this BMS or 2) There is an issue with some cells and I will need to pull them. I only have easy access to batteries 3 and 4, they sit on top of 1 and 2, so difficult to open up and do manual reading/testing/balancing and I the whole reason I have a BMS is to avoid that

 

[sunshine_eggo]

Doesn't this mean that bypass won't start until 3.50? I don't totally follow that one.

Sorry, on the CellMon page, not auto level. Set Bypass Volt (CV9) to 3.50

I don't totally follow this. When you say to set it above my max charge current, do you mean set the amperage to the max current I push through? I make around +5kw of charge on a sunny day, so at It's somewhere near 100A...so set it to 120?

Yes

What does this do? And as for discharge limit, not sure I follow here either.

The way you have it set for 20 means if you charge at more than 20A, it will not auto level balance.

The max I've seen is .72A. They state here that is has 500mAH of balancing power, which I don't get as I'm getting 720mAH per cell? https://wiki.batrium.com/product/cell-monitor/cellmate-k9 (under balancing)

I think 500mA is a holdover from the WM5 materials.

And again, thank you for the troubleshooting help. I know these aren't way out of balance, I'm just worried that either 1) They will continue to drift and I wont be able to keep them in balance with this BMS or 2) There is an issue with some cells and I will need to pull them. I only have easy access to batteries 3 and 4, they sit on top of 1 and 2, so difficult to open up and do manual reading/testing/balancing and I the whole reason I have a BMS is to avoid that

1) if they are actually drifting, then you're dealing with #2.

Assuming healthy, while they may drift in voltage, they're not drifting in SoC. The cells at 3.45V are going to INCREASE faster for 1Ah input than cells at 3.38V.

 

[sunshine_eggo]

Also worth noting that my battery is NMC, so balancing is valid at any state of charge due to the high voltage to SoC correlation, so I could balance 24/7. It may take you a week or so of just letting it run before it converges as you'll only be balancing above 3.40V.

 

[hummmingbear]

Also worth noting that my battery is NMC, so balancing is valid at any state of charge due to the high voltage to SoC correlation, so I could balance 24/7. It may take you a week or so of just letting it run before it converges as you'll only be balancing above 3.40V.

I think you're right here. We had 3-weeks of mostly clouds with sparse sun. We are getting back to some sunny days where I will reach a high enough voltage to balance. I'll keep these settings for a bit and see what happens. They are new cells, only put into service in August and QR code shows they were produced in January/Feb of 2022 so I would hope they don't have issues.

How can I increase the balance power of my k9's? This seems to be hardcoded at .72A from batrium

 

[sunshine_eggo]

I think you're right here. We had 3-weeks of mostly clouds with sparse sun. We are getting back to some sunny days where I will reach a high enough voltage to balance. I'll keep these settings for a bit and see what happens. They are new cells, only put into service in August and QR code shows they were produced in January/Feb of 2022 so I would hope they don't have issues.

How can I increase the balance power of my k9's? This seems to be hardcoded at .72A from batrium

You can't.

 

[sunshine_eggo]

Cell telemetry?

 

[hummmingbear]

Cell telemetry?

Thanks for checkin' in! Telemetry below..numbers are slightly higher from yesterday. How does it look to you?




My sense is that the low cells are getting better...slowly. Cell #25 used to be in the 3.37 range at this point of absorption and is now at 3.4. Cell #48 still struggling but this might be a multi-day process. Curious your thoughts on the tele above.

 

[sunshine_eggo]

There are a lot of cells that are getting about 500mAh of bleed off, so that's encouraging; however, the "Initial Byp date" has likely been reset again. I can't believe your cells have only balanced for less than 1 hour on average.

IIRC, the unit can burn 6Ah/day off, and I would expect you to be able to do at least 1-1.5h during your charge/float period.

I can't see all the cells, but those displayed look very consistent - all very near 3.46V.

Can you sort by Min V with the lowest displayed first and post?

Did you change the main Bypass Voltage to 3.50?

 

[sunshine_eggo]

Try turning off and unchecking these:

 

[OffGridInTheCity]

I'm a long time Batrium user.
Reading thru the thread, I don't recall reading that the cells were ever in balance so I ask.... Were the cells in balance when you 1st built the battery or did this imbalance occur after some time period? If they started out in balance and then dropped you might have some weak cells which would make balancing more difficult.

I use Auto Level on Lithium-ion 260ah packs and it can take 4-6 days to 'tighten the balance' by 20mv and you've got 90mv max difference in the 1st post. Patience is good in these situations - especially if things are not getting worse

FYI - the overall battery voltage does not mean the individual cells (in series) will have a specific voltage in relation to each other. They can vary widely and is one of the reasons BMS's are important so you know what's happening.