Connecting Balancing wires

I’ve got a question.

If you want to double up with say Pylontechs/ Revov’s, you would get a 2nd, 3rd bank and each bank has a BMS.

Now most BMS’s take a long time (week) to balance a cell/cells due to the wot, 30mA (?), current.

Now on the picture below, where you have 4 batteries connect to a 4 pole busbar, one balancing wire, how can that possibly work i.e. balancing not one cell but 4 cells with ±30mA balancing current per wire?

What makes sense to me is you take a balancing wire per cell, and if you want to double up on the bank, you duplicate the cells and the BMS, not so?

Or am I looking at this wrong?

Once your cells are all balanced, that 30mA will have nothing much to do. So in a setup like you shown above it will divide the 30mA into the 4 cells so it will take longer to balance, but then it should be fine.

The 4 individual cells that are connected in parallel will also be pulling at each other. If the once cell is lower the rest will want to pull him up to match them.

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Thank you Louis.
The part I forgot is the 4 cells “balancing”.

I am under the impression that the balancing of cells is an ongoing thing over the bank’s lifetime?

Firstly, that picture is a bank with 2 cells in parallel - where you see 4 cells connected to one bar, that is 2 sets of 2 parallel cells being connected in series.

So in this case, with 1 BMS, the balancing current gets shared between 2 cells.

You could also do the same thing by making 2 completely separate banks of 16 cells, each with their own BMS and connect them in parallel.

The problem with the second approach is that none of the cheap BMSs are approved for parallel connection. So you invalidate the warranty, and you still need to keep peak currents below the limits of each individual pack. Or get more expensive BMSs which are actually designed for parallel connection.

If you go for the first option (parallel cells with 1 BMS), you can add a separate balancer to operate in parallel with the BMS, if you need a higher balancing current.

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Correct … best picture I could find where those connections are being used.

AHA, so that is where those balancers come in. :cool:

Am I wrong to think one can add more lithium banks, each with their own BMS, as long as one connects each of them onto the same battery connect busbar, and the banks are identical, as are the BMS’s.

Each BMS will then sort the cells it is connected to, with nothing to do with the 2nd bank next to it.

The above is assuming the BMS’es are not interfacing with the system.

You can, and it will probably work. But for something critical to your infrastructure, do you really want to be using parts against the manufacturer’s recommendations? They likely have a very good reason for warning not to connect the BMS in parallel.

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The big issue from what I can see with using 2 BMS in parallel is that if the one BMS/battery shuts down all the current (charge or discharge) will want to go through the other BMS/battery and that can be higher than what it is rated for.

You have 2 ways to prevent this issue.

  1. Only spec the inverter’s current usage to what 1 BMS/battery can handle. If the one stop the other can handle that (more difficult to do if you parallel more than 2)

  2. Or the 2 BMS needs to talk to each other (and the inverter) so that they can reduce the charge/discharge currents as a set and not separately.

The expensive BMS that can handle parallel works like in option 2, so it is always safe.
The cheap alternative is to just have the one BMS manage all the cells, then it is like option 1

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Yes, but once they are already balanced you only need micro adjustments so even 1mA could work. It is for this reason that most BMS does not have a huge balance current capability. As you know high current anything is expensive to build, so it makes the BMS more expensive for something that is only used in the first 2-3 weeks of the battery’s life and then it only needs micro adjustments from there.

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The final few cents I’d like to add, is that the low balancing current of many BMSes is actually not as bad as you might imagine. If the cells are properly selected (so they are similar capacity), and once you have them balanced the first time round (good battery makers do this at the factory), you will rarely need more than 50mA or so to keep them in balance.

Lately I have started to observe – perhaps because of the volumes the factories do nowadays – that the batteries often arrive at the customer with an imbalance that has to work itself out. In this respect BYD has been by FAR the worst, I’ve had at least two cases now where there is a single high cell (the others all more or less at the same lower level)… where you end up taking 2-3 weeks pulling the other cells up because the moment the high cell gets to 3.65V… you have to stop charging and watch the balancer slowly lift the other cells at 50mA…

But once it is balanced, it doesn’t matter. And I’d say that paying more money for a faster balancer is a complete waste. It’s going to use that extra capacity in the first two weeks, and then for the next decade it will just sit there…

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