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Top Balancing using a PSU: is this exactly the other way around?

Sure, if your pack capacity isn’t acceptable, you will need to top up cells. This is simple to do without dismantling the series pack, and you can use the protection of the BMS.

You don’t know if your cells are SOC mismatched until you series charge and get the highest cell into its high voltage knee.
 
Sure, if your pack capacity isn’t acceptable, you will need to top up cells. This is simple to do without dismantling the series pack, and you can use the protection of the BMS.

You don’t know if your cells are SOC mismatched until you series charge and get the highest cell into its high voltage cell.

From experience, all of the ones you get from the current trusted sources are SoC mismatched on arrival, including ones you get from EVE directly (ask me how I know). If you look at most people getting these cells, they lack a decent 60V capable power supply (in case of a 48V system) to charge these in series, unless they hook it all up to their solar system and making sure the BMS and MPPT is configured correctly and they actually have sun. Once they then come to the conclusion that the cells are imbalanced, they need to take the battery apart (every time you do that you introduce risk) to top up the cells (yes, I know, you can do that without taking them apart as well - more risky - and how would you do that with a PSU while still having the protection of the BMS?). My point is that this is much more complex process, more potential for error, and requires more knowledge than just explaining an initial balance - preferable one cell at a time.
 
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It is most certainly not more complex - and i have tried both methods.

To get BMS protection while single cell charging use the high voltage disconnect of the BMS to shut off the PSU instead of the SCC.

Basically what you are saying is that parallel top balancing is good for people that don’t understand how their system works.

I would say these are the last people i would advise to connect all their cells to a cheap PSU with no secondary protection.
 
To get BMS protection while single cell charging use the high voltage disconnect of the BMS to shut off the PSU instead of the SCC.
How do you do this with a commodity fet based bms?
I have this one and Its not immediately clear to me how to control a typical bench psu with via the bms.
 
You connect the B- to the supply negative, the C- to the cell negative pole you try to charge and the + of the supply to the cell positive you're going to charge. With all the BMS leads as usual. This way, the BMS sits in the power supply current path and will interrupt this once it detects an over-voltage.

Personally, I think there is a lot of room for error. Setting the power supply to 3.6V, and then connecting to a cell is in my opinion much easier, you just have to understand not to touch the voltage controls on the supply after connecting to the cell.

Part of me wants to make a small dedicated device that disconnect the power supply based on the same principle as the BMS method, just simpler...
 
You connect the B- to the supply negative, the C- to the cell negative pole you try to charge and the + of the supply to the cell positive you're going to charge. With all the BMS leads as usual. This way, the BMS sits in the power supply current path and will interrupt this once it detects an over-voltage.

Personally, I think there is a lot of room for error. Setting the power supply to 3.6V, and then connecting to a cell is in my opinion much easier, you just have to understand not to touch the voltage controls on the supply after connecting to the cell.

Part of me wants to make a small dedicated device that disconnect the power supply based on the same principle as the BMS method, just simpler...
I can't seem to find it now but a recent video by Andy from off grid garage showed that the cell voltage registers incorrectly when b- is not connected to the pack. Kind of like b- is a sense lead or something.
 
How do you do this with a commodity fet based bms?
I have this one and Its not immediately clear to me how to control a typical bench psu with via the bms.

In the diagram below, connect a suitable relay with the coil windings connected to P+ and P-

Connect your PSU through the switched side of the relay, and connect to the cell to be charged.

If the voltage of that cell exceeds the BMS setpoints, the BMS will disconnect, and the coil circuit will be interrupted.
 

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For the next iteration, I could use a bms to drive an ssr to terminate the charge programmatically.
Should work fine.
Keep in mind that solid state relays typically fail in the on state.

You can buy fail-off ones but one only seen them once in a very expensive machine.

You should use a mechanical relay/contactor for this tbh.
 
These threads pop up constantly. It never ceases to amaze me how people take a simple concept and make it complicated.

Parallel top balancing is for people that are clearly not forward thinking.

If the self discharge variance between your cells is smaller than the balancing ability of your BMS, and your pack capacity is acceptable, a top balance is unnecessary.

If the self discharge variance of your cells is greater than the balancing ability of your BMS, a top balance will still not help you for long.

Nobody has ever given a good reason to parallel top balance. It is a quite recent fad bought about by a few youtubers that obviously had no long-term experience with LiFePO4 packs.

Of course you can get the SOC close with a parallel top balance, but that is pointless anyway because:
1. that is not going to be the exact SOC that the BMS invokes balancing / disconnect.
2. the instant any balancing takes place your “perfect” balance has been altered.
Almost everything you just said is wrong for large cells.

The point isn't really for the sake of balance itself. The point is to get the most capacity out of the assembled pack before the bms shuts off on the weakest cell.

You even touched on this with your "if the pack capacity is acceptable". For most of us "full capacity" is the only acceptable condition for pack capacity, and that's why we all do it.

If you're just using 18650s or something like that then sure. You're probably close to right. The capacity is low enough where these things don't take hundreds of hours.

When you have 300ah cells the BMS can't do any significant balancing and the balance/delta V currents are so low relative to capacity that it effectively never sorts itself out.

A passive balance BMS can correct half a dozen mv of imbalance or so on large cells within a reasonable span of time but anything more than that is painfully long and it's just going to be burning capacity while doing it. The point is to limit how many AH the thing has to turn into heat as much as possible.

As upnorth stated, you might have a 100ah difference with some cells fresh out of the box.

Are you prepared to wait weeks or months for the pack to balance itself well enough to get full capacity?

Cell and bus bar resistance should be low but a 0.050 volt difference is going to give you a fraction of an amp. If you have to make up 100ah with 0.1a that's 1000 hours to do so naturally. More when you factor in the reality that the delta will decrease over time, slowing significantly. It might take a over year to self balance.

Further, your point 2 is irrelevant when you realize balance shifts occur over time with cells that aren't perfectly matched to begin with and it only serves to again meet the criteria of getting the most capacity out before reaching the low cell cutoff.
 
I absolutely agree with top balancing to improve pack capacity.

The smallest cell i’ve ever used is 100ah, the largest is 450ah.

There is no way to determine the SOC imbalance without series charging.

Series charge first, individually top up cells as required. Easiest, fastest, lowest risk method.

The reason people parallel top balance is because they don’t understand how to top up cells in a series pack.

As i said, i have tried and understood both methods - have you?

You talk about large packs, that is where parallel balancing is the worst option!

If you have a 900ah / 48V pack (the largest i’ve assembled), and it is nominally 50% SOC on delivery, how long do you think you’ll be leaving the charger connected? ?

Also would you recommend leaving tens of thousands of dollars of cells connected to a charge source without a secondary high voltage disconnect in place?

Describe to me a single scenario (ie cell SOC when received) where parallel top balancing will get you to full pack capacity faster than series/individual cell top up.

If you are really in a rush, series charge with a high current active balancer in place.
 
I absolutely agree with top balancing to improve pack capacity.

The smallest cell i’ve ever used is 100ah, the largest is 450ah.

There is no way to determine the SOC imbalance without series charging.

Series charge first, individually top up cells as required. Easiest, fastest, lowest risk method.

The reason people parallel top balance is because they don’t understand how to top up cells in a series pack.

As i said, i have tried and understood both methods - have you?

You talk about large packs, that is where parallel balancing is the worst option!

If you have a 900ah / 48V pack (the largest i’ve assembled), and it is nominally 50% SOC on delivery, how long do you think you’ll be leaving the charger connected? ?

Also would you recommend leaving tens of thousands of dollars of cells connected to a charge source without a secondary high voltage disconnect in place?

Describe to me a single scenario (ie cell SOC when received) where parallel top balancing will get you to full pack capacity faster than series/individual cell top up.

If you are really in a rush, series charge with a high current active balancer in place.
So. I agree, and disagree. Depending on the individual aspect. You are definitely not wrong on the whole, but I disagree that the issues presented are important enough to go on a crusade against the matter. The issue is you are presenting it as parallel top balancing itself is the problem. Its really not though.

Parallel top balancing is not bad. The only actual issue that can cause damage or a real problem that you are presenting is the chance of overvoltage situation which is a concern, but the method presented doesn't actually eliminate it and might actually make it more likely to occur (bad) while reducing the possible damage to only one cell (good). So... people can do it either way.

The time thing is just an annoyance. Everyone wants faster, but more time wont hurt anything.


Regarding time to parallel top balance, that's a matter of equipment and individual opinion about it being "too long". Taking too long is not itself a "problem", its an annoyance. Get a larger power supply. Or....

Regarding your comment about it being better to series charge then top balance - that's what most of us are doing to skip the wait time. I agree and it works fine. You can still finish it with parallel just the same. However, time taken is an opinion rather than a problem to strictly avoid.

Regarding time spent parallel top balancing after series charging: if the top-off process takes, say 5 hours per cell, then it will take 20 hours for 4 cells - plus swap time. Unlike the series charge, we aren't saving any time doing them one at a time because the voltage delta is the same and the total power you need to put into it is the same. It also adds time with the disconnect and whatever "extra" time you wait while its sitting there at no current and are away. So once again the "time" argument isn't really a strike against it and your suggestion increases that time.

Regarding leaving it hooked up without a secondary disconnect - your followup suggestion is to do individual cells and that's the same scenario, but much more work to disconnect and reconnect multiple cells. Insisting on doing them one at a time does nothing to reduce the odds of power supply failure, but adds time and effort to the process while increasing the odds of a cheapo power supply biting the dust by introducing more handling to it, and more turning it on and off.

It does however force you to check on it more often, so that's a positive. Not much of an improvement if you are diligent though, but for some this is important.

And at the end of the day your only real reason to avoid it would be solved by having a high voltage disconnect that you can adjust to your preferred cutoff.

Perhaps some enterprising individual will create and sell such a thing with sufficient accuracy to protect the cells, while still remaining fairly inexpensive.
Ideally with just a cheapish voltage monitoring board with a dry contact for the user's choice of higher current relay/contactor. There are a few cheap ones on the market but the accuracy is rather low, usually like +/- 0.1v. Like this one here.

This can still work of course. You could set it to say 3.55v, and set your power supply there too. Then just the last bit of top balancing would require manual attention. You could likely set it to 3.7v as well, but you still risk just a bit of overvoltage there. However if the power supply has failed it would blow way past that anyways, and 3.7 or 3.8v is way better than 4+
 
For sure if you use a 3.65V HVD on your parallel pack and charge with a high current charger, it is no problems other than being slower.

I’m not seeing anyone doing that though, there is significant risk of equipment failure or operator error - hence a thread like this every few days.

With the method i have described there is zero risk to any cell because of PSU fault, (you mentioned it was more risky - underlining your misunderstanding of what i am explaining) and the pack gets assembled once only. I agree the time saved may not be important - but it is a nice bonus.
 
For sure if you use a 3.65V HVD on your parallel pack and charge with a high current charger, it is no problems other than being slower.

I’m not seeing anyone doing that though, there is significant risk of equipment failure or operator error - hence a thread like this every few days.

With the method i have described there is zero risk to any cell because of PSU fault, (you mentioned it was more risky - underlining your misunderstanding of what i am explaining) and the pack gets assembled once only. I agree the time saved may not be important - but it is a nice bonus.
How is there zero risk?

You're increasing the number of times the system is messed with and the power supply can still overvolt on a single cell.
 
PSU cannot overvolt, it is connected through the HVD which is controlled by the BMS.
 
PSU cannot overvolt, it is connected through the HVD which is controlled by the BMS.
Not when you're topping off indivdual cells, which is explicitly what you suggested.

Screenshot_20211016-090129_Chrome.jpg

You can't (edit: can?) do an "individual cell top up" through a BMS, and you can't balance them in series through said BMS.

I mean maybe I'm missing something, but if so it's because you've badly explained it.

Edit: I see I did in fact miss something which was badly explained. Upnorth seems to have covered it clearly enough for my tiny brain to comprehend.

I will have to attempt this method and document it. If it works, and I have many reasons to believe it will, a video is in order.

I have another set of cells coming in the future so why not.
 
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In the diagram below, connect a suitable relay with the coil windings connected to P+ and P-

Connect your PSU through the switched side of the relay, and connect to the cell to be charged.

If the voltage of that cell exceeds the BMS setpoints, the BMS will disconnect, and the coil circuit will be interrupted.
I think my issue came from this which shows the positive lead simply connected to the 4th cell implying you're just using a 14.6v charge to do this.

Combined with the need to hook up the bms negative to get the right cell readings it's unclear how this diagram suitably demonstrates the ability to disconnect on single cell, because it doesn't show the single cell leads in place.

I think I see how it can work this way but this diagram is not the whole picture.
 
I agree it would be easier to understand if i made a video. Where i live there are hundreds of people that have always had off-grid power. Over many years it becomes clear what methods work long term, and what methods don’t.

When i set up a pack i use busbar mounted aluminium 20mm 6x1mm full thread standoffs with the BMS sense leads in between the standoff and the busbar.

This leaves an unused threaded pad i can connect to for individual cell adjustments.

It is also easy to use bus bar clamps.

The main objective is to only torque up the cell terminal connections once.

If you read the text accompanying my DALY diagram - i clearly said the single cell charger is connected through a relay that has its primary coil switched by the P+/P- BMS circuit.
 
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