Why are my EG4 batteries charging/discharging at different rates?

[Marcello with 2 Ls]

Hi folks,

My two EG4 batteries are often way out of sync in terms of charge. Does anyone know why?

Example: Right now, battery 1 is 76% full, while battery 2 is 85% full. If battery 1 gets too low, it will trip the inverter into grid-bypass mode, even when Battery 2 has plenty of power.

They charge and discharge at varying rates (no logic) and I don't understand why the BMS doesn't try to equalize them.

I'm using the built-in BUS on battery 1 to connect to both the Growatt 12k inverter AND battery 2. The cables are short (<3ft), and similarly sized.

 

[42OhmsPA]

What are your cell voltages?

BMS soc is notoriously inaccurate.
Join the dark side and run open loop, control the inverter based of battery voltage (make sure your cells are well balanced).

 

[Mark-]

My two EG4 batteries are often way out of sync in terms of charge. Does anyone know why?

BMS soc is notoriously inaccurate.

And EG4, at the least the ones I have, (7) are horrible at SOC. Here is the current readings of 2 batteries:

I am using open loop and an F(x) function (also called a characterizer, FGEN or, look-up table), to get a better SOC "guess". Here is all the batteries, (4 brands) average state of SOC verse the F(x):

 

[chrisski]

Have you tried to contact EG4? I can only recommend a deep charge every morning, thats what works in my Outback Radian.

If you want to run in closed loop with BMS battery com, than keep trying.

IME, I run my system in Open loop with no option for BMS com and the grid tied settings have trouble giving the batteries a deep charge every day. This gives me problems just like your closed loop. What I decided was to charge my batteries for two hours every day by going out of grid tied. This gives me a good deep charge every morning.

I’m not sure the exact settings but basically once the battery reaches its sell back voltage, no more charging is done. Every hour I lose a few hundred what hours and in peak hours about a thousand an hour. This ends the day with me 10 kWh down. Without going and doing a deep charge every day I end the day with batteries Les and less every day until a BMS trips.

 

[Mark-]

My two EG4 batteries are often way out of sync in terms of charge. Does anyone know why?

Looking at your picture, is the battery wired from inverter to battery to battery or inverter to common bus bars to batteries?

 

[Marcello with 2 Ls]

@Mark-, its wired inverter to battery 1, then battery 1 to battery 2. I wanted to make use of the EG4's built-in bus bar. Thx

 

[Mark-]

@Mark-, its wired inverter to battery 1, then battery 1 to battery 2. I wanted to make use of the EG4's built-in bus bar. Thx

Will has a video about "series" battery connections. Might want to watch it.

 

[ossiemoore]

I am an amateur at this and the following probably has nothing to do with your issue but...

• From your picture, the one on the left does not have enough "breathing" room to the left and above it? If that one is not discharging as fast as the right, maybe the temp is a bit warm inside that one and the BMS is limiting the draw/charge current?
• Have you deep discharged both and then fully charged both? (Probably not relevant but I did see something about an internal imbalance in the manual that suggest that as a potential solution.)
• I assume you have inspected and/or reseated the cables (after powering things down obviously.. ha ha)

Forgive any dumb ideas above. I am new to all of this.

 

[ossiemoore]

To follow on to my comments.. the fridge to the left of the one on the left would actually generate additional heat behind the fridge, which might further "warm" the one on the left? Probably not related, but worth looking a temperature graphs of the batteries to compare?

 

[EG4TechSolutionsTeam]

Could you post BMS Tools screenshots to review?

https://eg4electronics.com/wp-content/uploads/2024/04/Bms-Tools-V1.0.zip

https://eg4electronics.com/wp-content/uploads/2024/04/BMS-TOOLS-SETUP.pdf

 

[wpns]

@Mark-, its wired inverter to battery 1, then battery 1 to battery 2. I wanted to make use of the EG4's built-in bus bar. Thx

So which is "1" (Left?) and which is "2" (Right?)? How many parallel cables do you have running between the batteries, and how many cables do you have running between the battery (which one?) and the inverter?
You might run equal-length cables one from each battery to the inverter, and then parallel cables between the batteries.

 

[Bananaman321]

I have 2-power wall batteries 4-server rack batteries and 1-golf cart battery all in parallel with an odd cable configuration.

To keep things in balance I periodically put charge controller in acid mode 56.8v absorb/bulk charge and 55v charge 48v low batt warning / back to grid 46v disconnect / shutdown.

I do this for a day or two this balance is all the packs and allow BMS to top balance cells. (once a week or so works for me)

Also should mention when I started doing this I put the bulk / absorption charge at 58v this will help bring your low packs up quicker as the high SOC battery's BMS will disconnect them and let the lower SOC batteries catch up. One more thing IMPORTANT when absorption set to higher than 56.8v set LOWER capacity in charge controller like 250Ahr so as batteries satisfy the last few don't get charged with too high of "C-rate".

BTW in Lithium mode I have comms with powerwall batts

Hope this helps

 

[Bananaman321]

I have 2-power wall batteries 4-server rack batteries and 1-golf cart battery all in parallel with an odd cable configuration.

To keep things in balance I periodically put charge controller in acid mode 56.7v bulk charge and 55v charge 48v low batt warning / back to grid 46v disconnect / shutdown.

I do this for a day or two this balance is all the packs and allow BMS to top balance cells. (once a week or so works for me)

BTW in Lithium mode I have comms with powerwall batts

Hope this helps

 

[Bananaman321]

What are your cell voltages?

BMS soc is notoriously inaccurate.
Join the dark side and run open loop, control the inverter based of battery voltage (make sure your cells are well balanced).

How often do your batteries get fully charged

 

[42OhmsPA]

How often do your batteries get fully charged

Almost daily in the sunny months, at least a couple times a week in the dark months.

 

[Brett V]

@Mark-, its wired inverter to battery 1, then battery 1 to battery 2. I wanted to make use of the EG4's built-in bus bar. Thx

Then battery 1 will supply more of the load to the inverter than battery 2. Your SOC readings are correct.

 

[Quattrohead]

Your voltage and current readings are suggesting the batteries are better balanced than that, they need to be fully charged at least once a week to enable the BMS to understand where 100% is and then count down from there. Percentage state of charge is not a good metric long-term but it is good for a day or two.
The Power pro battery likes to be charged up to 55/56 volts for full balancing.

 

[Bananaman321]

I've never seen my Powerpro's more than 2% difference SOC with or without Comms and they are connected just as manual describes.

I would get a snapshot of the cell voltages of both batteries the next time it cuts off on a low SOC.

At least this would let us know if we had a hardware problem.