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?
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.
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.
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.
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.
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)
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
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.
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…
My understanding of a Li-Ion BMS is that it doesn’t have balancing capability unless it specifies this. My experience with 18650 LiPo battery packs is that their BMSs don’t balance the cells. This is a major shortcoming because as @plonkster identifies all you need is one high cell to limit the capacity of the pack and it will never fix itself.
Since I first posted the question, I can confirm what Plonkster said. Top balance Grade A Batch Matched cells once and you are sorted. The balancer does very little thereafter.
I aim for a Delta of 0.005v when charged.
Can also confirm that you can mix 100ah cells with 280ah cells If the cells are of similar resistance, in a pinch. The cells can end up at a Delta of 0.005v when charged.
Can also confirm that if you have access to the cells, using a car headlamp bulb can work magic if a cell is out of whack. One bulb is like 50w, two are 100w, if all the filaments are on. Helps a LOT to quickly balance that cell/cells.
Here I’m helping the 2p cell no 16 a teeny bit, as I did not top balance the one 2p cell. Crude but more “flexibility”. The one cells is the new 6k one, the other the previous order 4k cell, hence the difference in looks. Am waiting to import 3 more cells.
Fact that the balancer does very little, means it is still there, cause I’m pretty sure the cells will go out of whack over time.
I go one step further, I can now see the cells all the time to check the Delta, as I have seen me arse once before when I could not see the cell voltages, the BMS could not balance the cells and the cells were not batch matched nor top balanced.
Really really cheap battery packs has no balancing. Just minimal electronics to ensure the thing does not blow up. You can get away with it (to some extent) by matching the cells well, and making sure they are balanced when you build the pack. They will stay in balance long enough that the user of such a pack (who expects maybe 2 years or so out of it, it is a cheap pack) won’t notice.
I’m not talking about big batteries used to power inverters and stuff. When you get to this level, most of them should have balancing. It doesn’t have to be an expensive balancer, you really just need the most basic pass-the-parcel type that can do a few tens of milliamps and you’ll be fine.
The legacy of lead acid batteries haunts us. These didn’t need balancing with fancy circuits. To balance those batteries you simply overcharged them and let them boil. (I recall it was called equalizing…) But they were forgiving.
Now we have far more sensitive batteries that need a hellava lot more attention.
I must be honest, having had to deal with quality 8 x T105RE’s 6v cells vs the individual cells volts of a 16/18 cells of Lifepo4 bank, you have much better control than (stand to be corrected) 1 x 6v battery containing 3 x 2v cells, or 1 x 12v battery, containing 3 x 4v cells.
And those green indicators on some “deep cycle” 12v batteries … it is on one of the cells in the battery, NOT an indication of all the cells in that battery.
Once the penny drops … like me insisting on a UART and an RS485 port on a BMS … lifepo4 is so much easier.
We went through the same learning curve with lead acids, having Victron or Chinese balancers some bought to help “balance” the bank.
Today we reminiscence, lead-acid was so much “easier”.
To be honest I have never worked on a bank of wet lead acid batteries. I have a hydrometer ready for the day when I do but I’m not betting the farm on it…
My AEE catalog has this procedure:
Check the electrolyte level in wet-cell, or “flooded” batteries, at least once every three months and top-off each cell with distilled water.
Do not add water to discharged batteries! Electrolyte is absorbed when batteries are discharged, so if you add water at this time and then recharge the battery, electrolyte will overflow and create a safety hazard. Keep the tops of your batteries clean and check that cables are
tight. Do not tighten or remove cables while charging or soon after charging! Any spark around batteries can cause a hydrogen explosion inside the case and potentially ignite a fire or an even larger explosion if the batteries are not properly vented.
An “equalization” charge should be performed on flooded batteries whenever cells show a variation of 0.05 or more in specific gravity from each other. This is a long steady overcharge, bringing the battery to a gassing or bubbling state. Do not equalize VRLA batteries!
With proper care, lead-acid batteries will have a long service life and work very well, in almost any power system.
They are talking about deep cycle wet cell batteries. (and this wasn’t so long ago!)
Pity these were so expensive.