My Parallel 24v Batteries drop out of Synch

hi,

I need some help troubleshooting an annoying issue I'm seeing on my solar setup.

I have a self built solar & battery setup on / in my garage to run a fridge and chest freezer.

Everything works, the solar panels charge the battery, the battery powers the garage when the sun goes does.



The batteries are self built from Docan Tech Lifepo4 cells to 24v each @280ah.

The only thing I can't get my head round is the fact that the 2 parallel batteries often get un-coupled. So, 1 cell is getting all the charge and all the draw, whilst the other sits idle. Clearly the voltages diverge as part of this.

The batteries are connected in parallel with each having an 8s 200a BMS on the negative end, and they join at a busbar as per the photo below.



the only way I get them back into synch is to reset both BMSs at the same time.

I'm assuming this HAS to be BMS related as I can't see how they aren't operating in parallel otherwise.

They will reach full and empty together.
Difference in resistance will make them deviate from each other in the middle.
All that you can do is get the resistance as close as possible.
Same length and size cables to each.
Internal cell resistance is out of your control.

timselectric said:

They will reach full and empty together.
Difference in resistance will make them deviate from each other in the middle.
All that you can do is get the resistance as close as possible.
Same length and size cables to each.
Internal cell resistance is out of your control.

Click to expand...

The do not reach full and empty together.

Currently Pack1 is sitting at 26.8v and idle, whilst Pack2 is at 26.42v and is receiving solar and powering the appliances. If I disconnect the solar Pack2 will continue to discharge right down until the mains kicks in. The only way I can get the idle pack to take over is to switch off the charge on the active pack and they swap over.

Without any intervention, 1 pack will do all the work for weeks. Given that the voltage difference can be quite large, and I am using the same length of cable and arranged the connections symmetrically on the busbar I can't see how it's down to resistance. I'm at a loss.

Kman68 said:

The do not reach full and empty together.

Currently Pack1 is sitting at 26.8v and idle, whilst Pack2 is at 26.42v and is receiving solar and powering the appliances. If I disconnect the solar Pack2 will continue to discharge right down until the mains kicks in. The only way I can get the idle pack to take over is to switch off the charge on the active pack and they swap over.

Without any intervention, 1 pack will do all the work for weeks. Given that the voltage difference can be quite large, and I am using the same length of cable and arranged the connections symmetrically on the busbar I can't see how it's down to resistance. I'm at a loss.

Click to expand...

That's definitely odd.
Are the BMS's communicating with each other in any way? If not, they should operate on the voltage that they see. There is some type of control that is causing this.

Maybe the cells in one or both of the batteries aren't balanced well enough and the BMS is cutting off charging and/or discharging because one of the cells went high/low?

Symbioquine said:

Maybe the cells in one or both of the batteries aren't balanced well enough and the BMS is cutting off charging and/or discharging because one of the cells went high/low?

Click to expand...

That could be on charging or discharging, but not both.
But definitely a control issue.

Look at the BMS for the off-line battery. For some reason, the BMS stopped the discharge. I'm guessing when it tried to resume discharge, the voltage differential created an overload, or some other situation, that took it offline again. You may need to rebalance the packs. Slowly charge pack A until it reaches pack B's voltage, and then bring B online. Complete the charge. Float for a while to complete the balance.

Do they get charged to 100% together on a regular basis?

DIYrich said:

Look at the BMS for the off-line battery. For some reason, the BMS stopped the discharge. I'm guessing when it tried to resume discharge, the voltage differential created an overload, or some other situation, that took it offline again. You may need to rebalance the packs. Slowly charge pack A until it reaches pack B's voltage, and then bring B online. Complete the charge. Float for a while to complete the balance.

Do they get charged to 100% together on a regular basis?

Click to expand...

It's not 1 specific battery that's seeing this. It's either. If I reset both BMSs at the same time they do re-synch (and one will charge the other up) until they are reasonably close in terms of voltage. After this, they can remain synched for days and then 1 will just stop.

Maybe the cells in one or both of the batteries aren't balanced well enough and the BMS is cutting off charging and/or discharging because one of the cells went high/low?

Click to expand...

the BMS is showing the cells are pretty close over the majority of the voltage range, with only close to 100% seeing any divergence. This issue occurs at mid range voltages, so I don't see this as a likely cause.

I think it must be a BMS thing - they are separate and don't have any communication between them.

It might be costly but perhaps I need to either get 2x replacement BMSs or a single BMS to over the 8s2p setup in one.

I would look into different BMS's.
The JK is working great for me. But I'm on 48v. So I can't really speak to the 24v ones.

Kman68 said:

Symbioquine said:

Maybe the cells in one or both of the batteries aren't balanced well enough and the BMS is cutting off charging and/or discharging because one of the cells went high/low?

Click to expand...

the BMS is showing the cells are pretty close over the majority of the voltage range, with only close to 100% seeing any divergence. This issue occurs at mid range voltages, so I don't see this as a likely cause.

Click to expand...

Ah, makes sense. I didn't quite catch that it was happening in the middle of the voltage range from your original post.

For future reference, what BMS brand/model do you have currently?

Symbioquine said:

Ah, makes sense. I didn't quite catch that it was happening in the middle of the voltage range from your original post.

For future reference, what BMS brand/model do you have currently?

Click to expand...

JBD 8s 200a