Sudden drop to 0% in SOC with EG4 6500Ex + EG4 LLv2

[solarhombre]

Hello,

Something strange happened today.

I topped off the 6xEG4LLv2 batteries earlier today and then started running off-grid with the 2xEG4 6500EX disconnected from the grid. AC was on and off all the time. A few minutes before 4pm one of the inverters peaked at ~120A which is what I have configured as maximum draw from the batteries (AC running (1/2), microwave and water kettle on the same leg probably did that). At that point 2 breakers tripped but I did not notice until ~4.40pm, which is when I went to take a look, and reset the breakers. Both inverters had 'bp' flashing as if they did not the battery monitoring connection so turned on the grid connection. Turns out all the batteries were drained to 0!? They are happily and slowly charging now.

What I don't understand is the drop from SOC 54% to 0% that suddenly happened at ~4.40pm. I'm attaching some screenshots of the monitoring data showing things like the minimum cell voltage dropping from ~3.15 to 2.75 for all the cells(!?) in an instant, the power delivered by the batteries in the few hours they were connected (~15kWh out of 30kWh for the six of them),...

In case it's relevant, I don't run on batteries often (once or twice a month drop then to like 70% and then charge them back up to 90%), no solar connection, and the batteries are usually on (all green LEDS are on, but batteries not in use). The system is almost to the date 1 year old.

Any idea of what happened here?
TIA

 

[PotusGobbleBottom]

I don't run on batteries often (once or twice a month drop then to like 70% and then charge them back up to 90%)

This may be your clue. This could be caused by SOC drift or perhaps they were never balanced to start with tho from the max voltage at time of event, this doesnt seem the case.

Good job with all the pics! Someone here will look at them for 1 minute and have an answer for you.

 

[BentleyJ]

The discharge curves starting at about 1 pm look normal. Voltage slowly falls, and responds to current changes, until the batteries are depleted and voltage falls off a cliff. This is normal behavior for LFP chemistry, both during charge and discharge cycles. The SoC values being calculated by the BMS or inverter are known to be of poor accuracy. Something like a Victron smart shunt is much better at actually counting coulombs.

If only 15kWh of 30kWh was used then either the batteries were not actually at 100% SoC to start with or the inverter is not totaling kWh accurately or the batteries are only holding 1/2 of the nameplate capacity. I'm going with choice #1.

At no time between 8 am and 1 pm did the battery voltage go above 3.4V. In fact it barely reached maybe 3.35V but hard to tell for sure, the graph is not that detailed. If the batteries were "topped off" when did that happen? LFP does not fully charge until voltage is minimum of 3.4V.

What are the Bulk and/or absorption charging voltages set at? The batteries are full only when the voltage is above the aforementioned min value AND the tail current has dropped off to lets say 3-5 Amps total with all 6 batteries.

 

[PotusGobbleBottom]

With your usage patter,If these batteries were not turned off routinely, that would explain everything. The Battery management system consumes power and I doubt its included in the SOC value displayed.

 

[solarhombre]

The discharge curves starting at about 1 pm look normal. Voltage slowly falls, and responds to current changes, until the batteries are depleted and voltage falls off a cliff. This is normal behavior for LFP chemistry, both during charge and discharge cycles. The SoC values being calculated by the BMS or inverter are known to be of poor accuracy. Something like a Victron smart shunt is much better at actually counting coulombs.

If only 15kWh of 30kWh was used then either the batteries were not actually at 100% SoC to start with or the inverter is not totaling kWh accurately or the batteries are only holding 1/2 of the nameplate capacity. I'm going with choice #1.

Interesting. I didn't leave the them at 100 and charging at 2A for long. But the SOC reported by the batteries was 100% except for 2 of them which were at 98%.

At no time between 8 am and 1 pm did the battery voltage go above 3.4V. In fact it barely reached maybe 3.35V but hard to tell for sure, the graph is not that detailed. If the batteries were "topped off" when did that happen? LFP does not fully charge until voltage is minimum of 3.4V.

I guess this is part of the problem, that they reported 100% but were lying.

What are the Bulk and/or absorption charging voltages set at? The batteries are full only when the voltage is above the aforementioned min value AND the tail current has dropped off to lets say 3-5 Amps total with all 6 batteries.

I just use the 'EG4' battery type on the inverters instead of configuring those values. Looks like I should switch to voltage based measurements instead of blindly trusting that the inverters know what they are doing. I'll find out the right values and set them.

Thank you so much!

 

[solarhombre]

With your usage patter,If these batteries were not turned off routinely, that would explain everything. The Battery management system consumes power and I doubt its included in the SOC value displayed.

From now on, I'll turn them off when. Not in use.

Thanks!

 

[BentleyJ]

For reference, see attached float charge cycle from yesterday. This is a good representation of a proper 2 hr & 45 min top-up float. Note how sharp the current tails off when well balanced cells all reach full charge simultaneously at constant voltage.

Its a small battery, only 270Ah and the charge current drops to about 0.5A, most of which is standby being consumed by the inverter. Until this very low tail current point is reached, your batteries are not fully charged.

 

[EG4_Jared]

It would be interesting to see if this issue reoccurred while set to user-defined for the battery charge and discharge. With the batteries not utilizing the 80% depth of discharge, it seems like the percentage reading needs to be calibrated through a deep cycle process.