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

[silverstone]

@upnorthandpersonal : did you experience/notice a 0.7 V difference between the JK BMS v10 and JK BMS v11 ? I am guessing there is really some MOSFET body diode involved in the measurement circuit in one version of the JK BMS and not the other.

It's a bit of a coincidence at least that it's 0.7 V ... I originally thought it was offset or measurement tolerances. But what if the measurement circuit is different after all ?

 

[silverstone]

@timselectric 4mV delta left

3.54 V / cell on battery 1 and 3.58 V / cell on battery 2. Apparently some voltage measurement offset/method that is going on causing this issue as also the total pack voltage is different (by 0.7V ).




I think I will keep it like this for a few hours hoping that all the cells will settle.

 

[silverstone]

@timselectric But what do you set as "float" after you let your "equalization" take place ? 54.6 VDC (3.41 VDC/cell) ?

 

[upnorthandpersonal]

did you experience/notice a 0.7 V difference between the JK BMS v10 and JK BMS v11 ? I am guessing there is really some MOSFET body diode involved in the measurement circuit in one version of the JK BMS and not the other.

No, the MOSFET body diode should only there when either charge or discharge are disabled. Did you calibrate the voltage?

 

[silverstone]

No, the MOSFET body diode should only there when either charge or discharge are disabled. Did you calibrate the voltage?

Weird then that's its 0.7V. I don't believe in coincidences.

No I did NOT calibrate the voltage. Was I supposed to do that? Oups

 

[silverstone]

At least now the pack seems balanced though. Not sure how long if will last though.

 

[upnorthandpersonal]

No I did NOT calibrate the voltage. Was I supposed to do that?

In general, they're ok out of the box - but it's not a bad idea to calibrate them just in case. Use a decent multimeter, and without load. You can also calibrate the current (a 10A resistive load is what I tend to use), which should increase the accuracy of state of charge.

 

[timselectric]

Are you also going to install a Deye ?

Yes

Were you in the US or EU, I cannot remember?

US

If US I assume you have this "split-phase" type 120V/240V ?

Correct

 

[timselectric]

53-55 mV left .

Awesome
Getting there.

 

[timselectric]

@timselectric 4mV delta left

More awesome

 

[timselectric]

@timselectric But what do you set as "float" after you let your "equalization" take place ? 54.6 VDC (3.41 VDC/cell) ?

I float at 56v. Might be too aggressive for some people.

 

[MattiFin]

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 ...

If one of the cells is low you probably better connect charger directly to that low cell. With some precautions it is even possible to power the charger from the same battery bank you are balancing. You need either isolated DC-DC power supply or run your inverter combined with adjustable laboratory power supply. Just make 100% sure that lab power supply output is isolated from inverter/system ground. As a added benefit your are draining the other cells and charging one.

Despite JK having 2A balancer it is active only for one cell at a time. If you have 15 cells that are full and 1 low cell the JK balancer has to shuffle between 15 cells, making the effective balancing current only 2A divided by 15.

 

[upnorthandpersonal]

Despite JK having 2A balancer it is active only for one cell at a time. If you have 15 cells that are full and 1 low cell the JK balancer has to shuffle between 15 cells, making the effective balancing current only 2A divided by 15.

But it would just fill up the internal capacitor with energy from whatever cell is highest, and then put it in the lowest cell. Even if it would take this energy from multiple cells. I don't think you can really equate this with 2A / 15.

 

[MattiFin]

But it would just fill up the internal capacitor with energy from whatever cell is highest, and then put it in the lowest cell. Even if it would take this energy from multiple cells. I don't think you can really equate this with 2A / 15.

OK, it is better than 2A/15 in bringing up the lowest cell. But worth noting is still that maximum charge current for the entire pack should be only 2A/15 = 133mA or the balancer can't keep up with this scenario. Charging at 2 amps won't work despite having 2 A balancer.
Badly mismatched pack with half of the cells lower than the rest would limit the balancing charge current to 2A/8 and for the full cells discharge at 2A/8.

 

[nc73]

Get the 5 amp active balancer...works fast. Just don't use it all the time.

 

[silverstone]

I think it's fine now. Too like a day and a half but now the pack is balanced .

Agree that charging just the lowest cell with a separate charger might be quicker, but thst also involves opening up the pack etc.

I'm lazy. Just charge slowly via software e up to 56.7 vdc andet the balancer run. Leave all cells at 3.54v + for like 5 hours. Done.

 

[silverstone]

I float at 56v. Might be too aggressive for some people.

That's also what I though, but at least your pack is well balanced all the time.

Otherwise you end up like me floating at 54.2-54.6v, absorb end equalize only at 55.2v (I think this is the same thst Andy from off grid garage does), but this is not enough to ensure all cells are properly balanced.

Indeed I had to lower absorbtion to 55.0vdc. And I STILL hit OVP a few days ago like that too (one cell went to 3.60v).

So I'm kinda... Well, at least I need to use EQUALIZE for a few hours to ensure pack is balanced. People say to turn it off for Lifepo4 but it seems I need it. Otherwise the unbalance just gets worsd and worse.

Float could be lower, but combined with this bug I found on the Deye... Well... Maybe 56V float is the only way to go actually.

To me it seems high for a float value. But what is really aggressive, as all cells are below 3.65v? @upnorthandpersonal any thought here?

 

[silverstone]

OK, it is better than 2A/15 in bringing up the lowest cell. But worth noting is still that maximum charge current for the entire pack should be only 2A/15 = 133mA or the balancer can't keep up with this scenario. Charging at 2 amps won't work despite having 2 A balancer.
Badly mismatched pack with half of the cells lower than the rest would limit the balancing charge current to 2A/8 and for the full cells discharge at 2A/8.

Well, it's actually worse than that, because of the "Charge Pump" effect. A little bit similar to what a Boost Converter would work with duty cycle 0.01 I believe. Because when the highest cell will get discharged, since the supply voltage for the whole pack is (still) present, a charge current (counteracting the discharging counter of the balancer) will take place.

But yeah, over a day and a half I managed to get it balanced. It was never balanced to begin with I guess.

 

[silverstone]

In general, they're ok out of the box - but it's not a bad idea to calibrate them just in case. Use a decent multimeter, and without load. You can also calibrate the current (a 10A resistive load is what I tend to use), which should increase the accuracy of state of charge.

But the question is ... is is better to calibrate for the OVP to work better or leave it as it is ?

The 0.7V pack "weirdness" I guess I can live with.

OVP set at 3.60 VDC might also be a bit conservative (cell max voltage is 3.65VDC) so a bit of margin is also there although not much (0.8VDC in total). And that battery is indicating HIGHER than it should, so protection-wise right now it's probably like OVP 3.55VDC in reality (corresponding to a readout of 3.60VDC).

 

[upnorthandpersonal]

I always just calibrate them and add the safety margin on the OVP. There is no need to go to 3.65, or even 3.6 for that matter - so why not put it at 3.55.