Yeah on this topic, it is pretty common to only balance while charging, and also pretty common to use the cheaper “passive” balancer that just bypasses a high cell and has fairly low current capacity. Lots of very good BMSes do that and there is no reason for concern.
This is one topic where I disagree with @TheTerribleTriplet , and I will probably regret bringing it up, but I don’t think a battery will just rapidly go out of balance on some random Tuesday unless a fairly serious failure arises in one of the cells and the capacity of that one cell is suddenly severely diminished. In most cases, once the battery is top-balanced, your balancer doesn’t need more than a few hundred milliamps, and it doesn’t need to balance at any other time than while charging.
It also doesn’t matter if the “balance” is out while discharging at higher current levels. Your cells will not have identical internal resistances (though they should be close of course), so at high power levels you WILL see different voltages. That is why you don’t balance while discharging.
Your more expensive “active balancer” can also balance when the battery is idle. Which is nice, but I’m not always sure if it is worth the extra cost.
But when we get to cost-cutting measures that are so severe that it can run a battery dead, such as the example of a current sense with poor granularity combined with a lack of voltage waypoints. That’s a bridge too far. Don’t use those BMSes.
We agree. Lifepo4 cells don’t just “break” instantly.
It creeps up on one … if you can watch the cells voltages in front of you, you can get a pretty good picture over weeks/months of which cells are more excitable than others. Or even better, have the data recorded for the lifetime of the battery, see which cells are slowly losing the “race” towards the end.
We agree.
And THAT is why one needs a tiny bit more “help” to keep that cell/cells from deteriorating faster than its “family”.
I believe that in due course, down the line, an Active Balancer WILL extend banks/cells lifetime.
What I’m ALSO noticing … each time a bank is replaced under warranty, there is no warranty left on that bank thereafter, right?
So I’m wondering … What are the actual true stats of how long brands last in their 10-year/ xxxx cycle warranty?
Low and behold … as we were chatting there, this came in, 2 banks in parallel, one having an issue the other one fine.
My first reply:
Are you ready? Hey
Are you ready for this?
Are you hanging on the edge of your seat?
Out of the doorway the voltages rip
To the sound of the beat, yeah
Another one bites the dust
Another one bites the dust
And another one gone, and another one gone …
OK, I think I can find a way to state this that makes us both right. I do think that the discovery of the failure will probably creep up on you. Probably after you’ve had a deeper discharge event that average.
The failure will however catch you unawares, and by the time it does, no amount of balancing will fix it anymore. A good balancer also won’t help you avoid it.
A good balancer MAY help to reconstitute the pack and get some residual life out of the diminished capacity. Maybe. Commercial BMSes are not going to bother with that option. If that happens, it is either a warranty claim, or not their problem
Yes, that happens most definitely(!), especially during bad bouts of LS. The weaker cells tend to go first.
However, with the care people are taking to NOT drain their banks hard, I find it tends to also happen as much between the “high LS stages”. Just my observation on this, here and on other forums when people ask for help with warnings on their batts.
Case point:
Watching the cells daily over the last year I can see cell 6 needs TLC. Probably as a result of me, way back, giving it too much TLC in top-balancing a running bank.
It always settles at 3.45v, the balancer doing its part, but it always “wins the race to the top” every time.
“The Bulb” will sort that one of these weeks, a booking, and Lynx installation, have been “tentatively penciled in.”
An active balancer should also only work near the fully charged voltage. If you balance the whole time, you are actually unbalancing the cells.
Active balancers (at least the common cheaper ones from china) use cheap capacitors in ways they are not really meant to be used, which could lead to problems in the form of flames. I would rather avoid those kind of problems.
I was actually just reading the LTC3300 datasheet last night, and this chat reminded me of:
There is no single balancing algorithm optimal for all
situations. For example, during net charging of the overall
stack, it may be desirable to discharge the highest voltage
cells first to avoid reaching terminal charge on any cell
before the entire stack is fully charged. Similarly, during
net discharging of the overall stack, it may be desirable
to charge the lowest voltage cells first to keep them from
reaching a critically low level. Other algorithms may
prioritize fastest time to overall balance. The LTC3300-1
implements no algorithm for balancing the stack. Instead it
provides maximum flexibility by imposing no limitation on
the algorithm implemented as all individual cell balancers
can operate simultaneously and bidirectionally.
Even among balancer manufacturers, there is no consensus on ideal balancing algorithms (and indeed there is likely no single balancing algorithm that works for all cases).
Do you want to maximise energy extraction (eg EV)? Then balance on charge and discharge. Want to maximise pack life? Balance on charge only.
Either way, in my experience, it is a very bad idea to balance LiFe cells between 3.0V-3.4V. In that range, cell voltage has so little relation to cell SOC that you just end up ‘balancing’ internal resistance…
One big problem with most BMSs, especially from China is that they only balance whilst charging, often without the ability to drop the voltage or current targets to compensate.
I have two banks, one made with donor cells from batteries that the Tianpower BMS has destroyed, where we repacked batteries with whole layers (4S per layer in this specific battery), then hacked layers for our personal use from the remainder, knowing these cells are not good at all.
Using the BMS we developed I am able to maintain the bank, getting reasonable capacity out of it, the BMS will drop the target voltage to stop a cell from going too high, whilst balancing the higher cells, so I typically have this kind of graph at the end of the day.
If you start with top balanced cells, you need very little current to keep them balanced, unless of course, you bottom out on one or more of the cells which would unbalance them again.
More aggressive balancing does make sense with NMC in car batteries that have many more cells in series, but LFP for home use 50mA passive balancing is more than adequate if implemented well.
@Paul did that similar with a Rpi using a JBD-AP20S003S BMS on 280ah cells.
Bluetooth for the App. RS485 to control the inverter. As the cells Delta exceeds a pre-set value, he drops the charge amps instantly. Goes a hell of a long way toward helping the BMS balance at 260mA.
He can share more.
Must say, as I typed this, a penny dropped.
If you have 50/100/120ah (C1) cells vs. 280ah cells (C1), then BMS balancer balancing at 50mA gives a whole new perspective.
One day when I get the mother cells, 560ah or whatnot, trust me when I say, that balancer needs to be a bit more souped up.
I stand by:
As the cells age over the years, a decade maybe, you will need a bigger balancer to wring the last out of them cells to get to say ±20 years due to TEMP being THE biggest bliksim over time ito cells subtly degrading faster.
Goal is to avert that “o my, too late, cells have bulged” because of a BMS not up to the task. Been there, done that, lost 16 brand new 120ah cells over months due to ignorance and a crap BMS.
This really isn’t a problem. Even the big brands do that.
This is more of a problem, yes. The really good BMSes drop the charge voltage. OrionBMS for example, few BMSes do it as well as the Orion. Some of the later BYDs as well, the Premium LV and the LV Flex as far as I know. Most of them at the very least drop the CCL (charge current limit).
Dropping the charge current limit is done precisely to slow the charging of the cells so that the balancer has more time to do the actual balancing, but even with a high current balancer you will usually need several cycles to get an imbalance worked out. The balancer cannot permanently bypass a cell. It bypasses it for a short period, usually burning off a little bit of energy in a bypass resistor, while allowing the neighbouring cell to receive a bit more charge. Once the bypass is removed, whatever charging current is flowing still flows through that high cell and still pushes it up. Higher bypass capacity really doesn’t solve the problem, at least not instantly. Balancing always takes time, and once the battery is in balance, you paid for a big balancer only to never really use it
TTT says you’re going to need it later. I disagree. If you need it… you already have bigger problems.
If you optimistically assume that the pack will spend 1 hour in the balancing zone (3.5V->3.65V, under charge), that gives you a maximum balancing capacity of 0.05% of the cell capacity.
If cell efficiency differs by more than this, then 50mA is no longer sufficient to balance the cells.
This is fine for high quality, batch matched cells - but in reality only a handful of top brands have this quality matching. For everything else, you need a bit more ‘oomph’ in your balancer.
Not cell efficiency, but the charge/discharge efficiency difference for a cell has to differ that much from the other, which is a much higher bar to clear.
My point though, BEFORE you NEED it, you WANT it … to give the cells the best possible help to last as long as they can, 20-25 years if they could.
@plonkster high-quality batch-matched cells, over time they will need more “oomph”, all I’m saying.
@plonkster let’s talk about this in another 2-5-10 years. By then we both will have seen a lot more, patterns, what actually works and what not.
We are ALL still learning.
We don’t know what we don’t know.
“There are known knowns. There are things we know we know. We also know there are known unknowns. That is to say; we know there are some things we do not know. But there are also unknown unknowns, the ones we don’t know we don’t know.”
At one stage I had two cell that was fairly out of balance , that was due to my own fault. Connected a Pi Zero with a buck converter over the cells and though what harm can it do…
I ended up using a 12v globe and at a set voltage would have a specific cell’s charge limited while the rest catch up. Used some code to detect the trigger and switch off time after that was fairly sorted…
Now I charge the bank at full blast or rather as much as the panels can supply until any one of the cells hits the 3.48 v mark , I then drop the charge amps to 5A and when the any of the cells hits 3.49v I stop charging of the bank.
You can clearly see the impact of this in the graph below
Morning peeps. Just an update on my eTower issues.
My battery is still with FW for the repairs. They cannot give me a estimate time when I’ll have it back and according to their last email “we are still currently busy inspecting the unit we will communicate with you once the unit is ready for collaction.”
I’ve got a loan battery from the supplier but I’ve got new issues that worries me. The Frimware on my batery’s Pace BMS is V1.07. This the latest supported firmware for that specific BMS. The loan battery’s BMS has got V5.07 on. One would say that seeing that it’s a stackable battery system, they should connect flawlesly but I keep having SOC issues. When the batteries are fully charged they both start up at 100% SOC but the lower the SOC% drops, the higher the difference between the SOC levels on the 2 batteries differ. The highest difference I’ve monitored so far was a 27% difference, with the loan battery at 11% and my battery at 38%. Cell voltages on both packs were very similar at the time. Also at the time my Smartshunt showed 44% SOC and VRM seem to split the SOC% of the batteries and showed 25%.
So to sum it up…
Old battery - 38% according to PBMS tools,
Loan battery - 11% according to PBMS tools,
VRM SOC - 25%,
Smartshunt SOC - 44%
Min and max cell voltages at the time was 3.16V and 3.18V with bank voltage at 50.73V.
I think the problem lays with the firmware differences but it doesn’t seem that the newer one can use the V1.07 firmware or that my old one can use V5.07 (waiting for confirmation ons that)
If this is the case, what do I do should I want to add more batteries in future? I specifically dicided on the eTowers because of the stackability and being able to add batteries as my finances will allow. Now with 2 eTowers with different mmanufacturing dates, it seems that this will not be possible at all? Whats’s the use then? I’m either going to be stuck with my 2021 build eTowers with V1.07 firmware and cannot add newer ones with newer firmware, or I’ll have to replace the bank with a different battery brand. I hope they can provide a solution before it comes to that.
The other option would be to ignore and lose the “warranty”, remove the Pace BMS’s myself and install something else, which might mean butchering the batteries and completely destroying their value.
Don’t worry about that, just ignore it. It’s the reason why some manufacturers lower their DoD limit to 95%. Over time they will equalize. Try not to spend too much time looking at PBMS tools if you don’t have plenty of experience - you will worry yourself to death about things that don’t matter.
The thing is that the loan battery goes offline when it’s SOC oes below 10%, with a bunch of warning lights flashing, then (it seems) current is going from the higher SOC% battery into the lower SOC% battery. I don’t want to make an issue out of it seeing that I hope it’s a temporary problem with the loan battery and would be resolved when I get my battery back but it’s quite a frustration when you lose up to 20% capacity (what the system sees) while the one battery still has ample capacity to go…
