Questions regarding current charge control between EG4 & Inverter Charger

[Old_Skewler]

I am trying to wrap my head around the dynamics between a lithium rack-type battery with internal BMS and a inverter charger which has NO communication with the battery BMS.

Who dictates the charging current to feed the battery? The battery BMS or the inverter charger?

I ask this because I have the following conditions:

- Schneider 4048SW Inverter Charger set up to a max current of 35A.

When battery is at 58% SOC and I run the charger, the battery intakes 35A. Once the battery reaches 100% SOC, it still manages to intake about 30A from the charger, to which at the point I turn off the generator.

I have the following questions I would like to hear from other members:

1. who should be in "charge" (pun intended) of the charging current going to the battery: the battery BMS or the Inverter Charger?
2. What eventually would had happened if I didn't turn off the generator once the battery hit 100% SOC? Would the same 30A still try to get into the battery or would the battery say "enough is enough" and reduce the intake current? How does this actually work?

Here are some screenshots I took today during the beginning of the charge cycle SCO 58% and another screenshot at 100% SOC.

 

[BentleyJ]

Quick answer: The Inverter is the controlling component in the system, the BMS is only a last and final safety device (on/off switch) that disconnects the battery if an out-of-range condition is imminent. (assuming the BMS is set up properly) Theoretically it should never have to intervene.
Its the Inverter that should be regulating charge current and voltage. This is true even if there is closed loop communication between the BMS and Inverter since the BMS is only sending data, the inverter settings determine how to respond to the data.
If the Constant Voltage (Absorption) portion of the charge cycle is set properly the battery will create its own voltage "back pressure" and the current should fall off significantly once near the 100% SoC level.
I'm remembering something about the SW4048, it has more than one charge current setting in the user menus depending on what battery chemistry is chosen?? Need to look into this more.

Will review the screen shots and go into more detail if anything jumps out.

 

[BentleyJ]

The cells need to be top balanced, there is an 83mV difference between cells as its approaching full charge and there is an over voltage alarm on cell 4. I would recommend float charging at 56.0V and 10A or less for several hours to give the BMS time to top balance the cells.

Could you post screen shots of the SW4048 user menus, specifically the battery type and charge parameters we can work through them and get the system dialed in.

Edit: Forgot to mention it takes a full charge cycle to get the SoC calibrated. Just because it says 100%, this cannot be true since the current is still at 30A. The battery will perform so much better with a good top balance. When all the cells reach a full charge simultaneously there is a sharp break in the charging current and it falls off rapidly as the battery voltage climbs and resists the current flow. I was going to post a screenshot of the last charge cycle from our XW+ 6848 but the ComBox only holds 7 days of data and InsightCloud doesn't seem to have any of the battery data.

 

[Old_Skewler]

Quick answer: The Inverter is the controlling component in the system, the BMS is only a last and final safety device (on/off switch) that disconnects the battery if an out-of-range condition is imminent. (assuming the BMS is set up properly) Theoretically it should never have to intervene.
Its the Inverter that should be regulating charge current and voltage. This is true even if there is closed loop communication between the BMS and Inverter since the BMS is only sending data, the inverter settings determine how to respond to the data.
If the Constant Voltage (Absorption) portion of the charge cycle is set properly the battery will create its own voltage "back pressure" and the current should fall off significantly once near the 100% SoC level.

This is great and it helped me see this dynamic. Thank you! Very clear.

Will review the screen shots and go into more detail if anything jumps out.

Here are the two relevant settings under the SW:

Charger Settings




Battery Settings:

 

[Old_Skewler]

The cells need to be top balanced, there is an 83mV difference between cells as its approaching full charge and there is an over voltage alarm on cell 4. I would recommend float charging at 56.0V and 10A or less for several hours to give the BMS time to top balance the cells.

I have tried to top balance this battery but the vendor, which by the way happens to sell both products - inverter & the battery, would not provide me any guidance in regards to what setpoints (voltage, current, stages, etc) to run the inverter to top balance the battery. It puzzles me that they would not just have that already detailed by their technical team and all over their support team.

How would I go about programming the Conext inverter to float charge 56V @ 10A? Currently I am setup for 2-stage only.

Could you post screen shots of the SW4048 user menus, specifically the battery type and charge parameters we can work through them and get the system dialed in.

I would appreciate feedback on the settings.

When all the cells reach a full charge simultaneously there is a sharp break in the charging current and it falls off rapidly as the battery voltage climbs and resists the current flow.

Okay, but what happens at the time with the generator/inverter trying to charge the batteries,? In other words, what happens to the generator & inverter when charging current actually hits zero? I suspect they keep on running, but instead of provide AC power to inverter charger+pass-thru, at that point, when the battery no longer can take any more charge, that the inverter stops charging mode and simply pass-thru AC power to the cabin since the generator was left running. Did I get this correct?

I ask because I believe I will need to do this top balancing manually, forcing the generator to run and setting the charging voltage & current. If I do that, and there is nothing to automatically turn of the generator, two things jump to mind:

1-. How will I know that the top balancing is actually done and I can finally turn off the generator? I suspect I will need to keep an eye on the Battery BMS software, looking out for current to approach zero?

2- Is there anything dangerous in trying to top balance? What are the scenarios that I need to understand and avoid?

TIA!

 

[RCinFLA]

Charging a 100 AH LFP battery at 35 amps will have significant absorb time above 3.45vdc before charge current tapers off. At very low charge current rate a cell is fully charged above about 3.45v. Some inverter/chargers keep the bulk current very constant until absorb voltage is reached. This is more likely to cause a BMS cell overvoltage shutdown if charger absorb voltage is too close to overvoltage setting.

Since you have BMS overvoltage per cell set at 3.623v if cells are not well balanced there will likely be a BMS shutdown with 35 amps of bulk current and absorb at 57.6v. Balancing on BMS likely does not start until a cell exceeds 3.40v and the BMS likely only dumps 100 to 200 mA per cell during balancing. For a 100 AH battery that will take 5 to 10 hours to remove a 1% (1 AH) difference in state of charge between cells. A bit less than 1% SOC balance difference will get you BMS shutdown problems when trying to fully charge battery.

You can get more absorb balancing time before tripping an overvoltage cell shutdown by lowering charge current. Once cells get balanced you can raise the bulk charging current back to 35 amps. If you don't really need to have 35 amp charging I would recommend backing it down to 20-25 amps to be less stressful on battery. You can always crank it up if you need to get charged quicker for some situation like charging from a generator.

If you are going to use 35 amp bulk charge rate I would recommend you set charger absorb voltage to 3.55v x 16 = 56.8v. This will keep you away from overvoltage cell trip voltage. 57.6v absorb setting is too close to overvoltage cell trip voltage.

The SW4048 has a maximum absorb timer and it needs to be long enough to complete the absorb charge current taper down time. 30 minutes to 1 hour should be long enough. The bulk termination time is the length of time the voltage must be at or above absorb voltage limit before bulk constant current phase is exited to constant absorb voltage phase. It is to prevent a momentary grid voltage spike from creating a premature exit from bulk mode.

For LFP battery you never want the charger to accidentally get into lead-acid equalization charge so I would recommend you set the equalize voltage to float voltage setting. A float voltage of 3.35v x 16 = 53.6v is fine. You want float on all cells to stay below 3.4v per cell and with any imbalance you need to keep total float voltage a bit below 54.0v to ensure no single cell floats above 3.4v.

Charge bulk, absorb and float voltages are set by inverter/charger. A comm link between BMS and inverter/charger should only limit inverter/charger user settings if the BMS limits are less than inverter/charger settings.

An inverter/charger should not raise charge limits set by user in inverter/charger based on info read from BMS.

The BMS's battery state of charge likely resets to 100% when battery voltage reaches a set level during charging.

 

[BentleyJ]

How would I go about programming the Conext inverter to float charge 56V @ 10A? Currently I am setup for 2-stage only.


I would appreciate feedback on the settings.


Okay, but what happens at the time with the generator/inverter trying to charge the batteries,? In other words, what happens to the generator & inverter when charging current actually hits zero? I suspect they keep on running, but instead of provide AC power to inverter charger+pass-thru, at that point, when the battery no longer can take any more charge, that the inverter stops charging mode and simply pass-thru AC power to the cabin since the generator was left running. Did I get this correct?

I ask because I believe I will need to do this top balancing manually, forcing the generator to run and setting the charging voltage & current. If I do that, and there is nothing to automatically turn of the generator, two things jump to mind:

1-. How will I know that the top balancing is actually done and I can finally turn off the generator? I suspect I will need to keep an eye on the Battery BMS software, looking out for current to approach zero?

2- Is there anything dangerous in trying to top balance? What are the scenarios that I need to understand and avoid?

TIA!

Lets start with the second question and get the settings corrected. This will tie in with question 1 since we will also set charging to 3 stage, but only for the top balance then it will get set back to 2 stage.

The below will refer to the Charger Settings screen shot you provided, going from top down, then move over to the right hand column.
Charge Cycle: 3 Stage (once 3 stage is selected there may be a max Float current setting that shows up, set to 5A if applicable)
Maximum Charge Rate: 75% is acceptable.
Equalize is disabled: good
Equalize Voltage Set Point: Is inactive but I would set it at 54V to be safe in case equalize were to become active, your battery will not get overcharged.
Bulk/Boost Voltage Set Point: Reduce to 56.8V (this needs to be 0.8V higher than Bulk Termination Voltage setting)
Bulk Termination Voltage: Reduce to 56.0V
Bulk Termination Time: 180 sec is OK
Absorption Voltage Set Point: Reduce to 56.0V
Absorption Time: Increase to 90 min (This can be reduced later if necessary, we don't want charging to terminate too soon while testing)
Float Voltage Set Point: 54.8V (Run the generator with the above settings. You should see the charging cycles go from Bulk to Absorb to Float. Keep an eye on the BMS cell voltage readings. It may take a few hours but you should see the mV deviations between cells start to get smaller. Ideally reaching only 10mV difference between the high and low cell.)

 

[Old_Skewler]

The below will refer to the Charger Settings screen shot you provided, going from top down, then move over to the right hand column.
Charge Cycle: 3 Stage (once 3 stage is selected there may be a max Float current setting that shows up, set to 5A if applicable)
Maximum Charge Rate: 75% is acceptable.
Equalize is disabled: good
Equalize Voltage Set Point: Is inactive but I would set it at 54V to be safe in case equalize were to become active, your battery will not get overcharged.
Bulk/Boost Voltage Set Point: Reduce to 56.8V (this needs to be 0.8V higher than Bulk Termination Voltage setting)
Bulk Termination Voltage: Reduce to 56.0V
Bulk Termination Time: 180 sec is OK
Absorption Voltage Set Point: Reduce to 56.0V
Absorption Time: Increase to 90 min (This can be reduced later if necessary, we don't want charging to terminate too soon while testing)
Float Voltage Set Point: 54.8V

Very clear and I was able to update settings to match the recommended setpoints. Float Current option never appeared. Thank you.




But I still need some help understanding the following:

(Run the generator with the above settings.

Force generator on thru InsightHome, correct?

You should see the charging cycles go from Bulk to Absorb to Float.

Based on my settings and conditions (100ah capacity, 50% SOC and 35A charger) how long should I expect for each stage?

Keep an eye on the BMS cell voltage readings. It may take a few hours but you should see the mV deviations between cells start to get smaller. Ideally reaching only 10mV difference between the high and low cell.)

Got it. How about charging current? Will it safely get to zero? Or is this like singularity and never hits zero, kinda of thing? How will I know when to turn off the generator? It is a 14kW propane generator, so I suspect running it for longs periods of time with near zero load could be a problem? Should I be concerned about performing this toping with this generator?

 

[Old_Skewler]

For a 100 AH battery that will take 5 to 10 hours to remove a 1% (1 AH) difference in state of charge between cells. A bit less than 1% SOC balance difference will get you BMS shutdown problems when trying to fully charge battery.

Ouch... I didnt see this coming. I suspect it will be cheaper and easier for me to bring the battery back home with me for the winter, buy an external charger and charge it back home with utility power than trying to achieve this in the winter with a propane generator. I appreciate the warnings.

If you don't really need to have 35 amp charging I would recommend backing it down to 20-25 amps to be less stressful on battery. You can always crank it up if you need to get charged quicker for some situation like charging from a generator.

Yes, I currently rely only on the generator to charge the battery. Next spring I plan to add another battery and about 5kW of solar.

If you are going to use 35 amp bulk charge rate I would recommend you set charger absorb voltage to 3.55v x 16 = 56.8v. This will keep you away from overvoltage cell trip voltage. 57.6v absorb setting is too close to overvoltage cell trip voltage.

Thank you!

Charge bulk, absorb and float voltages are set by inverter/charger. A comm link between BMS and inverter/charger should only limit inverter/charger user settings if the BMS limits are less than inverter/charger settings.

An inverter/charger should not raise charge limits set by user in inverter/charger based on info read from BMS.

It just happens that today I was also able to connect the EG4 BMS to my InsightHome. And something is not kosher because the inverter throws a fault event when the BMS is connected to the Conext:





Any ideas what this fault is about? As soon as I disconnected the EG4 BMS from the system, everything has been back to operational. I was not able to find any setting related to the warning message.

 

[BentleyJ]

Force generator on thru InsightHome, correct?

Yes, need to Force start the Gen and trigger a bulk charging cycle. SW/InsightHome menu is a bit different than my XW+ so not sure of the steps you need to take, sounds like you know what to do.

Based on my settings and conditions (100ah capacity, 50% SOC and 35A charger) how long should I expect for each stage?

Guestimate?? Little over an hour for Bulk, when the voltage hits the Bulk Termniation of 56V, SW should switch over to Absorb, 30 to 45min. Once the current falls off the SW should switch over to Float automatically. Not sure what the SW algorithm is, the manual may have that info. I'll take a look, I'm curious. Initially when the SW exits absorb and goes into float the float voltage will drop to 54.8 and charging current may drop to zero for a period until the battery settles down then you should see a small float current appear. Keep watch of the cell voltages to verify they are getting balanced and the cell Delta Voltage is going down.

Unfortunately using a generator to top balance is not ideal as its going to use fuel and put hours on it. RCinFLA is correct it could take several hours in float to see a significant change in the cell imbalance. Using an external AC charger would be a better choice.

The best way to top balance the cells would be to take the cover off the battery and use a 5 or 10A active balancer attached to the individual cells. However, I cannot recommend doing that since it may void your warranty or create a safety hazard if the metal case shorts a couple of cells in the process of removing it.
I use this JK-BMS Bluetooth active balancer on my DIY battery when it gets out of balance. https://www.jkbms.com/product/jk-b10a24s/
Its a bit pricey at about $500 but it sure works well, rebalances a 120Ah 48V pack in a couple of hours.

 

[BentleyJ]

The SW manual states that Absorb mode exits and Float begins when absorb current falls to 2% of batt capacity, so in your case that would be 2A. Same algorithm as the XW.

 

[Old_Skewler]

Unfortunately using a generator to top balance is not ideal as its going to use fuel and put hours on it. RCinFLA is correct it could take several hours in float to see a significant change in the cell imbalance. Using an external AC charger would be a better choice.

Totally understand it, I'll just move the Charger Settings back to 2-Stage.

I use this JK-BMS Bluetooth active balancer on my DIY battery when it gets out of balance. https://www.jkbms.com/product/jk-b10a24s/
Its a bit pricey at about $500 but it sure works well, rebalances a 120Ah 48V pack in a couple of hours.

Would a simpler charger like this works as well? I suspect I would just need to keep an eye on the EG4 BMS software looking for current to get near zero?

Battery Charger

 

[Old_Skewler]

The system is acting in a weird behave recently and I suspect part of it is due to the very cold weather these days - battery is outside in the shed and it is about 35F outside. I was hoping that one of you @BentleyJ & @RCinFLA could help me reviewing these settings below:




The condition has been the following: inverter + battery working properly for small LED loads thru the night. Battery at approx. 60% SOC. When I turn on the water pump (1/2HP @ 240V) the battery carries the load until the very end of the pump cycle, then the generator turns on (I suspect the battery hits the 51V voltage trigger because of the cold weather). The generator then runs for a about 1 or 2 minutes at full 33A charging rate then it instantly reduces to 3.5A for about 30 secs then it turns off entirely. I suspect the generator is turning off because of the Stop Voltage point of 55.5V. I suspect this number should be higher? Also, I can't tell if the settings "AC Current Level to Stop/Start" are relevant. I suspect they are currently disable and not relevant to my setup?

Lastly, could one of you explain a bit how the cold temperature will affect the battery? Is it expected that voltage will drop faster under load when in cold temperatures? Is it advisable to warm up the shed (kerosene heater) when charging the batteries? This is my very first winter running this system and I have the feeling the weather does have an impact.

Generator Running while pump running, full charge current for about 2 mins:


Generator running but low charge current, for about 30 secs:



Generator Off due to High DC Current:

 

[RCinFLA]

The impedance of LFP batteries goes up at cold temps. This causes greater voltage slump under heavy load current.

The rate of increase in impedance start to rise quicker below about 15 degs C.

This is not the conduction resistance (typically <0.2 milliohms at 25 degs C for 280AH cell), it is the overpotential voltage slump caused by lithium ion migration getting sluggish at cold temps. It has an exponential time decay that takes a couple of minutes to reach steady state equilibrium for a given load current.

You will also get less extractable capacity at cold temp from battery due to the sluggish lithium-ion migration. At 32 degs F you can expect about 20% less available capacity from battery. At -4 degs F you may get 60-70% less extractable capacity from cells. When you have high discharge current the cell will have some internal heating that will improve voltage slump but the result is it first slumps more then reduces as cell warms up due to the high current demand. It does take 10-20 minutes at moderate current to warm up a large 280 AH cell.

This impedance also effects charging causing cell voltage to rise greater for given amount of charge current that may cause a premature charge termination due to hitting absorb voltage early.



The chart below shows the terminal voltage slump for various load currents at 25 degs C (for a new cell). At cold temp multiply the 25 degs voltage slump at given load current times the degradation factor in above chart. As cells age their overpotential voltage required to support a given cell current rises. An aging cell may be 3 to 5 time greater than a new cell.

 

[Old_Skewler]

A lot of background stuff that I have personally been going thru recently, during this cold season. Very informative. For instance, I understand why I am not able to charge the batteries late night but fine during the day. The battery specs limits charging at 32F.

With the knowledge you describe above, is there any action I should be doing under my control to mitigate the effects of the cold weather? Is there any setpoints I should be adjusting because of the weather? For instance, since cold weather affects the battery impedance creating high voltage alarms, should I reduce charging voltage during the cold weather?

 

[RCinFLA]

A lot of background stuff that I have personally been going thru recently, during this cold season. Very informative. For instance, I understand why I am not able to charge the batteries late night but fine during the day. The battery specs limits charging at 32F.

With the knowledge you describe above, is there any action I should be doing under my control to mitigate the effects of the cold weather? Is there any setpoints I should be adjusting because of the weather? For instance, since cold weather affects the battery impedance creating high voltage alarms, should I reduce charging voltage during the cold weather?

For best performance try to keep LFP above 10 degsC.

At least, try to keep the discharge and charge currents lower at cold temps. You can probably solve your colder charging issue by backing down charge bulk current rate.

Although it is stated you can discharge below freezing, to me, the performance degradation is too great. When you stack up a little aging and low temps, the voltage slump from rested open circuit voltage gets pretty bad.