Top Balanced Needed?

bobdelso

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I bought 8 grade 8 cells from Amy. They all tested at the same voltage when they arrived. Since they are matched and at the same state of charge, do I need to do a top balance charge each cell?
 

Short_Shot

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Voltage is not an indication of state of charge for a lifepo4 cell.

You still need to fully charge and top balance them. The fastest way to do this is to assemble them into a pack with the bms and charge until one cell triggers the 3.65v disconnect then disassemble the pack and top balance everything from there.

Some folks will even go so far as to introduce a relay into the mix to let the bms monitor each cell and cutoff the top balance voltage in case of power supply error but that's a bit on the advanced side.
 

Short_Shot

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Oh and I wouldn't suggest assembling them into a pack if your plan is to have cells in parallel inside the pack because you can't detect if one of them goes too high.

Only do that after the top balance and individual cell discharge characteristics are known.
 

time2roll

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I recently assembled my cells with the BMS and they were close enough as they went to 3.45 so I put them in service. Worked fine in the RV. The solar and active balancer can work on getting the balance closer to perfect now that all is back in storage for a while. Good enough for me.

Most others prefer the parallel top balance to 3.650 before assembly. The process can take a week or more was time I did not have.
 

Short_Shot

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3.45v is decently into that upper knee so I'd expect that to be reasonably acceptable and the bms should finish the balance.

As before this is also largely fine if you rarely charge past that anyways.

This would not be true at lower voltage.
 

toms

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Oh and I wouldn't suggest assembling them into a pack if your plan is to have cells in parallel inside the pack because you can't detect if one of them goes too high.

Only do that after the top balance and individual cell discharge characteristics are known.

Weird thing to say, what is the difference between parallel balancing with the cells by themselves, or parallel balancing within a series pack?

You realise that one of the cells can’t go high without tripping the BMS right? If the BMS or individual cell charger is holding the voltage to 3.65V the two cells effectively end up parallel top balanced.
 

Short_Shot

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Weird thing to say, what is the difference between parallel balancing with the cells by themselves, or parallel balancing within a series pack?

You realise that one of the cells can’t go high without tripping the BMS right? If the BMS or individual cell charger is holding the voltage to 3.65V the two cells effectively end up parallel top balanced.
If one cell suddenly hits top while the other is still at a lower state of charge the full one will of course start discharging into the other but it's still certainly possible for parallel cells within a pack to come out of balance and one of them exceed 3.65v when though the "group" reads lower to the bms.

In parallel the cells average together as far as the BMS can tell.

In reality it shouldn't be a huge difference unless you're charging with a rather substantial current that can exceed any passive balance obtained between a cell at 3.65 and one at say 3.4v. Both full or nearly full but definitely still room for current to move and will not necessarily read 3.65 to the bms.

The difference is when assembled into a pack you're not top balancing with 3.65v but rather 12, 24, 36, 48, whatever voltage and there's nothing truly preventing one cell from from going higher than 3.65 because higher voltage is available.

Simply top balancing them beforehand, properly, is normally enough that it isn't an issue in an assembled pack with decent cells.

If they started out wildly imbalanced you risk that undetected over voltage scenario in one cell. You'd never even notice it happening because the bms will just report the average voltage of the two.
 
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Short_Shot

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Weird thing to say, what is the difference between parallel balancing with the cells by themselves, or parallel balancing within a series pack?

You realise that one of the cells can’t go high without tripping the BMS right? If the BMS or individual cell charger is holding the voltage to 3.65V the two cells effectively end up parallel top balanced.
I just realized you may have missed the context that "assembling them into a pack" is referring to putting them together in the series assembly with BMS and charging to the final series voltage with the BMS in place.

I mean. You mentioned series pack but... just in case I'll cover that base:

This is just to speed up the initial top balance process, but will not itself balance anything within the pack and the previously mentioned phenomenon of cell voltage averaging will result in cells exceeding 3.65v if it's partner is at a lower soc still.
 

toms

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Short_Shot

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Nothing when the voltage is only 3.65v during top balancing.

A lot when the pack is getting 14v and the bms can only measure the average voltage between the two.
 

toms

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Are you serious, or just trolling?

With a BMS connected each parallel pair can only reach the BMS high voltage cutoff.

As an example, if the BMS high voltage cutoff is set at 3.55V/cell, and you have a parallel pair within the series pack that started with one cell at 3.5V, and one cell at 3.0V the following happens:

The parallel pair averages voltages as per the paper you linked, so the pair has a voltage of 3.25V. They will now charge (as per the paper you linked), with the higher initial capacity cell charging slower and the lower initial capacity cell charging faster until they reach the charge controllers target voltage or the BMS high voltage cutoff.

The voltages of both cells will be the same always, even if the SOC is different. It is exactly the same way as putting a 90% SOC and 10%SOC cell in parallel and connecting them to a 3.65V PSU. The high cell doesn’t go overvoltage.

(actually curious if you aren’t trolling exactly what you think IS happening when you parallel top balance cells)
 

Short_Shot

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... do you think you can't overvolt a single cell in a pack?

Because you can.

And if you can do it to one on its own you can do it to one in a parallel group. The difference is you can't detect it unless you check each cell independently because, and I'll repeat for the third time, two cells in parallel will average their voltages together.

Do you think that the BMS reading is indicative of 2+ cells in each group 100% of the time?

There is a reason you need to match cells very closely when building large parallel groups inside a series pack. This is known widely.

Because they absolutely can get out of balance.

As I said, top balancing them properly largely solves the issue for our needs in this context but not doing so can *potentially* cause problems with one being over 3.65v.

It is not the same as connecting a 90% cell and a 10% cell because that scenario is a high voltage delta. If you have a 98% charged cell and a 100% cell however the voltage delta (and thus passive balance current) is miniscule and the already fully charged cell won't be giving up much current to the lower one. In this scenario the full one will overvolt and the BMS won't pick it up.

If one cell is at 3.5v and the other is at 3.65v the bms will read the pair as ~3.575v. *This does not mean that both cells are 3.575v.* It means you cannot tell that one is higher than the other until you measure them independently.

If the full cell is at 3.8v and the low one is at 3.5 the bms still sees 3.65v and will then cut off but you've already overshot.

As I said, the actual overvolt will be fairly low but why not put in a little bit of work to avoid it?

This isn't rocket surgery. The cylinder cell guys have known and applied this understanding of it forever. That's (part of) why you see them all building bank testers to match the things.
 
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Short_Shot

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Eventually, yes. Both cells will average their soc.

But you're forgetting that reading the average voltage does not mean that each cell is actually that voltage.

If you take a look back at the chart I posted you see there is a time factor to this assumption you're making where they do not immediately converge in voltage. This is exaggerated by the very properties of lithium cells that make them desirable. Two cells at different SoC may hit the knee at different times and if they are not able to self-balance before the more full cell is itself full the potential to overvolt still exists within the group, however short a duration or or low the overvolt delta actually is, it can still occur.


If one single cell in a 4s pack can overvolt even though you're at 14.6v, and it can, then a group also can. It's possible to overvolt a single cell at 14.4v too, or even much less, but you should know that also.

And if a group can overvolt, then it's possible that a single cell in that group reaches full charge first, the bms continues to allow charging, and the overvolt situation occurs despite being undetected strictly because it's averaging that extra voltage out to a lower value.

The lower cell will drag that group down even though the pack/charger is presenting sufficient power and voltage to overvolt the full one.

Just like how 14.6v/4 is still only 3.65v per cell and yet somehow magically it's possible to exceed 3.65v on a given cell in a 4s pack, sometimes significantly without protection in place. The potential is still present.
 
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Short_Shot

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Parallel cells within a series pack will not go over volt any more than a full stack of cells in parallel.
Except you can because its possible to provide the potential to overvolt just like its possible to do so with a single cell in that series pack.
The difference is the BMS wont detect it if the group average is still below 3.65v.

EVENTUALLY this will iron itself out. My point was just to avoid the possibility at the start with a good top balance and then just dont worry about it anymore because from then on any imbalance will self-correct over time unless the cells are particularly bad.
 
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