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

[silverstone]

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

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 .



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

 

[1201]

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

 

[silverstone]

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.

 

[1201]

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.

Ok, so don't get the smart shunt
It's a nice tool to have but your system is working fine without it

 

[timselectric]

So I'm debating whether I should install a "Smart Shunt" in my future batteries.

This is the best way to get accurate SOC measurements.

 

[740GLE]

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?

 

[Checkthisout]

Balance your cells first.

 

[silverstone]

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.



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


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.

 

[silverstone]

Balance your cells first.

Easier said than done when it's in operation. That's like saying "Turn on your time travel machine" ...

 

[silverstone]

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.

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).

 

[Checkthisout]

Easier said than done when it's in operation. That's like saying "Turn on your time travel machine" ...

Yes. It's difficult to go back and install a foundation after the rest of the house is built.

 

[silverstone]

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 ...

 

[Checkthisout]

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.

 

[silverstone]

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 ...

 

[Checkthisout]

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.

 

[Checkthisout]

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.

 

[silverstone]

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 .

 

[silverstone]

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.

 

[timselectric]

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.

 

[silverstone]

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.