How to safely rebalance banks?

[aforismo]

Hello,

I posted a few days ago, but the moderation system blocked my message. Since it still hasn’t been approved, I’m trying again with a shorter version.
I have an off-grid installation with Pylontech batteries (2× US5000 and 6× US2000C), arranged as two parallel banks, each consisting of 1× US5000 and 3× US2000C.

However, in July one of the banks stopped charging (and discharging) normally. It has been slowly discharging simply from being switched on for almost a month. Since its SOC is included when calculating the overall average, it keeps bringing the whole system’s reported SOC down. At the moment, the maximum charge level I can reach on any given day is already only around 77%, which means this bank itself is probably at roughly 55%. It seems to be losing around 1% SOC per day.

Using BatteryView, I’ve noticed a significant voltage difference between the two banks, as well as a couple of US2000C modules with what appear to be problematic cells.

The problem is that I’m not an electrician. The installation was being set up for us by someone who then disappeared, so I’ve ended up in a situation where I’m not really sure how I should proceed.

My specific questions are: How should I safely shut down the entire system -i.e. in exactly what order should I switch off the different devices, considering the voltage difference and avoiding voltaic arcs or whatever risks- and how can I safely rebalance the batteries/banks?

I’ve never done this before, so I’d much rather get advice from people with experience like yourselves before doing something wrong and ending up with the system completely down.

Best regards, and thanks in advance!

[DirkW]

Please tell us more about your complete installation.

[BjoernK (Björn Kellermann)]

All 8 batteries shall be parallel, else if you put 4 batteries in series you do not have a 48V system.

Please provide a drawing.

To include all BMS k to Venus OS, all 8 battery communication must be connected. One US5000 must be the master and connected to your Venus OS / Cerbo GX.

I assume you have two banks which are internally parallel. Each bank is connected to your DC bus bar. On the bank with with dropping voltage, the fuse on the bus bar is blown, why it does not charge.

For shutdown and more important switch in procedure pls refere to Pylontech manual.

1. Shutdown
2. Measure not voltage in DC bus bar
3. Disconnect “good” bank
4. Ensure “bad” bank has connection / fuse on
5. Switch in with only bad bank
6. Load until same voltage like good bank

Shutdown, add good bank, switch on

[lxonline (LX)]

You probably just need to check the fuse to that bank.

[aforismo]

Thanks for all your replies, I’ll try to answer the questions as detailed as I can and I provide a schema of the installation, I wrote on the diagram the item names in Catalan because is my mother tongue but it’s fully understandable, I hope:

There are 2 physical parallel branches, there are no series-connected batteries. There are 8 48V modules distributed in the right/left bank/branches, as you can see. All of them share the positive and negative bars. The right bank is the one that’s connected to the Color Control GX (CCGX) and also the one that’s not charging/providing energy.

Inverter: Multiplus-II 48/5000

MPPT: SmartSolar 150/60.

Since july 20th 1-4 modules show 0A, read with BatteryView on every module.

With ~1100W PV production and ~900 going to battery I saw:

• Address 1: 49.153V, 0A, idle
• Address 5: 50.596V, +10,34A, charging

The right branch fuse doesn’t seems to be burnt or inflated but I can see the fuse terminal for this right branch slightly away from the busbar, although it still touches the stud. None of the eight nuts appears properly tightened; they seem to be threaded only a few turns.

I have not touched anything, waiting to learn and understand a secure way to proceed without risks.

The right bank is the one that shows problems, with this ~49.15V (the working one is at ~50.60V)

Your proposal is that me to shutdown the full system, then, after measuring the voltage and fuses I have to disconnect the good (left, address 5-8) bank, check then the bad bank (right, address 1-4), switch only this last bank and ensure charging until loads are similar, then shutdown everything again, reconnect both banks and that’s all?

I have a question on the faulty branch: this is already at the lower voltage. I don’t understand the instruction “load until the same voltage as the good bank” because applying a load to the lower-voltage branch would reduce its voltage further. When you wrote that “load until…” did you mean “charge until…” or discharge the “good” branch to the same level of the “bad” one? Maybe it’s something obvious but I want to be sure

Excuse my poor English and the questions, I want to be sure that I understand everything before touch anything.

I assume that every energy source, including PV/MPPT and the Multiplus, must be isolated, turned off, etc., before accessing the busbar. I need to understand the exact isolation points and how to verify that the busbar is at 0V befoure touching or removing any fuse or cable. One of my main doubts are about the order of the steps to proceed, because I saw different videos and tutorials and not always the steps seems to follow the same order.

Thank you very much for your help!

[Karl-Heinz]

Please take photos of the screw connections in question!

That could be one of the problems (the connection heats up until the fuse itself melts as well!)

[BjoernK (Björn Kellermann)]

@aforismo

The amount of current flow depends on voltage difference.
If you connect two batteries with high (+1V) voltage difference the equalizing current might be more than 100A which can destroy US2000 batteries.

ATTENTION / DANGER

Before starting anything, please switch off your whole system including every single batterie.
Measure there is no voltage on DC bus and DC terminals.

Loose connections which get contact suddenly can result in very high currents like a short circuit.
LFP batteries can deliver very high currents and weld or even melt metal!

Remove all DC cables. Then start checking further steps like fuses burned.
Use a torque messing device to fasten

General procedure:
Disconnect higher voltage good batteries.
Connect lower voltage batteries, but do not connect all if they have more than 0,5V difference.
Then start batteries and the system.
Charge batteries via PV or net or generator until same voltage as good/disconnected batteries.
Switch off system and batteries.
Add good batteries (they have same voltage, no current will flow)
Switching system and continue charging until 100% SoC. Keep batteries on 100% SoC for one day yo top balance all cells.
Now you can use you system normally.

[aforismo]

Thank you, I’ve attached three close-up photos.

The first photo shows the busbar and fuse holder. From top to bottom, the four connections are: MPPT, inverter, left branch (address 5-8, operating normally) and right battery branch (address 1-4, not operating).

The second photo shows the fuse connection for the right battery branch. As mentioned earlier, the metal terminal of the fuse link appears to sit slightly away from the busbar instead of being clamped flat against it.

The third photo shows that the nuts appear to be only finger-tight and are not clamping the fuse links firmly against the busbar.

For clarity, I’ve not touched or tightened anything while the system is energised.

The full history of why these connections were felt like this is long, but in short, we hired a technician to install all the equipment but at some point he desappeared before completing the work, leaving us with an unfinished installation that I don’t know how to handle or correct by myself..

Thanks to everybody for taking the time to help us!

@BjoernK Thanks for your detailed reply, the previous images confirm the way you suggest I have to proceed or there’s something new to have in mind or consider?

[BjoernK (Björn Kellermann)]

@aforismo

The connections are complete loose!
The spring washer shall be completely flat.

Please shutdown the system right now.
Loose connection are a fire risk.

If you have M8 connections I recommend changing bus bar to Lynx Distributor and tighten the nuts according manual with a torque wrench.

Torque in Lynx M8 is 14nm. Other bus bars might have different torque requirement.

[Karl-Heinz]

The entire fuse holder is also mounted on a wooden board!!

This can be thermally affected by the heat and could therefore catch fire even at relatively low temperatures!!

Take some photos of the entire installation!! All the wiring and connections should be clearly visible and easy to follow!