John Frum
Tell me your problems
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Cells can be brand new but not be top quality.
Or they can be top quality but not go through the entire QA process.
Top balance Required for fresh cells?
- Thread starter 7Days_Late
- Start date
Or they can be top quality but not go through the entire QA process.
Absolutely!!
sunshine_eggo
Happy Breffast!
These are very dismissive sweeping statements that add no value to the discussion in terms of addressing real-world problems. Often, the key first step in a well designed system is starting with a top balanced battery, which isn't a given even with premium cells.
Will's experience is that of Will Prowse - Internet solar guy with a big reputation. It's very clear on this site that Will's experience is better than MANY posters who follow his advice, process and design approach.
In order for "matched and balanced" cells to not need a top balance, they must all be essentially exactly the same Ah from full or at least no more deviation than can be addressed by the paltry 30-70mA balance current typical of a passive balancing BMS.
To claim that these cell-based issues are the results of a bad design is disingenuous.
So, I can agree with the "if it pulls full capacity..." directive; however, there's an important step before that:
"If it reaches full charge without issue on your first full charge, then you don't need to top balance."
Per the guide I linked and encouraged them to re:
5. Let the battery charge till the BMS does an over-voltage disconnect. Depending on the Cell size and SOC, this could take several hours. Note: If the voltages stay close together toward the end of the charge cycle, then the cells are already balanced, and top balancing is not necessary.
I stand by my recommendation to follow the guide.
The value i’m adding is to let the original poster know that fixing the issues he’s having right now with regard to the way his system handles an imbalance will result in less stress over the duration of his system life.
Fix the way the BMS handles cell balancing so that it won’t be an issue in the future - OR, bandaid the cell balance and repeat as required by manually balancing the cells multiple times as the system ages.
I’m already seeing a lot of people asking how to stop their one or two year old battery (that was initially top balanced) from disconnecting while charging. What does “the guide” recommend for that?
7Days_Late
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Wow I didn't mean to start such a big discussion with all these heavy hitters, but I really appreciate all the information and feed back in this thread.
I was referring to this video
@ 1:35. Where its stated they will naturally balance over time, but it may take weeks or months. @Will Prowse
Maybe I took this the wrong way or out of context?! (because I'm so fresh to this!)
These cells are supposedly
I was referring to this video
Maybe I took this the wrong way or out of context?! (because I'm so fresh to this!)
These cells are supposedly
- Brand New, Grade A Cells
- Matching Voltage, Capacity, Internal Resistance
sunshine_eggo
Happy Breffast!
He doesn't have a system yet. He is asking about what to do with his new cells.
are you just going to pound your chest about "bad design," or are you going to give the answer?
Guide is specifically for manually top balancing. It's strongly implied as the first step for a new battery.
Again, are you going to divulge the tightly held secret of design, or are you just going to dance around it?
Here's my "good design":
Set BMS to balance at or above 3.40V, 20mV deviation and only during charge.
Charge to 13.8-14.4V
Float at 13.6V
If your cells are good, they should never go out of balance under typical operation. If they go out of balance, you may need to re-top balance, or you may need active balancers, or you may need new cells.
I retract my statement. @Will Prowse is wrong in that video - at least based on the auto CC generated text.
I bolded the key word. that's the tough part. Dozens (hundreds, thousands?) of people have been told that they are getting primo, top notch, grade A, matched, balanced cells of pure awesomeness. Sometimes it's true. Sometimes it's almost true. Sometimes it's close enough to true that you don't get mad. Sometimes, it's such an egregious lie, it makes you want to punch babies.
You won't know until you receive them.
Follow the guide. If you charge to full without issue, no top balancing is needed. Top balancing may not be needed, but may have to change the way you use your battery until the BMS can passively balance to an acceptable degree... which may take weeks or months.
Even with the same supplier, you may find your 16S cells might be different from your 4S cells in a non-trivial way. Since you're building two separate batteries and not mixing them in the same battery, it shouldn't be a problem. You may find that your 4S pack goes together perfectly with no balancing needed... but you may find that your 16S pack needs to be top balanced... or vice versa.
Nearly there. Only charge to 13.8, and set your BMS to reduce charge current when the first cell goes over 3.4V. Disconnect charge source(s) when any cell reaches 3.5V. Disconnect load(s) when any cell reaches 3.0V.
No need to ever take a cell out of the range 3.0 - 3.5V
It is completely pointless to connect a charger to a cell that isn’t part of a fully functioning system - get that right first.
Of course you can spend hours playing with cells before pack assembly, or disassembling your pack multiple times over its lifespan. Your insistence that every other way (ie the overwhelming majority) of pack assembly is wrong speaks volumes.
sunshine_eggo
Happy Breffast!
Fan of that.
Most BMS can't do this.
Pretty conservatively, but no disagreement
Don't agree with that as an absolute.
That truth is context specific. I don't think you're willing to pontificate about the variety of varyingly right methods to a complete beginner. I'm not either.
Technically they should but the voltage differential between each cell is so minimal that the amount of current flowing is nearly non-existent. Practically zero. That's why I was saying you need to top balance. But read the comment in that video that was pinned and also in the description. If you're pulling full capacity there is no reason to do a top balance at all.
The number one most important thing here is if you pull full capacity there is absolutely zero reason to do any type of balancing at all. Absolutely no reason. I also made that very clear in the videos description and the pinned comment. Check it out. There is no reason to obsess over balancing if you have brand new cells. Put the cells together. At a bms. And use the battery. All of these active balancers and excessive top balancing are just plain silly. There is zero benefit. If you pull full capacity use the battery and that's it.
This is the pinned comment and the description under that video:
Also, if we wish to be technical, there is no such thing as a truly matched cell or balancing them all the way. All that matters is getting them close enough and letting the bms do its job. This is determined by the capacity when the cells are in series. If they are imbalanced or not matched, they will not pull full capacity. So like I mentioned earlier, if you pull full capacity, leave it alone. Do not balance them any further. There is zero benefit.
Also understand that with differences in internal resistance, some cells will warm up slightly more than others, and this will also cause imbalances over time. Cell drift occurs naturally. This can cause one cell to spike for a few cycles. This is normal. If you pull full capacity with standard cycling threshold voltages, you are good to go.
In a grade B and or used cell, the internal resistance values will differ significantly, and the cell drift will be excessive. You may benefit from a solid top balance. Even in this situation, I do not see a need for an active balancer. You will have a set capacity that will be limited and lower performance, but thats it.
Also understand that with differences in internal resistance, some cells will warm up slightly more than others, and this will also cause imbalances over time. Cell drift occurs naturally. This can cause one cell to spike for a few cycles. This is normal. If you pull full capacity with standard cycling threshold voltages, you are good to go.
In a grade B and or used cell, the internal resistance values will differ significantly, and the cell drift will be excessive. You may benefit from a solid top balance. Even in this situation, I do not see a need for an active balancer. You will have a set capacity that will be limited and lower performance, but thats it.
Another cause for excessive cell drift over time is use at cold temperatures. A battery engineer told me that this can be the #1 cause of drift over time for systems used in cold temps. But he also said that a basic passive dissipative balancer is more than enough to fix this imbalance after a couple cycles. Again, not an issue. And the engineers plan for this. Every mass manufactured pack I know of, they depend on the cell supplier to ship them at the same state of charge, then they slap them together in a case. There will always be a cell that rises in voltage before the others. Always. But if it is pulling full capacity and the bms has a balancer, which 100% of them do, it will balance it over the next few weeks. The improvement in performance is not noticed. You may get .01Ah more on a 100Ah pack. It is very very small. If you are using used cells, and they are not balanced at all, you will increase the capacity by a lot. But keep in mind that a series string of cells is always limited by the lowest capacity cell. And you will find out that cells capacity with a capacity test.
This is why a basic capacity test with basic cycling thresholds will tell you if the cells are matched/equalized enough to be used. If it does not pull full capacity, you either got ripped off, or your cell supplier did not do their job.
This is why a basic capacity test with basic cycling thresholds will tell you if the cells are matched/equalized enough to be used. If it does not pull full capacity, you either got ripped off, or your cell supplier did not do their job.
I have also seen people destroy $1000's of dollars of lifepo4 cells from trying to top balance new cells and failing. Typically, they overcharge them by setting the PSU voltage too high. I can assure you that this loss of capital has been greater than the benefit of equalizing new cells. If I get a batch of cells now, I connect them together and do a quick capacity test. If I pull full capacity, they are good. If they don't, I send them back or make a video telling you guys not to buy them and who the supplier is.
As a new owner of this type of cells...watch all you can of the YouTube channel called "off grid garage". The creator experiments with, and addresses questions about, these very cells along with larger and smaller versions. Plus he is as interesting as can be. You might even find yourself binge-watching his content. Good luck.
Will's advice is sound because in most cases - the possibility of doing it wrong is far greater than using the techniques us experienced eggheads use, despite videos, pdf's and so forth. Ie, know your audience.
@7Days_Late - do me a favor. When constructing your bank, take the 10 seconds out of your day to wrap the shafts of the screwdrivers, wrenches and whatnot with electrical tape just leaving the working head exposed.
It's the *first* step. Like putting on your underwear. Going commando with batteries can lead to complacency later even if you have other safety measures in place.
@7Days_Late - do me a favor. When constructing your bank, take the 10 seconds out of your day to wrap the shafts of the screwdrivers, wrenches and whatnot with electrical tape just leaving the working head exposed.
It's the *first* step. Like putting on your underwear. Going commando with batteries can lead to complacency later even if you have other safety measures in place.
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