Heltec BMS' (up to 350A) with Active Balancing & Independent Active Balancers

fafrd

Solar Enthusiast
The Heltec Smart BMS with active balance will balance any time the difference between cells exceeds the "Trigger Diff Volt" parameter setting.

From the manual -
e) Trigger Diff Volt - Triggered balance difference Voltage is the only parameter that controls balance. When the balance switch is on, when the maximum difference voltage of the battery pack exceeds this value, the balance starts, and the balance ends when the difference voltage is lower than this value. For example, the balance trigger voltage difference is set to 0.01V. When the battery pack voltage difference is greater than 0.01V, balance is started, and when the battery pack voltage difference is lower than 0.01V, the balance is ended. (It is recommended that the balance trigger difference voltage of the battery above 50AH is 0.005V, and the balance trigger difference voltage of the battery below 50AH is 0.01V)

This is from the Alibaba Heltec
View attachment 28123
Trigger Diff Voltage is a key parameter for understanding how Active Balancing will function and the Smart Heltec BMS you have appears to be very different than the non-Smart Heltec BMS I am considering.

That non-smart BMS appears to have a Trigger Diff Voltage of 100mV, meaning it will only have any impact above the knee.

A Trigger Diff Voltage of 10mV or 5mV would mean Active Balancing would likely be effective through practically the full charge/discharge cycle which may not cause a problem but pretty much guarantees that charge will be added to the weak cell during discharge which will then need to be pulled back out near the top of the charge cycle.

Ideally, I’d like all my cells to reach 3.575V (for my 28.6V Charger) at the same time, or if not, for the weak cell to lag and get boosted up to the others with Active Balancing.

The situation I want to avoid is to have the weak cell reach 100% SOC before the rest of the cells, continue charging up to high-voltage Shut-Off (3.75V) and have the rest of the cells slowly inch their way closer to 100% SOC as the weak cell relaxes below high voltage reconnect.

If that ends up happening, I would rather have no additional charge going into the weak cell during discharge.
 

fafrd

Solar Enthusiast
I find this interesting as well. The passive balancing on my BMS doesn't kick in until the voltage is more than 3.4 volts per cell or 27.2 volts for the pack voltage. Above that is when I start to see higher than a 10mv difference between the cells. Last night I realized the delta setting for balancing was set to 30 mv's. So I set it to 15mv's which is the recommended setting for my Overkill BMS. As I recall the balancing current is 70ma's.

Anyhow, I set the charger to 27.2 volts and it went into CV properly and continued charging like it's supposed to. The current came down gradually. But I noticed the voltage of the pack rising above 27.2 when the charger displayed around 300ma's. So just for kicks, I started bumping up the voltage on the charger and I kept bumping it up until the highest cell voltage of 3.65 volts caused the BMS to cut off. There was just less than a 60mv difference between the cells. And the charger displayed very little mah's that went into the cells when the BMS cut off.

I know when the pack voltage reached 27.10 volts, the Ah's in was 265. SOC was 95%. I had the BMS's nominal capacity set to 280ah's and have since set it to 275ah's since the total I have been able to get from the pack is 272ah's.

My BMS can be set to balance while charging or to keep balancing on. Since it needs in excess of 200ma's going to the cells to balance while charging, I set the balancing to remain active with the charger disconnected. It took about 2 hours for the cells to come within 15mv's of each other.

My conclusion is most people stay between the knees and there is very little difference in voltage between the cells in this area. Mine vareis between 6mv's and 20 mv's between the knees. I have found the sweet spot of my pack to be between 25.35 and 27.2 volts per cell. I don't know if this would improve with a higher amperage charger than the 12 amp charger I am using, and drawing higher currents than the 17 amps I am drawing. And I don't know what effect all of this would have on the cells drifting over time. I do know for now, I don't need any balancing as long as I stay within the knees. And I am not sure how effective any balancing would be using non capacity matched cells because the voltage on my pack shoots up once 27.2 volts is reached, and drops quickly once 25.35 volts is reached.

What I see on the top is a voltage difference of 0.083mv's when the highest cell is 3.65 volts. What I see on the bottom is a .5 voltage difference when the lowest cell is 2.50 volts. I would say my parallel top balancing worked really well....lol
Yes, I agree that most people likely never have any engagement of Active or Passive balancing (especially those limiting themselves to within 20% and 80% SOC).

Can you share a link to the BMS you are using?
 

Gazoo

Dumb Dumb
Yes, I agree that most people likely never have any engagement of Active or Passive balancing (especially those limiting themselves to within 20% and 80% SOC).

Can you share a link to the BMS you are using?
It's on his website which I will link to. I realize it's more expensive than ordering from China. I purchased from him because of his awesome support and warranty, and I didn't want to mess with crimping lugs. Also the instructions are very comprehensive and even explain top balancing.

Overkill Solar – More is Better…

It can also be purchased from Will's website. The link redirects to Overkill Solars website but Will get a small commission.
 

Gazoo

Dumb Dumb
If you discharge at a very low rate, Active balancing to the bottom of the discharge cycle could theoretically deliver the full charge capability of the battery, dispite cell mismatch.
As I understand it the active balancer will draw current down from the other cells. Capacity is being lost because the active balancer is taking current from the other cells until they all match.

When my BMS cuts off the lowest cell is 2.50 volts and the highest cell is 3.00 volts. If I charged the 2.5 cell to 3 volts what would happen when the cells started to reach the top of their charge? The cell I charged would reach the top before the others. If I equalized my the cells at the bottom then the delta will be far greater at the top. I am happy with my cell delta of 0.083 volts at the top of my charge thanks to parallel top balancing.

With mismatched cells there is a choice...parallel top balancing or parallel bottom balancing. The goal is to achieve a low delta depending on which one is used. I do not believe a low delta can be obtained at both ends of the spectrum unless the cells are all capacity matched, and one can't change the capacity of a cell no matter what they do.

Having said that in my 8S pack cell number 4 is the one that always goes the highest during charging, and cell number 7 is the one that goes lowest during discharging. When I finalize my build I am going to move cell 7 into position number 1, and cell 4 into position number 8. I don't know that will make any difference but what the heck...lol.
 

CerealKiller

New Member
I too am interested in their active balancing, especially for a potential 8S x 3P x 280AH arrangement ... I'm concerned that 80ma might be light on for balancing 3 x 280AH cells in Parallel .. But I'm leaning to 3P x 8S arrangement with 3 BMS's to share load.
The Heltec website sucks.. the way they arrange their products and limited info online worries me. The 300A unit only supports 50A charging ?
and likewise down for lower current units. Perhaps its better to just add their active balancers in parallel with a BMS with up to 5A(?) support for 8S in a simple unit ? I must say my head is spinning from reading about all these and some negative comments about DALY and other cheap Chinese units. Often its not the cheap that is the issue , but communication, documentation and lack of consistency that worry most.

For ref : https://heltec-bms.com/project/8s-b...-300a-350a-3-2v-lifepo4-same-port-split-port/
says a 300a unit has 50A max charging current. The 350A spec has nothing stated.

But the AliExpress site is a bit better but I couldnt find this one from the entry level of that site !


And of course states the charging current is same as discharge limits. Which means 3 x 150A units would be great for my application.
I saw this 200A BMS with 2A balance current current at Alibaba. The seller offered a DC booster similar to this one to enable the BMS work with an 8s system. Attached are images showing the schematic diagram and image of the DC to DC booster converter.
 

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  • Schematic diagram on how to connect the DC booster to enable the use in an 8s system.png
    Schematic diagram on how to connect the DC booster to enable the use in an 8s system.png
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  • 24v to 48v converter.png
    24v to 48v converter.png
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dRdoS7

Solar Enthusiast
Hi,

I bought the same via Aliexpress:


Quite a bit cheaper. Happy with it. So far.

This was my 2nd choice:


Also, are you sure it's usable with the above?


They show that on the JK site, but with a Balancer. It's not mentioned to be able to do the same in the BMS manual. It is called a 14S-24S, maybe for a reason? The balancer is 2-24S.



dRdoS7
 
Last edited:

CerealKiller

New Member
Hi,

I bought the same via Aliexpress:


Quite a bit cheaper. Happy with it. So far.

This was my 2nd choice:


Also, are you sure it's usable with the above?


They show that on the JK site, but with a Balancer. It's not mentioned to be able to do the same in the BMS manual. It is called a 14S-24S, maybe for a reason? The balancer is 2-24S.



dRdoS7


Just relaying what the seller says, yet to buy it.
 

fafrd

Solar Enthusiast
When my BMS cuts off the lowest cell is 2.50 volts and the highest cell is 3.00 volts. If I charged the 2.5 cell to 3 volts what would happen when the cells started to reach the top of their charge? The cell I charged would reach the top before the others. If I equalized my the cells at the bottom then the delta will be far greater at the top. I am happy with my cell delta of 0.083 volts at the top of my charge thanks to parallel top balancing.
[/QUOTE]
This is my concern as well, any boosting near the bottom of the discharge curve will result in the boosted cell reaching full charge first and just needs to be ‘undone’ near the top of the charge cycle.

I’m asking Heltec if they can deliver a customized Active Balance cut-off voltage of 3.4 or 3.5V so the Active Balance is disabled during discharge.
With mismatched cells there is a choice...parallel top balancing or parallel bottom balancing. The goal is to achieve a low delta depending on which one is used. I do not believe a low delta can be obtained at both ends of the spectrum unless the cells are all capacity matched, and one can't change the capacity of a cell no matter what they do.

Having said that in my 8S pack cell number 4 is the one that always goes the highest during charging, and cell number 7 is the one that goes lowest during discharging. When I finalize my build I am going to move cell 7 into position number 1, and cell 4 into position number 8. I don't know that will make any difference but what the heck...lol.
Having different cells top out and bottom out is the worst possible situation and represents lost capacity.

Ideally you want the weakest cell to be first to the top and first to the bottom. This provides the maximum capacity you can expect.

My first LFP battery suffers from a weak cell and I’ve seen that over repeated cycles, it strays from being the first to charge to lagging the stronger cells (so I’m losing potential capacity, like you).

What appeals to me about Active Balancing is that that weak lagging cell can be boosted up to the level of the others rather than just waiting endlessly for the many stronger cells to passively drain down to that lower-voltage level.

I just don’t see any benefit to that Active Balancing being in effect during discharge (especially near the bottom of the cycle).
 

Gazoo

Dumb Dumb
Having different cells top out and bottom out is the worst possible situation and represents lost capacity.
Going to the top doesn't matter. If you noticed I posted there is only a 0.083mv difference between the cells when the charger cuts off. There is no capacity lost in that area when discharging. My cells are very closely matched at the top because I top balanced.
Ideally you want the weakest cell to be first to the top and first to the bottom. This provides the maximum capacity you can expect.
Ideally yes. I don't understand how that would be possible with mismatched cells.
What appeals to me about Active Balancing is that that weak lagging cell can be boosted up to the level of the others rather than just waiting endlessly for the many stronger cells to passively drain down to that lower-voltage level.
But wouldn't that result in the cells being bottom balanced?
I just don’t see any benefit to that Active Balancing being in effect during discharge (especially near the bottom of the cycle).
Honestly I am not concerned about my weakest cell since I will stay well within the knees after I am done playing around with the testing. With my pack I have estimated 240ah's capacity between the knees and that is with a 10mv difference in the upper and lower area of the knee. Since I will be using my cells for a UPS, I plan to keep them at a 70% SOC to avoid permanent capacity loss from sitting at a high SOC. That is equal to around 190ah's. Of course I will cycle the pack probably on a monthly basis.

The upper area of my pack is good with there only being a 0.083 difference in the voltages of the cells when fully charged. The low area of the knee is where the problem is with the highest cell being 3.010 volts and the lowest being 2.50 volts when the BMS cuts off. And the highest cell is cell number 1 when the BMS cuts off.

I have wondered if I would achieve more capacity if I bottom balanced? Not that I am going to go there but I honestly don't know.

The voltages on my last test shortly before the BMS cut off were as follows in order of cell number 1-8.
3.010, 2.813, 3.008, 2.973, 2.934, 2.983, 2.625, and 2.848

And the following are some interesting measurements I noted when the cells drifted more than 10mv's.

Amp hours remaining at 25.35 volts = 25.24. Voltage Difference = 0.010 Capacity = 9%
Amp hours remaining at 25.30 volts = 21.25 Voltage Difference = 0.020 Capacity = 8% Cell 7 volts lower than others.
Amp hours remaining at 25.25 volts = 19.72 Voltage Difference = 0.033 Capacity = 7%
Amp hours remaining at 25.20 volts = 17.59 Voltage Difference = 0.057 Capacity = 6%
Amp hours remaining at 25.15 volts = 16.98 Voltage Difference = 0.066 Capacity = 6%
Amp hours remaining at 25.10 volts = 16.10 Voltage Difference = 0.079 Capacity = 6%
Amp hours remaining at 25.00 volts = 14.64 Voltage Difference = 0.102 Capacity = 5%
Amp hours remaining at 24.90 volts = 13.57 Voltage Difference = 0.110 Capacity = 5%
Amp hours remaining at 24.80 volts = 12.71 Voltage Difference = 0.122 Capacity = 5%
Amp hours remaining at 24.70 volts = 11.96 Voltage Difference = 0.132 Capacity = 4%
Amp hours remaining at 24.60 volts = 11.20 Voltage Difference = 0.140 Capacity = 4%
Amp hours remaining at 24.50 volts = 10.58 Voltage Difference = 0.150 Capacity = 4%

As you can see when the pack of the voltage approached 25.30 volts is when the voltage of cell 7 dropped below the other cells.
 

GXMnow

Solar Addict
My Heltec (JK) did work on just 4S so 8S won't be a problem, but there is one issue. You do need a 40 volt power supply to run the electronics. This can be a separate supply, or a DC-DC boost converter off of your 8S pack. It does not need much current at all. I have not measured it, but it is on a #22 awg wire. And when my pack sits at rest, I do not see any drop in pack voltage for days.

The current numbers are a bit more of a concern. Mine is rated at 200 amps. I am fairly confident it will take that as it barely got warm at 80 amps. I have not pushed it further, but the voltage drop and temp looks good. The newer rating I have seen for 300 amp appears to be a surge rating, and not continuous. It seems to have the same FET's as my 200 amp rated unit that does say 300 surge in the instructions.

On the full JK/Heltec BMS, you certainly can set a minimum voltage where it will shut off balancing. So if the pack is low, it won't waste energy trying to move charge around until you charge up again. All of the parameters are fully adjustable. If it comes with a password, try "123456" to get into the advanced settings menu.

For the current you are looking at, I would suggest doing it as 2 packs in parallel with 2 BMS units so the constant current is always under 200 amps per BMS. My maximum steady current should never exceed 150 amps, 6,800 watts on a 48 volt system, so the 200 amp rating is fine for me. For 300+ amps continuous in a single BMS, I would probably go with something like the Chargery with the external DCC contactor. Then you can add a Heltec balancer if you wanted the active balance feature. The JK works great and seems well built, but the current path is only a pair of #7 awg wires which seems suspect for anything over 100 amps for long term. I shortened them a bit. On the input they go to a buss bar withthe cables from the pack. And spliced them to a 2/0 cable on the output side to the output buss bar. The wires did not get warm on my 80 amp test, I have not pushed to 140 yet as I don't have a large enough load to use all that power. My current setup is limited to a 20 amp breaker feeding into my main panel. Pushing just over 16 amps back was how I hit 80 amps at the BMS.
 

GXMnow

Solar Addict
As for the one "weak cell" situation. I agree, you may want to set the minimum voltage for balancing a bit higher so once you start running the pack below 50% it just let's the weak cell drop. As long as the system shuts down before that weak cell gets too low and becomes damaged, you should be fine. As it charges back up, that low cell should climb up faster, and once it passes the balance threshold, the active balancing will start pulling from it and pushing to the stronger cells so it can top balance again.

If your one weak cell is so weak that it is dropping out before the end of a day and you have to resort to the generator, then I would say that cell just needs to be replaced. If it is just dropping 150 mv more than the rest, then it's probably okay for now.

My pack is way better balanced and matched than I ever expected. I have the active balancing set to operate at 3.2 volts and above, and when the balance from lowest to highest cell is over just 0.006 volts. And even with those settings it rarely triggers any balance activity. One of my banks of cells is a full year different on build date, so this was not expected. NMC cells have a distinct curve in the discharge graph near the 50% charge point, and all of my cells seem to hit it and curve together. On the first few cycles I did see it hit 0.010 balance error, but after a few cycles, I have not seen it get past 0.008 at any point during discharge.

In theory, the active balancing can make up for a fair amount of imbalance and mismatch, but I would say, you should never expect the pack to perform any better than the absolute worst cell. My balancer can move up to 2 amps. But at best, it is a bit less than 1/2 the time to any one cell. My bank is 360 AH. If I was running at 120 amps, that would mean 3 hours from full to dead. Pushing 2 amps for 1.5 hours means it could in theory compensate for a difference of no more than 3 AH of cell capacity. That means the cells would need to match within 0.8% of capacity for the active balance to mask the problem. The lower the current you run, the more it can correct, but you get the idea. Matched cells are important.

If I had discovered my packs did not match well, my plan was to get small but similar chemistry cells to parallel with any weak cells to bring up the capacity. If one of your LFP 260 AH cells comes up 10 AH short of the average, then parallel two 5 AH 26650 cells with that weak cell. It may not be ideal, but it sure should help. I had a few leftover 2.5 AH 18650 Samsung cells I was going to use to do this if I found a weak cell in my LG pack, but so far, I have not needed to do it.
 

ArthurEld

Solar Addict
On the full JK/Heltec BMS, you certainly can set a minimum voltage where it will shut off balancing. So if the pack is low, it won't waste energy trying to move charge around until you charge up again. All of the parameters are fully adjustable. If it comes with a password, try "123456" to get into the advanced settings menu.
Are you sure there is an option for turning off balancing a certain voltages? I only found On/Off and Trigger Balance Differential Voltage in the parameters.
 

dRdoS7

Solar Enthusiast
Hi,

Are you sure there is an option for turning off balancing a certain voltages? I only found On/Off and Trigger Balance Differential Voltage in the parameters.

I can't find any option to set a voltage for balancing either.

Maybe there's an other version of the app. I have 2, but neither have the 3rd. option mentioned by GXMnow.

dRdoS7.
 

GXMnow

Solar Addict
On my JK BMS App it is on the advanced options page "Start Balance Volt.(V)"
here is a screen shot. It is right after the "Max Balance Cur.(A)"
IMG_8674.PNG
 

ArthurEld

Solar Addict
Maybe it's because I have balancing turned off.
I don't plan to turn balancing on until it finishing discharging.
That will be in 3 days. lol
 

ArthurEld

Solar Addict
I turned on balancing and the Start Balance Voltage parameter didn't show.

The manual that I put in Resources doesn't show that option either.

The Heltec website gave me the impression that it balances on charge and discharge.1606126970287.png
 

ArthurEld

Solar Addict
Hi,



I can't find any option to set a voltage for balancing either.

Maybe there's an other version of the app. I have 2, but neither have the 3rd. option mentioned by GXMnow.

dRdoS7.
I think you might be correct. And I bought mine more recently and got my app direct from Heltec.
Where did you buy your BMS and where did you get your app? And when?
 
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