BMS destroyed my battery?!

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fatty   10 kW

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Re: BMS destroyed my battery?!

Post by fatty » Apr 03 2021 4:07pm

goatman wrote:
Apr 03 2021 3:43pm
i disagree, if youre pushing amps into a cell at 4.2v where capacity no longer exists, i believe it degrades cells faster...
Well, sure, if the capacity of that p-group has degraded so low relative to the rest of the pack that charging current exceeds their max C rate -- but at that point, I think you'd call those cells functionally dead as the pack would never stay balanced upon discharge anyway.
goatman wrote:
Apr 03 2021 3:43pm
...and throws the pack out of balance even quicker.
Again, sure -- if not balance charging. But I'd say top balancing every charge would be necessary on such a marginal pack.
goatman wrote:
Apr 03 2021 3:43pm
probably wrecks cheap bms's quicker also depending on how many amps the charger is pushing
Doubtful: area under curve during constant current between 4.1-4.2V is minimal, and some BMSs need to be held at 4.2V to balance.

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Re: BMS destroyed my battery?!

Post by RTLSHIP » Apr 03 2021 4:20pm

RTLSHIP wrote:
Apr 03 2021 12:08pm
goatman wrote:
Mar 21 2021 10:12pm
if 1 cell in the p-group is self discharging it will pull the others down to 0volts

you can think of the 0 volt cell as leaving the lights on in your car, it will drain the battery down
by separating the cells you remove the load (self discharging cells)

ive been trying for a week to zero volt some laptop cells so i can dispose of them and they keep bouncing back up in voltage, wont stay at zero volts, they kept bouncing back to about 0.96v

they were no good for a bike pack, i shouldve kept them to run a flashlight

i just grabbed them

click on a 12v bulb, it wont light up but it will take it down to zero

remove load and voltage starts climbing back up


your cells that went to zero volts are probably no good for a bike pack anymore, they will fail

i would tear that whole pack apart, slug tape all the cells together to equalize them with each other for a week, take the tape off and toss any cells that arent the same voltage as the rest and you might be safe but you should do a resistance test to each cell to see if you have enough matching cells to build a pack with
So if I have 1 bad cell in a 9 parallel, the bad cell works as a parasitic drain on the other 8? I ask this because my old pack has 9 & 9 cells out in 2 p. The other 14 series are fine. In other words, I get 18 zero volt. And 126 cells ~ 3.5 v. Then the bms is not the cause.
Sorry to confuse things, I was referring to a lifepo4 48/15 pack. I just charge it up using a 55 or 58 v. Once I disconnect, it gradually drops to 49 rather than the 55 or 56 if it were 100% good 16s instead of the 14s.
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RTLSHIP   1 kW

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Re: BMS destroyed my battery?!

Post by RTLSHIP » Apr 03 2021 4:31pm

fatty wrote:
Apr 03 2021 1:40pm
RTLSHIP wrote:
Apr 03 2021 12:08pm
So if I have 1 bad cell in a 9 parallel, the bad cell works as a parasitic drain on the other 8?
Yes
RTLSHIP wrote:
Apr 03 2021 12:08pm
Then the bms is not the cause.
Not necessarily -- the BMS could have overdischarged those two p-groups.
But the bms did not harm the other 14s x 9 p? Do you know what the bms lvc is on a lifepo4 pack? Mine cuts out at around 44v, but I can't tell yet if it is controller or bms. I would imagine that the lvc is higher on controller as manufacturer supposes you may have SLA's.
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Re: BMS destroyed my battery?!

Post by fatty » Apr 03 2021 5:44pm

RTLSHIP wrote:
Apr 03 2021 4:20pm
Sorry to confuse things, I was referring to a lifepo4 48/15 pack. I just charge it up using a 55 or 58 v. Once I disconnect, it gradually drops to 49 rather than the 55 or 56 if it were 100% good 16s instead of the 14s.
You're thread-jacking this thread and making it impossible to follow.
Please delete your replies here and re-post your question to you own new thread.

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Re: BMS destroyed my battery?!

Post by RTLSHIP » Apr 03 2021 6:26pm

fatty wrote:
Apr 03 2021 5:44pm
RTLSHIP wrote:
Apr 03 2021 4:20pm
Sorry to confuse things, I was referring to a lifepo4 48/15 pack. I just charge it up using a 55 or 58 v. Once I disconnect, it gradually drops to 49 rather than the 55 or 56 if it were 100% good 16s instead of the 14s.
You're thread-jacking this thread and making it impossible to follow.
Please delete your replies here and re-post your question to you own new thread.
OK, I apologize but was unable to remove above as it has 2 parties. Maybe you should not have replied
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Re: BMS destroyed my battery?!

Post by john61ct » Apr 03 2021 7:28pm

maarten_almighty wrote: I think that the high voltage cutoff in the bms should still come into play in some cases even when charge voltage is set correctly: example: let's image a really badly balanced pack, let's say 19 of the twenty parallel groups are at 3.5 volts and 1 out of twenty is at 3 volt.
You should be regularly checking per-cell voltages to see that they are not allowed to get too far apart, say 0.03V - 0.05V would show owner negligence.

For me the pack charger is never set to the max 4.20V anyway, 4.05V usually.

That way if the BMS is actively backing up the charger regulation, there's a nice delta between the normal stop-point and the failsafe.

Most BMS only trigger HVC off the highest cell, and that is fine, since if only the highest one reaches the setpoint, the pack level (average) will be nowhere near there.

The usual scenario of the BMS not showing per-cell voltages, so you never know if / when balancing is complete, IMO is idiotic.

As is non-adjustable setpoints.

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Re: BMS destroyed my battery?!

Post by john61ct » Apr 03 2021 7:39pm

fatty wrote:Doubtful: area under curve during constant current between 4.1-4.2V is minimal, and some BMSs need to be held at 4.2V to balance.
Never buy such a stupid BMS, or at least avoid using it for balancing packs where doing so takes more than a few minutes.

Your tools should suit **your** care parameters, not v/v

Holding above 4.15V is harmful to longevity, nothing to do with current rate, just voltage. 4.05V even better if trying to get maximum life cycles.

Now, if your use case is "abusive" anyway (e.g. high C-rates) then the impact of that one factor becomes much less significant

but really there is no point in going that high, does not actually add any significant range anyway!


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Re: BMS destroyed my battery?!

Post by goatman » Apr 03 2021 8:12pm

i dont want to jam up this thread with pictures, it kills the flow of a thread with a huge picture that fills the page, be nice if they had those little thumb nail pictures that can be clicked on and off
goatman wrote: ↑Apr 03 2021 1:43pm
i disagree, if youre pushing amps into a cell at 4.2v where capacity no longer exists, i believe it degrades cells faster...
Well, sure, if the capacity of that p-group has degraded so low relative to the rest of the pack that charging current exceeds their max C rate -- but at that point, I think you'd call those cells functionally dead as the pack would never stay balanced upon discharge anyway.
i dont agree, theres so little capacity up top from 4.15v to 4.2v its easy to throw the pack out of balance. just by lowering the charge to 4.1v will give some dead packs a new life.

the op lost 18 cells, the rest are sitting at 3.5v right now
if he used those cells to build another pack and only charged to 4.1v he would probably have a balanced pack, no bms required

could wire an Ant-bms with bluetooth, leave balancing at 4.2v but only charge to 4.1v

the bluetooth will show the parallel group voltages

if it goes out of balance at 4.1v then enable the balancing function to balance it

i have to order one so i can use it to desulfate my packs in line with a load tester

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Re: BMS destroyed my battery?!

Post by fatty » Apr 03 2021 10:19pm

john61ct wrote:
Apr 03 2021 7:39pm
Never buy such a stupid BMS, or at least avoid using it for balancing packs where doing so takes more than a few minutes.

Your tools should suit **your** care parameters, not v/v

Holding above 4.15V is harmful to longevity, nothing to do with current rate, just voltage. 4.05V even better if trying to get maximum life cycles.

Now, if your use case is "abusive" anyway (e.g. high C-rates) then the impact of that one factor becomes much less significant

but really there is no point in going that high, does not actually add any significant range anyway!
Oh, I agree. My point was that current load on the BMS between 4.1-4.2V is minimal and not likely to contribute to BMS failure.

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Re: BMS destroyed my battery?!

Post by fatty » Apr 03 2021 10:40pm

goatman wrote:
Apr 03 2021 8:12pm
i dont agree, theres so little capacity up top from 4.15v to 4.2v its easy to throw the pack out of balance.
Balancing with unmatched cells is inherently temporary. It doesn't matter if you top balance at 4.1 or 4.2V, or bottom balance at 3.0V -- the pack will only be balanced at the voltage you balanced it at, and necessarily must be unbalanced everywhere else. If there's little capacity between 4.15-4.2V, then there is necessarily little capacity variance, and thus little imbalance that occurs between these points.
goatman wrote:
Apr 03 2021 8:12pm
just by lowering the charge to 4.1v will give some dead packs a new life.
By what mechanism, necromancy? No, there is no theoretical nor experimental data to support this.
goatman wrote:
Apr 03 2021 8:12pm
if he used those cells to build another pack and only charged to 4.1v he would probably have a balanced pack, no bms required
Well, it would be balanced at 4.1V/cell -- but otherwise just as unbalanced everywhere else. Charging under max is for cycle life, not balance.
goatman wrote:
Apr 03 2021 8:12pm
i have to order one so i can use it to desulfate my packs in line with a load tester
LOL WUT? You know lithium ion batteries don't have sulfur in them, right?

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Re: BMS destroyed my battery?!

Post by john61ct » Apr 03 2021 10:54pm


fatty wrote: not likely to contribute to BMS failure.
Instances of most BMS models just fail, they are usually the weakest link and live up to Battery Murdering System

No reason to look outside their own build quality for failure reasons


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Re: BMS destroyed my battery?!

Post by john61ct » Apr 03 2021 10:56pm


fatty wrote:You know lithium ion batteries don't have sulfur in them, right?
He just (mis)uses the term for some protocol to help "resuscitate" old worn cells.

Has a whole thread with that in the title, and yes he has acknowledged it's an improper use of the term


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Re: BMS destroyed my battery?!

Post by john61ct » Apr 03 2021 11:00pm


fatty wrote:Well, it would be balanced at 4.1V/cell -- but otherwise just as unbalanced everywhere else. Charging under max is for cycle life, not balance.
For those doing top balancing, most use BMS that only **start** balancing at a too-high voltage

and have a very very slow / low mA balancing current

and offer no way for the user to know what the balancing status is, so many users need to keep the too-high voltage going for literally days

All this considered "normal", to the point many users aren't even aware that alternatives exist


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Re: BMS destroyed my battery?!

Post by goatman » Apr 03 2021 11:08pm

john61ct wrote:
Apr 03 2021 10:56pm
fatty wrote:You know lithium ion batteries don't have sulfur in them, right?
He just (mis)uses the term for some protocol to help "resuscitate" old worn cells.

Has a whole thread with that in the title, and yes he has acknowledged it's an improper use of the term
still waiting for someone to tell me the right term :D

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Re: BMS destroyed my battery?!

Post by john61ct » Apr 03 2021 11:11pm


fatty wrote:Balancing with unmatched cells is inherently temporary. It doesn't matter if you top balance at 4.1 or 4.2V, or bottom balance at 3.0V -- the pack will only be balanced at the voltage you balanced it at, and necessarily must be unbalanced everywhere else.
Truth, and another one many do not grok

> If there's little capacity between 4.15-4.2V, then there is necessarily little capacity variance, and thus little imbalance that occurs between these points.

That does not follow.

Balancing is often only required because of the way people use cells.

Assuming good model, Grade A quality brand new

if you just leave the cells "middle balanced" as they come from the factory

never bottom balance nor top balance

implement both LVC and HVC at the per-cell level so triggered by the weakest links (lowest capacity)

with setpoints closer to the center voltage/SoC range rather than out past the voltage shoulders at the extreme edges

then the pack may **never** need balancing.

And longevity will be greatly extended.

By pushing your charge voltage out past 4.05 - 4.10V

and top balancing out there

you are indeed **causing** the imbalances that now require frequent "fixing"

and reducing longevity.

If you want to maximize range, you should bottom balance, but going down to a lower LVC drastically shortens lifespan, much more than charging too high

maybe as much as high C-rates, but some use cases that is unavoidable.

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Re: BMS destroyed my battery?!

Post by john61ct » Apr 03 2021 11:12pm


goatman wrote:still waiting for someone to tell me the right term :D
It's your baby, you name it.

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Re: BMS destroyed my battery?!

Post by goatman » Apr 03 2021 11:23pm

fatty wrote:
Apr 03 2021 10:40pm
goatman wrote:
Apr 03 2021 8:12pm
i dont agree, theres so little capacity up top from 4.15v to 4.2v its easy to throw the pack out of balance.
Balancing with unmatched cells is inherently temporary. It doesn't matter if you top balance at 4.1 or 4.2V, or bottom balance at 3.0V -- the pack will only be balanced at the voltage you balanced it at, and necessarily must be unbalanced everywhere else. If there's little capacity between 4.15-4.2V, then there is necessarily little capacity variance, and thus little imbalance that occurs between these points.
goatman wrote:
Apr 03 2021 8:12pm
just by lowering the charge to 4.1v will give some dead packs a new life.
By what mechanism, necromancy? No, there is no theoretical nor experimental data to support this.
goatman wrote:
Apr 03 2021 8:12pm
if he used those cells to build another pack and only charged to 4.1v he would probably have a balanced pack, no bms required
Well, it would be balanced at 4.1V/cell -- but otherwise just as unbalanced everywhere else. Charging under max is for cycle life, not balance.
goatman wrote:
Apr 03 2021 8:12pm
i have to order one so i can use it to desulfate my packs in line with a load tester
LOL WUT? You know lithium ion batteries don't have sulfur in them, right?
by bottom balancing my packs to 2.5v but running them from 4.1v to 3.0v im not exceeding any of the p-groups capacity and when i desulfate i bring capacity from down around the 3.2v area back upto the 4.2v area.
all the data is here on ES to back what im saying.

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Re: BMS destroyed my battery?!

Post by fatty » Apr 04 2021 3:02pm

john61ct wrote:
Apr 03 2021 11:11pm
Truth, and another one many do not grok
Well, I should have cited you :wink:
john61ct wrote:
Apr 03 2021 11:11pm
> If there's little capacity between 4.15-4.2V, then there is necessarily little capacity variance, and thus little imbalance that occurs between these points.

That does not follow.
There's under 10% capacity between 4.2 to 4.1V. That's much smaller than the larger capacity between 4.1 to 4.0V. So if you top-balance at 4.2V and discharge to 4.1V, the cells will necessarily be closer than if you top-balance at 4.1V and discharge to 4.0V. Not practically relevant, just refuting the claim that top-balancing at 4.2V would "throw the pack out of balance".
john61ct wrote:
Apr 03 2021 11:11pm
if you just leave the cells "middle balanced" as they come from the factory
never bottom balance nor top balance
implement both LVC and HVC at the per-cell level so triggered by the weakest links (lowest capacity)
with setpoints closer to the center voltage/SoC range rather than out past the voltage shoulders at the extreme edges
then the pack may **never** need balancing.
And longevity will be greatly extended.
Sure, middle-balancing minimizes how far out of balance the pack gets at each end of SoC, and by setting LVC and HVC by the lowest capacity cell, the other cells will necessarily be kept in a healthier range, since only the lowest capacity cell will experience those LV and HV extremes. But is this meaningfully advantageous at the pack level? You're just hitting the weakest cell the hardest at every charge and discharge. The other cells will live longer, but pack life is only as good as the weakest link, so I think the actual useful pack life might actually be shorter.

Cell capacities will also drift over time, necessitating balancing, though it may be less frequent.
john61ct wrote:
Apr 03 2021 11:11pm
By pushing your charge voltage out past 4.05 - 4.10V
and top balancing out there
you are indeed **causing** the imbalances that now require frequent "fixing"
and reducing longevity.
I'm not sure about this. If you mean charging up to 4.2V reduces longevity first, then I understand and agree, since that loss of capacity will not be uniform and the cells will be increasingly mismatched.

And I guess if you artificially constrain capacity, then capacity variance in absolute mAh will be reduced: say a 2s pack of 8Ah cells with 1% capacity variance have capacities of 8039 and 7973mAh, delta 66mAh. Charging to 4.1V (95%) gives 7637mAh and 7574mAh, delta 63mAh. But really, this argument would be equivalent to saying that smaller cells experience less imbalance, which in practical application is specious, since smaller cells necessarily get hit harder.

I don't see how termination voltage alone (independent of capacity) can directly cause imbalance -- indeed, this disagrees with our "balance at one point" concept.
john61ct wrote:
Apr 03 2021 11:11pm
If you want to maximize range, you should bottom balance, but going down to a lower LVC drastically shortens lifespan, much more than charging too high
I also don't follow this. If we agree that the high and low "shoulders" (the first and last 5-10% of capacity, respectively) are equally harmful to lifespan (and thus balance), then capacity is capacity and it shouldn't matter if you top-balance and use LVC on the weakest cell, or bottom-balance and use HVC on the weakest cell. The capacity would be equivalent.

However, top-balance would give a higher average voltage and thus higher pack Wh than bottom-balancing, and so I would expect that top-balancing would actually maximize range, not bottom-balancing.
This is why I advocate for top-balance node charging, where every cell is brought to top-balance -- not just the weakest cell.

This is an excellent discussion.

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Re: BMS destroyed my battery?!

Post by fatty » Apr 04 2021 3:12pm

goatman wrote:
Apr 03 2021 11:23pm
all the data is here on ES to back what im saying.
Yeah, I skimmed those thread previously, but total energy was the same, and results otherwise were within an expected margin of error, so I think the conclusion of resuscitating "dead" cells is spurious.

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Re: BMS destroyed my battery?!

Post by goatman » Apr 04 2021 5:19pm

fatty wrote:
Apr 04 2021 3:12pm
goatman wrote:
Apr 03 2021 11:23pm
all the data is here on ES to back what im saying.
Yeah, I skimmed those thread previously, but total energy was the same, and results otherwise were within an expected margin of error, so I think the conclusion of resuscitating "dead" cells is spurious.
actually what im looking at is whether to do pack maintenance every 6 months will actually keep the cells from degrading as quick

if you buy a new car
theres a warranty
if you dont change your fluids and filters and wind up blowing your motor at 40,000 miles then thats on you
no warranty coverage
so lets say every 100 cycles you do the capacity test/desulfate the pack
its going to extend the life of the pack

youll see that in the 30Q and 40T tests

take the Luna Wolf Pack for example
they dont believe in Bluetooth
https://lunacycle.com/52v-luna-direwolf-ebike-battery/
[quoteWIRELESS PACK
This is a truly wireless pack, and we do not mean Bluetooth (we believe Bluetooth communicating batteries are unnecessary and will lead to reliability issues).][/quote]

they have a fully potted pack that cant be maintained
if they had a bluetooth bms like the Ant bms
you could set the lvc at 2.5v
program balancing to start at 2.8v
hook up a load tester and the bms will turn off when it reaches 2.5v
let the bms bottom balance the cells, charge it back up and your done

instead you wind up with a paper weight

maarten almighty is using salvaged cells
his question was
BMS destroyed my battery????

i think charging to 4.2v killed it
he could build another pack with the remaining cells to find out

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Re: BMS destroyed my battery?!

Post by maarten_almighty » Apr 04 2021 5:26pm

Thank you to everyone for their input/replies, highly appreciated!

@ Goatman
Regarding charging to 4.2 volt. I agree this is not good practice since it reduces cycle life of the cells ,however I highly doubt that in my circumstances (only 5 charge/discharge cycles after having built the pack within the timespan of approx 1 month) that the charging to 4.2 volt would cause almost half the groups in my pack to die. If it would have happened over the span of many more cycles (for example 100) I could see that charging to a too high voltage could be the issue. I am indeed using recycled cells which have already endured many cycles and might have been quite close to their end of life, however the scenario that 7/20 packs died within 5 cycles seems like something with a low probability (if it were 1 or perhaps 2 packs that would have died I would accept that perhaps those packs were near end of life but with 7 it seems unlikely to me, hence my initial conclusion regarding the bms having caused the issue.(by discharging those packs too far)

If charging to 4.2 volt were the issue, wouldn't it be unlikely only 7 out of 20 parallel groups failed?

Please correct me if I'm wrong. Thank you :thumb:

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Re: BMS destroyed my battery?!

Post by fatty » Apr 04 2021 8:26pm

maarten_almighty wrote:
Apr 04 2021 5:26pm
@ Goatman
Regarding charging to 4.2 volt. I agree this is not good practice since it reduces cycle life of the cells ,however I highly doubt that in my circumstances (only 5 charge/discharge cycles after having built the pack within the timespan of approx 1 month) that the charging to 4.2 volt would cause almost half the groups in my pack to die. If it would have happened over the span of many more cycles (for example 100) I could see that charging to a too high voltage could be the issue. I am indeed using recycled cells which have already endured many cycles and might have been quite close to their end of life, however the scenario that 7/20 packs died within 5 cycles seems like something with a low probability (if it were 1 or perhaps 2 packs that would have died I would accept that perhaps those packs were near end of life but with 7 it seems unlikely to me, hence my initial conclusion regarding the bms having caused the issue.(by discharging those packs too far)

If charging to 4.2 volt were the issue, wouldn't it be unlikely only 7 out of 20 parallel groups failed?
Charging to 4.2V certainly did not kill those groups.

The BMS certainly could have. The Battery Tech forum is littered with threads where this occurs, to the point that we refer to cheap Chinese BMS as "Battery Murdering System"

However, dead cells could have also killed those groups. You're using "second hand" cells that you didn't test individually. All it would take is one bad cell to kill each p-group, so just 7 cells out of 340 --a failure rate of just 2%-- could have killed your pack. 2% failure rate is actually conservative for untested reclaimed cells -- published anecdotes are usually much higher. That's why reclaimed cells must be tested individually before assembly into a pack.

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Re: BMS destroyed my battery?!

Post by john61ct » Apr 05 2021 2:01am

fatty wrote:if you top-balance at 4.2V and discharge to 4.1V, the cells will necessarily be closer than if you top-balance at 4.1V and discharge to 4.0V. Not practically relevant, just refuting the claim that top-balancing at 4.2V would "throw the pack out of balance"
But it does do that!

In my case irrelevant, since I would not want the packs to ever get that high anyway.

> Sure, middle-balancing minimizes how far out of balance the pack gets at each end of SoC

By avoiding both shoulder areas, you may never see any significantly imbalanced state at all. . .

> by setting LVC and HVC by the lowest capacity cell, the other cells will necessarily be kept in a healthier range, since only the lowest capacity cell will experience those LV and HV extremes

The goal afaic is no cells approach any such extremes.

> But is this meaningfully advantageous at the pack level? You're just hitting the weakest cell the hardest at every charge and discharge.

That inevitably happens no matter what, in the sense of "hardest" meaning compared to every other cell.

So, the healthiest strategy for every cell, is to make sure to coddle the weakest links; that ensures the rest are coddled too.

Just giving up (another) 10% capacity utilization can triple cycle lifespan or more.

> Cell capacities will also drift over time, necessitating balancing, though it may be less frequent.

Since the weakest cells are under greatest stress, they wear fastest, only QA anomalies would result in any "switching places."


> And I guess if you artificially constrain capacity, then capacity variance in absolute mAh will be reduced: say a 2s pack of 8Ah cells with 1% capacity variance have capacities of 8039 and 7973mAh, delta 66mAh. Charging to 4.1V (95%) gives 7637mAh and 7574mAh, delta 63mAh.

Not "artificial", capacity and SoC% are not objective attributes, tgey are **defined** arbitrarily by the test parameters selected.

A data sheet saying a cell has 20Ah is meaningless without the company's definition of Full and Empty

if those parameters are followed for every cycle in normal daily usage, the user will need to replace the pack 5x or 10x more frequently - that is not a coincidence!

By defining capacity more realistically, using **your** gentler / healthier definitions of Full and Empty to achieve decent longevity and thus value for money

you aren't really "sacrificing capacity", you are just acknowledging what the realistic capacity **actually is** , there is no reason to acknowledge the datasheet values as anything other than marketing-driven fantasies - those are what is "artificial"!!


john61ct wrote:
Apr 03 2021 11:11pm
By pushing your charge voltage out past 4.05 - 4.10V and top balancing out there you are indeed **causing** the imbalances that now require frequent "fixing" and reducing longevity.
> I'm not sure about this. If you mean charging up to 4.2V reduces longevity first, then I understand and agree, since that loss of capacity will not be uniform and the cells will be increasingly mismatched.



> I don't see how termination voltage alone (independent of capacity) can directly cause imbalance

Imbalances never revealed, in effect do not exist functionally.

john61ct wrote:
Apr 03 2021 11:11pm
If you want to maximize range, you should bottom balance, but going down to a lower LVC drastically shortens lifespan, much more than charging too high
> I also don't follow this. If we agree that the high and low "shoulders" (the first and last 5-10% of capacity, respectively) are equally harmful to lifespan (and thus balance), then capacity is capacity and it shouldn't matter if you top-balance and use LVC on the weakest cell, or bottom-balance and use HVC on the weakest cell. The capacity would be equivalent.

No, the avoidance of the top shoulder is a much smaller range and much less impact tgan at the bottom.

Bottom balancing only is better for range when using **pack level** LVC, because you have greater peace of mind and don't need to be as conservative to prevent immediate gross damage.

Different from lifespan optimization, which requires a judgment call, giving up significant range, pack-level vs cell-level implementation is irrelevant.


> top-balance would give a higher average voltage and thus higher pack Wh than bottom-balancing, and so I would expect that top-balancing would actually maximize range, not bottom-balancing.

Wh are not nearly as accurate for battery capacity discussions, Ah match up with actual observed behaviour.

And "higher average" voltage is irrelevant, tge whole point is, voltage deltas represent much lower energy deltas once you go past 4.0V for EV chemistries, or 3.3V for LFP.

A very high % if the coulomb flow is not getting stored, just increasing useless even harmful chemical activity and producing waste heat.

That is why the standard for measuring energy deltas is not voltage nor charging Ah, but **discharged** Ah from an at-rest voltage down to an LVC

at a very slow current rate to give a bigger number for the nameplate and data sheet.

The SoC point at which you balance does not affect pack capacity at all, if HVC and LVC are measured at the per-cell level.

john61ct   100 GW

100 GW
Posts: 7675
Joined: Dec 18 2018 2:06pm

Re: BMS destroyed my battery?!

Post by john61ct » Apr 05 2021 2:11am


goatman wrote:if they had a bluetooth bms like the Ant bms
you could set the lvc at 2.5v
program balancing to start at 2.8v
I've never heard of a protective BMS that bottom balances.

Only dedicated balancers ( marketed as "non-protective BMS") or node charging can middle or bottom balance afaik

> i think charging to 4.2v killed it

No way, that has a relatively minor impact on longevity.

Using second-hand cells is inherently risky even if only 1P

Given that many will be past EoL, putting them in parallel groups astronomically increases the odds of quick pack failure

even with rigorous testing of each cell individually and discarding the majority.

If you want a reliable pack you have to buy a known good cell model

new, Grade A from a reliable source

maarten_almighty   100 mW

100 mW
Posts: 47
Joined: Jan 10 2020 7:41pm

Re: BMS destroyed my battery?!

Post by maarten_almighty » Apr 05 2021 4:07am

@fatty sorry if I didn't make myself clear earlier:
I did in fact test all individual cells in the pack for capacity, however I did not test them for internal resistance.

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