Disappointing SOK Rack Balance Issue

goldserve said:

Float is actually hidden when using custom lithium.

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Then have some patience, it may take a while like most packs do when new. At least you don't have a runner that is kicking off the BMS. The good news is your cell delta has already reduced by 100mv. At that rate you will be close within days. If you have an urgent need to see results you could go to custom settings briefly to enable Float now that you have the balancing start voltage adjusted.

Ampster said:

Then have some patience, it may take a while like most packs do when new. At least you don't have a runner that is kicking off the BMS.

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Yup, I will continue to monitor the pack and even use an external PSU to hold a 55V float voltage to see if cell 2 will rise in voltage.

goldserve said:

to see if cell 2 will rise in voltage.

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That is not how passive balancing works. All the other cells have to be pulled down so that cell can catch up. That is why it will take some time. As I said earlier, at least you do not have a runner.
EDIT: To clarify, when a passive balancer is operation during a charge cycle the lowest voltage cell should rise faster than the other cells that are being shunted. When a passive balancer is operating when there is not a charge current then the shunted cells should be reduced in voltage and the low cell should stay the same, other that initial settling as the surface charge dissapates.

Ampster said:

That is not how passive balancing works. All the other cells have to be pulled down so that cell can catch up. That is why it will take some time. As I said earlier, at least you do not have a runner.

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OKay, I have some gap in my understanding then. From the image above, all the cells except for #2 are 'BL'ancing so the BMS is going to siphon current away in the form of heat. This happens slowly as to not generate too much heat a once. The BMS still expects current to be going into the battery (charge cycle) so wouldn't Cell 2 be accepting the charge still?

The difference between the SOK and EG4 BMS is that I've only seen the BL when the battery is in a charge cycle and the EG4 will balance even when it is not charging.

The first picture are two EG4 packs all charged up, sitting there overnight and Balancing is still occurring in one of the packs.

The second picture is the SOK battery sitting in the same conditions, all paralleled together and Cell 2 delta is quite high with no balancing happening on this BMS.

I have found a fix for you:

goldserve said:

OKay, I have some gap in my understanding then. From the image above, all the cells except for #2 are 'BL'ancing so the BMS is going to siphon current away in the form of heat. This happens slowly as to not generate too much heat a once. The BMS still expects current to be going into the battery (charge cycle) so wouldn't Cell 2 be accepting the charge still?

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You have it correct, assuming the BMS is a passive balancer. You are also correct that some current will be shunted through the resistors so those cells will not receive as much current as the cell that is not being shunted. Yes cell 2 should be getting more current than the shunted cells. I do not know the details of how much resistance is used in that BMS but if the charging current is too low it is possible that it will not get much. I don't know how or why you picked 2 Amps but if you divide that by 16 cells without any resistance that would be only 125 mAmps per cell. I would have to review how Ohms law works in that situation but the important issue is the lowest cell should be getting enough current so the voltage will continue to increase. If the delta decreased by 100 mv in the course of one charge cycle then it should not take too long.

Thanks for your reply. 2A is chosen as the Absorb End Amps and from what I understand, the skybox will hold voltage constant at 55V and whatever current is drawn, it will continue until the current drops BELOW the Absorb End Amps in which the cycle will terminate. From the image posted earlier, the current fell to around 3.7A within 20mins of Absorb. I will give the battery a few more cycles and Dexter is getting me an external supply that I can keep the battery in charging state to allow the balancer to work.

Kudos to Dexter!

Thanks for the kudos.

From our emails I only saw 1 or 2 cells that may be a bit lower than the rest, but nothing to worry about at this point. We changed a few settings in the BMS to allow a more suitable balancing window for the connected equipment.

After 35 cycles, still horrible balance. 3 cells are hot and will reach 3.65v while cell 2 will still be 3.409V.

I finally decided to open up the pack, carefully charge up cell 2 to be inline with other cells and then I played wack a mole with a few resistors in parallel to draw 400mA from the hot cells to bring them down. I got tired and the difference between the cells when charging was under 100mV. I’ll monitor again tomorrow when it goes through a charge cycle.

Conclusion is that the BMS has horrible balance capabilities (current too low) and it only operates when charging and will not give enough time to balance the cells when the charger is set to cut off after absorb is less than 2A (0.02C).

I’ve decided not to buy any server rack batteries anymore and instead, make my own with a premade rack case. I hope the choice of the JK bms is a good one. I don’t plan on top balancing any of the cells and just put it into service. Let the bms take care of things for me.

Well 3.4 is basically full you might add 1 AH to reach 3.65

Alkaline said:

Well 3.4 is basically full you might add 1 AH to reach 3.65

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Yes, that has been my experience. Not much beyond 3.4.
I once spent lots of time with a resistor and alligator clips to find that it was loose connections. One would not expect that in a rack battery.

Yeah and as soon you put a load on they drop to like 3.35 anyway and then stay at 3.33 for pretty much the whole time.

GoldServe is cross posting on 2 threads, so I will leave this here as I replied to the topic there.