24v Battery Pack Discharging the other Pack

One battery pack is discharging the other at 3.2A and killing it.

My system is a 24v. Battery connected 8S2P (16 Eve Cells). Each 24v pack is 280AH for a total of 560AH.
24v Multiplus 3000
Victron Charge Controller 100|50
Lynx Distributor w/ shunt

I performed this test to show one battery pack is indeed discharging the other at 3.2amps. I had both DC and AC disconnected and the solar breaker cut off. When I disconnected one battery pack, the BMS showed the discharging stopped to zero current proving this is happening. I always wake up to dead batteries and finally found the reason. What am I doing wrong?

i am no expert by any means but i do know that electricity will take the lowest path of resistance which maybe draining your bank.
i have (due to budget) several mini power walls,tesla battery's, golf cart lead acid,18650 power blocks and catl batteries all various voltages on all from nominal around 19-29 volts on my 24 system, i do have battery disconnect on each bank set to battery type and disconnected at their perimeters, but my small catl 70ah battery will such up all the power real fast instead of putting power where its needed, i know its better to have all same types of batteries but i really think having a lead acid bank in my mix real helps quite abit as it keeps my several charge controllers busy (making my lifepo topped off )and running my mini split AC 18 hrs a day in AZ heat is a blessing. that said i am keeping a constant eye on all the batteries all the time ,from my LA boiling,lifepo from over discharging etc ,now i am thinking of finding a relay breaker system controlled by BMS's high powered diodes and solid state relays,
sorry about the rant but i do hope that gives you some clue to your problem

Chael said:

One battery pack is discharging the other at 3.2A and killing it.

Click to expand...

You've provided a really helpful set of pics. What you've shown looks right.

What happens when you turn off either of the BMS's in the app? What does the other one do?
Can you verify current flow out of one bat and into the other with a clamp meter?

The battery voltages are nearly identical, i would not be surprised if Daly manufacturing "tolerances" is behind this somehow.

If you can simplify anything, like by taking the shunt out that might reveal something.
I am not familiar with the Lynx but can you show how its wired under the cover?

Can you post i screenshot of each BMS setting? Just grasping here...

MisterSandals said:

You've provided a really helpful set of pics. What you've shown looks right.

What happens when you turn off either of the BMS's in the app? What does the other one do?
Can you verify current flow out of one bat and into the other with a clamp meter?

The battery voltages are nearly identical, i would not be surprised if Daly manufacturing "tolerances" is behind this somehow.

If you can simplify anything, like by taking the shunt out that might reveal something.
I am not familiar with the Lynx but can you show how its wired under the cover?

Can you post i screenshot of each BMS setting? Just grasping here...

Click to expand...

I looked further into this and spent hours trying to figure out if this is normal and I think I have come to the conclusion that it is. I believe the packs are trying to balance each other as (referencing the screenshots) one pack shows 25.2v and the other shows 25.3v. Energy is being transferred between packs. I had cut off the charge controller and all AC/DC connections to make sure. What do you think? This is still all new to me. I built this 3 months ago (top-balanced) but am using it full time in my rv.

To answer your question - When I turned one BMS off, the other BMS's current went to zero. That's how I knew one pack was discharging the other. I will definitely be ordering a clamp meter. The Lynx is just a fancy busbar with fuses. Ill post a pic of it opened when I finish my work.

I shared screenshots of each BMS setting, which is the same for both packs.

Chael said:

I always wake up to dead batteries

Click to expand...

If this is the case its more than the batteries trying to balance.

If your system can tolerate using only one battery at a time, I would see what happens with just one battery connected to your system loads. Evaluate the performance then swap batteries and repeat the test.

Mike

Waking up to dead batteries instead of balanced batteries makes me think you may have a bad battery. It should only drain power from the other until it balances but if it never balances and just draws power until the other is dead it makes me think the one drawing the power from the other is broken.

Do you have a voltmeter? If you do measure each cell of both packs and post the voltages. You could have a BMS that is bad and discharging the system or a bad cell in one of the packs.

Thanks for all the suggestions! I purchased a clamp meter to help measure current. Will also measure each cell with voltmeter soon!

I would float at full charge for a few hours. Then disconnect and separate the batteries for a few days and check if voltage is dropping on one of them.

Chael said:

One battery pack is discharging the other at 3.2A and killing it.

My system is a 24v. Battery connected 8S2P (16 Eve Cells). Each 24v pack is 280AH for a total of 560AH.
24v Multiplus 3000
Victron Charge Controller 100|50
Lynx Distributor w/ shunt

I performed this test to show one battery pack is indeed discharging the other at 3.2amps. I had both DC and AC disconnected and the solar breaker cut off. When I disconnected one battery pack, the BMS showed the discharging stopped to zero current proving this is happening. I always wake up to dead batteries and finally found the reason. What am I doing wrong?

Click to expand...

Sounds like a current leak somewhere on that battery pack. Perhaps one of the cables, or even the blue plastic wrap on a cell has a hole. Disconnect the one pack and see if you have a bad cell.

Question, why do you have frame ground connected to the negative busbar?

Just John said:

Sounds like a current leak somewhere on that battery pack. Perhaps one of the cables, or even the blue plastic wrap on a cell has a hole. Disconnect the one pack and see if you have a bad cell.

Question, why do you have frame ground connected to the negative busbar?

Click to expand...

That is my plan this weekend! Going to look at every cell. It is sort of a pain because they are all packed tight sitting under the dinette sofa.

The batteries are also feeding to a 24/12 converter for everything else on the rv like the tongue jack. The tongue jack is grounded through the frame. Dont I need to make that ground to the bus bar to complete the circuit or is it not needed?

Chael said:

That is my plan this weekend! Going to look at every cell. It is sort of a pain because they are all packed tight sitting under the dinette sofa.

The batteries are also feeding to a 24/12 converter for everything else on the rv like the tongue jack. The tongue jack is grounded through the frame. Dont I need to make that ground to the bus bar to complete the circuit or is it not needed?

Click to expand...

Someone like Filterguy, Hedges or Snoobler knows more about it than I. I do know if the tongue jack has 2 wires feeding it power, the first thing I would try is removing that frame ground from the negative busbar.

I'm wondering what the result was!

Because all this might be bad news for me, I tentatively intend to run between two and four 8S1P 24V 55AH batteries, each with a Daly J10A-GJ21, in parallel. I was sort of hoping it would "just work". I have one running, in parallel with four 7S 1P batteries made of Tesla 2170 cells (my initial test setup). It seems to be working well enough so far, even though the chemistry and curves differ, I assume because the voltages match pretty well with a 24V system. I'm about to add the second 8S1P 24V 55AH battery, maybe next week.

I've been careful about having the voltages pretty close before making the parallel connections, but, I admit I haven't thought very carefully about what could happen after one or more BMS has gone open, or if they get a little too crazy charging each other.

Maybe I should add fuses to each battery, to start?

I hadn't seen this thread when it was originally posted, but I do have some thoughts about parallel packs.

Early this year, when I joined this forum and was learning more and more about LiFePO4, I was reading about people who were putting packs that were the same voltage but different Ah in parallel. I'm a retired MSEE, but I didn't have any engineering sense of how this would work. My gut was that it would cause some problems, and that invariably current would be moving between packs. However, what I heard from those that were doing it was that it worked fine.

Since then I think I've realized the key factor: LiFePO4 (and probably all Li chemistry cells) are nearly 100% efficient. That is, if you put 100Ah in, you likely will get 98-99Ah out. That makes this current moving between packs so that it really doesn't matter, as long as all the cells are healthy. In the OP's case, the two packs shouldn't be at different voltages (may indicate a sub-optimal connection somewhere), but the fact that current is coming out of one pack and into the other is of little consequence. The energy being stored in the lower pack is going to come back out at almost 100%, meaning there isn't much lost in moving it.

Of course, all this assumes that you have very low resistance between the packs, meaning short distance and large enough wire. Otherwise, even though the batteries are in fact very efficient, there will be lost energy in the transfer itself.