Questions on LL-S battery behavior

Posted by: @jlankford

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What is the expected EPS standby current load on this battery bank from the 18Kpv? Is it a continuous value, or is there a switching function? I ask this because I typically see Batt4 status on the LCD panel alternate between Standby and Discharging. When discharging it is usually showing 0.54A current. The alternating modes seems to be random, sometimes as frequently as 5 sec, sometimes the display remains in Standby for a minute or more. All other batteries show Standby constantly and remain at 100%. Is it possible that all six batteries are contributing to the EPS standby load, but only one of them is sensitive enough to measure and indicate this current draw?

With the rated <70W in normal mode, you should expect to see roughly around 1.37A from the battery bank when powering idle consumption.

Posted by: @jlankford

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What is the discharge current measurement threshold for the LL-S BMS? I have encountered other LiFePO4 battery brands that have a relatively high threshold for the discharge current used to calculate SOC. There is one brand that does not include any discharge current below 1.0A in its SOC calculations, according to their engineering department, and so that battery can slowly discharge but will continue to report 100% SOC. Is it possible that my LL-S batteries are each slowly discharging at something less than 0.5A due to the inverter’s continuous EPS standby current load, and the batteries (all but one) are not changing the SOC value from 100% because the current is below some threshold?

A lot of BMS information can be found from the spec sheet. However, I will be reaching out to the battery team for this as well since I do not see this documented.

Thank You @Jared for your responses here and in my other thread.

So this is beginning to make sense to me. The EPS idle current of 1.37A DC, distributed across six LL-S units, means each battery is supplying around 0.23A DC, which is less than the BMS current measurement threshold of 0.5A you just stated. This would explain why I am seeing most batteries remaining at 100% SOC when in fact they are discharging slowly. One battery happens to be more sensitive, or it is sourcing slightly more idle current, and that unit is therefore showing a decreasing SOC.

These slight numbers are not insignificant. Over time, my battery bank is decreasing by 1,680 Wh per day due to the 70W idle consumption. The data below represents the battery bank 4 days after the last full charge cycle performed by the 18Kpv. We expect around 6,720 Wh of capacity to be consumed over that time, which is 22% of my total bank capacity. The system SOC is reported at 98%. I should have included that figure in my screen shot, but it’s obvious from the individual batteries' reported SOC. As illustrated below, the actual system SOC is about 77%, not 98%.

From the data above, the cell voltages are indicating a large disparity in SOC for every battery. Note that the screenshot was taken after all the batteries' individual circuit breakers had been switched OFF for one hour. This gave the individual batteries time to settle to a true idle voltage. In the table below, note the large difference between "BMS SOC" and what I'm calling "calculated SOC," which is actually a value interpolated from a discharge curve, not a calculation. I realize the 77% result is not very accurate.

I think this explains the large amount of charge I described in my other thread – where the bank at 98% accepted an additional 30% of charge energy from the Chargeverter. The system had been sitting in EPS Idle for a few days before I performed that charging test. I can live with this SOC inaccuracy – I don’t care as long as I know what’s really going on here. But, what I do care about is keeping my system above a certain stored capacity. In the event of needing emergency backup, I want at least 90% to be available at any given time. I want to keep the actual SOC above 90%, so I have the Start Charge SOC value set to 90% and Stop is at 100%. With the slow idle discharge process illustrated above, this system may discharge to as low as 20% actual before it hits the 90% system SOC that will trigger a charging cycle.

What settings need to be configured to keep this battery bank topped up and available for EPS at all times?

@jared is there any configuration to reduce the minimum?