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JK BMS quite inaccurate at SoC (and also Current/Power ???) Measurement

silverstone

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It seems to be a common issue. Yeah, at first I didn't think it was important. Duh :fp.

I mean, the way to ensure that the batteries get fully charged is to keep a "reference voltage" for the inverter/charger of say 55V-56V for a 16s Pack (3.43VDC/cell - 3.50VDC/cell although at 56V I will hit OVP on one of the cells if I do it "open-loop", i.e. without feedback and lowering the voltage when I get "close" to say 3.55VDC).

So typically at the end of the day, that's what I do (open-loop only, so the "tail current" can get quite low at 55VDC reference voltage).

But I see quite wide variety between my 2 batteries. And the SoC reporting I believe it's quite inaccurate. If I trust the BMS readout, I'd say that my inverters have a no-load (idle) loss of about 400W-600W (3 inverters x 3ph Deye 12kW/each Inverters in parallel in "off grid mode" only). That's 9.6kW-14.4kWh / day o_O.

1709310708841.png

It can be simply because they were NOT top balanced in the beginning (JK BMS has an active balancer though), 1 cell is from a ~10 month old batch in the second battery, simply the SoC is determined by the lowest SoC cell which impacts a lot one battery and the voltage measurement "tolerance" are quite high (one BMS has approx 0.7V total pack voltage more than the other). Yep ... I do NOT think that's a body diode voltage drop though (VF).

So I'm debating whether I should install a "Smart Shunt" in my future batteries. Originally I was thinking about a DC Current Transducer, but these can have quite HIGH DC offset. Not an issue if you want to measure AC current but for DC measurement the DC offset can be critical, especially when using it for "coulomb counting" (SoC measurement based on integration of the current measurement over time).

I guess a Shunt + Precision Op-Amps Circuitry can get around that problem (DC offset) better.

So the question is: is it really worth to get the Victron Smartshunt (500 A is quite high ... I max out at 100 A long term, maybe 150 Apeak for OCP on the BMS), so I'm a bit disappointed that 500A is Victron's smallest Smart Shunt ? Or are there good Chinese clones ? I don't really need all the bells and whistles. And 200A shunt would be probably more than enough.

Preferred connection method would be RS232, RS485, Wifi/MQTT. BLE/Bluetooth I'm not too fond of (unless there is something easy to use like https://github.com/syssi/esphome-jk-bms) that I can then use to interface with MQTT etc. And some community "driver" or python script wrapper etc available would of course be a plus.

Thanks for your help :)
 
Get the smart shunt. You know you want to.

I have 4 jk bms. They work very well. I check them once a week and cell voltages are always very close but I rarely see the soc reported on any of them 😲
 
Get the smart shunt. You know you want to.

I have 4 jk bms. They work very well. I check them once a week and cell voltages are always very close but I rarely see the soc reported on any of them 😲
But the Victron SmartShunt is quite expensive. 95 EUR from Germany each. And 500A is way too much than I need, possibly leading to inaccuracies of their own. A 200A Smart Shunt would probably have better resolution.
 
I’d be more curious at your charging profile that’s causing a BMS into OVP.

A smart shunt won’t fix that.

When do you turn on balancing? What’s the delta between cells when >3.4v?
 
I’d be more curious at your charging profile that’s causing a BMS into OVP.

A smart shunt won’t fix that.

When do you turn on balancing? What’s the delta between cells when >3.4v?
As I said that happens if I have absorption/boost voltage set to 55.2V or 56.0V. 55.0V they all stay happily below OVP.

Or at least that's what I thought ... yesterday it didn't turn out that way. 55.0V was enough to trigger a cell to hit OVP 8 times and I only noticed it this morning ...

Not good. But turning off at 40A charge for a BMS rated 200A ... I don't think that's very stressfull for the BMS.

1709360885262.png

Delta Voltage can get up to approx. 160mV (!) when one cell hits OVP.
1709360832249.png

In "closed loop" control I plan to use the charger set to e.g. 57V in Current-Limiting mode, and of course reduce the voltage as one (or more) cell starts getting close to Overvoltage status.

IIRC I turn balancing on at 3.45VDC based on what Andy from Off Grid Garage did.
 
As I said that happens if I have absorption/boost voltage set to 55.2V or 56.0V. 55.0V they all stay happily below OVP.

Or at least that's what I thought ... yesterday it didn't turn out that way. 55.0V was enough to trigger a cell to hit OVP 8 times and I only noticed it this morning ...

Not good. But turning off at 40A charge for a BMS rated 200A ... I don't think that's very stressfull for the BMS.

View attachment 199466

Delta Voltage can get up to approx. 160mV (!) when one cell hits OVP.
View attachment 199465

In "closed loop" control I plan to use the charger set to e.g. 57V in Current-Limiting mode, and of course reduce the voltage as one (or more) cell starts getting close to Overvoltage status.

IIRC I turn balancing on at 3.45VDC based on what Andy from Off Grid Garage did.
And actually if you look at the individual cell voltage you could say that one cell is probably missing at least (Charging power) x (delta time) capacity.
1709362268880.png
Since charging power was roughly 1200W so say 20A, I'd say that one cell is missing at the very least 0.5h x 20A = 10Ah.

I actually might need to order a new cell and replace cell #11. This basically means disassembling the whole pack (and reassembling it into another battery box - right now 2 batteries in one box is a nightmare for maintenance).
 
Yes. It's difficult to go back and install a foundation after the rest of the house is built.
Well ... nothing constructive there, sorry to say.

The argument of top balancing of the JK BMS with active balancer 2A is a bit debated too ...
 
Well ... nothing constructive there, sorry to say.

The argument of top balancing of the JK BMS with active balancer 2A is a bit debated too ...

Not an expert but top balancing I assume is super-critical when running parallel batteries.

If nothing else, I would bite the bullet and do it just to see if I get better results.
 
Not an expert but top balancing I assume is super-critical when running parallel batteries.

If nothing else, I would bite the bullet and do it just to see if I get better results.
I assume that the active balancer could do its magic if I slowly raise the voltage to say 55.2V or more.

Problem is, right now at least, I quickly charge the cells between 14h00-17h00 using the charger (that's barely 2kW/battery pack so ~ 40A/battery pack), and at 17h00 when the price gets more expensive I stop charging. So either I hit OVP or there just isn't enough time for the balancer to do its magic.

If the battery would be almost fully charged at say 12h00, then I could raise the voltage by say 0.05V every 15 minutes or so until all cells gets to say 3.5V. Right now, just not enough time for that.

Or I could turn the breaker of the affected battery OFF and rely on a small AUX supply to provide 0-5A and slowly charge from there.

But then I'd be left with only one battery pack and 15kWh during normal operation ...
 
I assume that the active balancer could do its magic if I slowly raise the voltage to say 55.2V or more.

Problem is, right now at least, I quickly charge the cells between 14h00-17h00 using the charger (that's barely 2kW/battery pack so ~ 40A/battery pack), and at 17h00 when the price gets more expensive I stop charging. So either I hit OVP or there just isn't enough time for the balancer to do its magic.

If the battery would be almost fully charged at say 12h00, then I could raise the voltage by say 0.05V every 15 minutes or so until all cells gets to say 3.5V. Right now, just not enough time for that.

Or I could turn the breaker of the affected battery OFF and rely on a small AUX supply to provide 0-5A and slowly charge from there.

But then I'd be left with only one battery pack and 15kWh during normal operation ...

I had the ability to top balance my pack but didn't because I wanted to see what would happen. A couple cells in the pack would go to very high voltages and the bms would cut out while the remaining cells still needed a lot of charge.

I went ahead and disassembled and top balanced and at that point it just worked.

Was kinda shocked at the difference it made.

I have bitten the bullet so many times on other things.

I'll just say that when it comes to this stuff, it's way more time consuming and work than you want it to be. Especially when starting out, things fail due to both defect and installation mis-steps that can be quite a bit of work to fix.

Grid power is easy. Solar power is not. It's akin to raising and butchering your own cattle vs just going to the store and buying the meet. Or buying a boat and going fishing instead of just buying fish at the store. You end up having to be a boat and boat trailer mechanic, buy and maintain rods bait reels etc......

It's work. Resign yourself to taking the pack apart and top balancing and don't look back. 🤩
 
I think guys that are not top balancing are running fairly low absorption voltages so they aren't pushing the cells to those higher voltages where a lack of top balancing makes itself apparent.
 
I had the ability to top balance my pack but didn't because I wanted to see what would happen. A couple cells in the pack would go to very high voltages and the bms would cut out while the remaining cells still needed a lot of charge.

I went ahead and disassembled and top balanced and at that point it just worked.

Was kinda shocked at the difference it made.

I have bitten the bullet so many times on other things.

I'll just say that when it comes to this stuff, it's way more time consuming and work than you want it to be. Especially when starting out, things fail due to both defect and installation mis-steps that can be quite a bit of work to fix.

Grid power is easy. Solar power is not. It's akin to raising and butchering your own cattle vs just going to the store and buying the meet. Or buying a boat and going fishing instead of just buying fish at the store. You end up having to be a boat and boat trailer mechanic, buy and maintain rods bait reels etc......

It's work. Resign yourself to taking the pack apart and top balancing and don't look back. 🤩
Agree to most of the points.

The main killer (for me) is that I put 2 batteries in one expensive metal box. That was a big mistake I plan to fix (after I'll have at least 2 new batteries operational with the new design in a wooden box and probably also Victron Smartshunt).

Costed over 500 EUR in this metal box I will now have to thrash away ... That's frustrating but better than the current situation I guess :confused: .
 
I think guys that are not top balancing are running fairly low absorption voltages so they aren't pushing the cells to those higher voltages where a lack of top balancing makes itself apparent.
I thought 55.0V was safe ... Until yesterday it was. 55.2V was always too much.
 
I think guys that are not top balancing are running fairly low absorption voltages so they aren't pushing the cells to those higher voltages where a lack of top balancing makes itself apparent.
I don't top balance manually before building my batteries. With active balancers it's not necessary.
And I charge to 3.5v per cell, daily.
It takes 3 or 4 charge cycles to get the cells balanced the first time, and then I'm done. (Well, not me. The balancer is done)
Quality cells make a big difference. But that probably doesn't matter either.
As long as you have active balancing. Top balancing before assembly is wasted time.
You can spend 3 days top balancing, before putting your battery to use. Or you can put your battery to use. And let the active balancer take care of it during the first 3 days of use.
Top balancing with passive balancers is beneficial. Because it's the only time your cells will be balanced.
 
I don't top balance manually before building my batteries. With active balancers it's not necessary.
And I charge to 3.5v per cell, daily.
It takes 3 or 4 charge cycles to get the cells balanced the first time, and then I'm done. (Well, not me. The balancer is done)
Quality cells make a big difference. But that probably doesn't matter either.
As long as you have active balancing. Top balancing before assembly is wasted time.
You can spend 3 days top balancing, before putting your battery to use. Or you can put your battery to use. And let the active balancer take care of it during the first 3 days of use.
Top balancing with passive balancers is beneficial. Because it's the only time your cells will be balanced.
It still doesn't work that well in my case.

Yesterday cell 11 hit OVP. Today it was cell 9/7 that was staying higher.

As such I reduce to 54.8V until I'll have some "closed-loop" control algorithm to actively lower/increase the voltage based on the highest/lowest measured cell voltage.

Why the hell one JK BMS always measures approx. 0.7V more than the other (almost 40-50mV/cell) ... I don't know.
 
It still doesn't work that well in my case.

Yesterday cell 11 hit OVP. Today it was cell 9/7 that was staying higher.

As such I reduce to 54.8V until I'll have some "closed-loop" control algorithm to actively lower/increase the voltage based on the highest/lowest measured cell voltage.

Why the hell one JK BMS always measures approx. 0.7V more than the other (almost 40-50mV/cell) ... I don't know.
I would guess that it's a cell or wiring issue.
Or possibly a faulty BMS.
My cells are balanced to within 0.003v delta at 3.5v , every full charge.
No closed loop crap here.
 
I would guess that it's a cell or wiring issue.
Or possibly a faulty BMS.
My cells are balanced to within 0.003v delta at 3.5v , every full charge.
No closed loop crap here.
Full charged is like 150mV around here when cell 11 yesterday shot up, as you could see in one of the pictures.

Today it seemed it was going to be cell 7 or 9 though.

Faulty BMS ? Possible. The only explanation I had is that this is an older FW / HW version. The other "healthy" battery has v11, this weird one is v10.

Anyways though, given the rapid increase in voltage, it's pretty safe to assume that it's a physical phenomenon (the cell is almost fully charged).

So it's probably BMS Tolerances / Offset combined with a cell that is severly unbalanced.
 
Full charged is like 150mV around here when cell 11 yesterday shot up, as you could see in one of the pictures.

Today it seemed it was going to be cell 7 or 9 though.

Faulty BMS ? Possible. The only explanation I had is that this is an older FW / HW version. The other "healthy" battery has v11, this weird one is v10.

Anyways though, given the rapid increase in voltage, it's pretty safe to assume that it's a physical phenomenon (the cell is almost fully charged).

So it's probably BMS Tolerances / Offset combined with a cell that is severly unbalanced.
It really needs enough time to balance, regularly.
I make sure that I get to balance voltage at least once a week. The longer you go between balancing, the longer it takes to get balanced again.
 
It really needs enough time to balance, regularly.
I make sure that I get to balance voltage at least once a week. The longer you go between balancing, the longer it takes to get balanced again.
That's probably the issue then ... I also figured that you cannot just "peak" at 55V or whatever. You need to leave it there for a while until it finishes balancing. So I don't know how long it really takes, but 2A balancing current x 50% duty cycle (assuming you only discharge 1 cell, then recharge the others) is only 1Ah. It's not a lot ... I guess leaving them doing their thing for 12h or so would already be good.
 
That's probably the issue then ... I also figured that you cannot just "peak" at 55V or whatever. You need to leave it there for a while until it finishes balancing. So I don't know how long it really takes, but 2A balancing current x 50% duty cycle (assuming you only discharge 1 cell, then recharge the others) is only 1Ah. It's not a lot ... I guess leaving them doing their thing for 12h or so would already be good.
Yeah, it differs on each charge cycle. Depending on how long it's been since it was last balanced.
On a daily cycle, an hour is enough.
After a week, it usually takes 3 or 4 hours for mine.
 
Yeah, it differs on each charge cycle. Depending on how long it's been since it was last balanced.
On a daily cycle, an hour is enough.
After a week, it usually takes 3 or 4 hours for mine.
Today I'm staying up late at night doing "small steps" of voltage change and slowly bringing the battery to those magic 56V without hitting the limit.

Currently at 55.4V or so. It starts to get faster and faster to do steps of 0.1V at least 🤣 .
 
Time is the solution, even a 2a active balancer will take time. I adjusted my “absorbtion” time for 4 hrs on my passive JBD and once “balance only while charging” was turned off that passive balancer was able to get the delta down to .002v at 3.45 per cell.

But also being said the best active balancer won’t fix a bad cell. It’s only a tool to polish our half decent grade b turds we buy, but it has its limitations.
 
Time is the solution, even a 2a active balancer will take time. I adjusted my “absorbtion” time for 4 hrs on my passive JBD and once “balance only while charging” was turned off that passive balancer was able to get the delta down to .002v at 3.45 per cell.

But also being said the best active balancer won’t fix a bad cell. It’s only a tool to polish our half decent grade b turds we buy, but it has its limitations.
For sure. Right now I set my charger to 55.7V and it's (still) charging at like 2A. Still 123mV to balance (and 300mV of voltage to increase to get to the "magic" 3.5V/cell average).

But basically in the past I guess I kinda gave up and did the balance at 55.2V (then 55.0V, then 54.8V). Now I'm already doing a much better equalization than that.

So actually I was never doing a proper equalization for the past 4 months. Duh :rolleyes: .

I wonder why some people had absorbtion set to 55.2V (and equalization disabled completely).
 
I don't top balance manually before building my batteries. With active balancers it's not necessary.
And I charge to 3.5v per cell, daily.
It takes 3 or 4 charge cycles to get the cells balanced the first time, and then I'm done. (Well, not me. The balancer is done)
Quality cells make a big difference. But that probably doesn't matter either.
As long as you have active balancing. Top balancing before assembly is wasted time.
You can spend 3 days top balancing, before putting your battery to use. Or you can put your battery to use. And let the active balancer take care of it during the first 3 days of use.
Top balancing with passive balancers is beneficial. Because it's the only time your cells will be balanced.

To clarify, what I meant was you're more gentle on the cell to give the BMS time to balance the cell.

I also assume your BMS voltage cutoff is like maybe 3.65 while your charging voltage is set to 3.5.

This means the pack stays operational while giving some headroom for the new/unbalanced cell to balance.
 
I'm kinda stuck at 55.7 VDC with like 4 cells at 3.580 VDC (OVP is set at 3.600 VDC). I am afraid this will take a while, especially considering cell #1 (minimum cell voltage) is like 3.432 VDC ...
 
I also assume your BMS voltage cutoff is like maybe 3.65 while your charging voltage is set to 3.5.
3.6 and 3.5
This means the pack stays operational while giving some headroom for the new/unbalanced cell to balance.
On a new battery, it hits high voltage charging cutoff regularly.
Until the cells are balanced.
I don't baby my batteries at all. Bulk charge to 57v and float at 56v.
Balancing only above 3.45v per cell.
 
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I'm kinda stuck at 55.7 VDC with like 4 cells at 3.580 VDC (OVP is set at 3.600 VDC). I am afraid this will take a while, especially considering cell #1 (minimum cell voltage) is like 3.432 VDC ...
Give it time. The first time takes the longest.
 
Give it time. The first time takes the longest.
1709476440741.png

I also think that the BMS causes a "Charge Pump" or "Capacitor Multiplier" effect.

When it's discharging the highest cell, it's lowering its voltage, thus causing some current to flow from the main supply, which kinda counteracts the charger.

I'm a bit surprised that the orange cell (lowest #4) charges before the lowest blue (lowest #1) though.

I would assume that if the balance leads were miswired would cause the delta voltage to INCREASE, not Decrease though, so I still think it's a "Charge Pump" kind of effect that happens during balance.
 
View attachment 199707

I also think that the BMS causes a "Charge Pump" or "Capacitor Multiplier" effect.

When it's discharging the highest cell, it's lowering its voltage, thus causing some current to flow from the main supply, which kinda counteracts the charger.

I'm a bit surprised that the orange cell (lowest #4) charges before the lowest blue (lowest #1) though.

I would assume that if the balance leads were miswired would cause the delta voltage to INCREASE, not Decrease though, so I still think it's a "Charge Pump" kind of effect that happens during balance.
As long as the cells are in good condition, it will balance out.
The charger should maintain the overall pack voltage.
The active balancing will move small amounts of energy from the highest cell to the lowest cell.
And yes, each cell will bounce back a little after the balancer releases each time.
It just takes time. You just have to be patient.
 
As long as the cells are in good condition, it will balance out.
The charger should maintain the overall pack voltage.
The active balancing will move small amounts of energy from the highest cell to the lowest cell.
And yes, each cell will bounce back a little after the balancer releases each time.
It just takes time. You just have to be patient.
I think the lowest cell went from 3.43 V (below the "steep" curve point of the voltage vs soc characteristic) to like 3.47 V (above the "steep" curve point of the voltage vs soc characteristic). And the delta_V went from 150+mV to like 105mV now. I also raised the pack voltage to 56.1 V now to have a bit more on the "well behaving" battery, since it was a bit low on the cells.
 
I think the lowest cell went from 3.43 V (below the "steep" curve point of the voltage vs soc characteristic) to like 3.47 V (above the "steep" curve point of the voltage vs soc characteristic). And the delta_V went from 150+mV to like 105mV now. I also raised the pack voltage to 56.1 V now to have a bit more on the "well behaving" battery, since it was a bit low on the cells.
You're making progress.
 
You're making progress.
I'm just a bit confused about the fact that, while there is definitively not sun and almost fog, the inverter seems to be getting all the power from the battery (and thus the charger and thus the grid). It's like it's ignoring completely the PV. Weird stuff on these Deye Inverters. Grid power is 1.6kW right now, should be much less given that a bit should come from the PV.

I think I also have "load first" set in the settings, but apparently if the battery voltage is ABOVE the Inverter Absorbtion / Equalization Voltage, then it just takes the energy from the battery :confused: .

A bit expensive equalization, but I guess it had to be done. I'll leave it full time tomorrow as well.

In the future maybe I just disconnect the breaker from the affected battery and come in through another connector for auxiliary power supply and charge from there. Then I can let it equalize for 3 days or so without losing on the energy production.

This just confirms me that I need that extra connector that I thought I was kicking out of the "low-cost" battery box v2 🤣.
 
1709490398138.png

72-74 mV left.

But I am worried about the last finding (Inverter ignoring PV when battery voltage is above Float/Absorbtion) ... That will have HUGE impacts on my control strategy :confused: .
 
I can't help much with the inverter.
My Deye is still in its box.
Are you also going to install a Deye :oops: ? Were you in the US or EU, I cannot remember ? If US I assume you have this "split-load" type 120V/240V ?

53-55 mV left 🤣 .
 
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