Author Topic: Battery balancing - big cells  (Read 1890 times)

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Online mtdoc

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Re: Battery balancing - big cells
« Reply #25 on: April 23, 2018, 04:34:36 am »
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Bottom balancing has become the method of choice. You can read more about the rationale and concept HERE and HERE.

I was intrigued to see what 'bottom balancing' is.

Turns out that the guy is destroying cells because his system is NOT balancing, ie trying to keep all the cells in a series stack all at the same voltage (ie all at the same SOC) and therefore some cells would get reverse biased and be destroyed.

Your are not understanding the concept of bottom balancing correctly. It does not mean trying to keep all the cells at the same voltage.   The whole point of bottom balancing is to not try to keep the cells at the same voltage. They are only at the same voltage once - that is a the "bottom".

He is not "destroying cells". In fact the opposite. When he penned that article (2009) the concept of bottom balancing was new.  It has since become widely adopted and found by many to be the best way to avoid destroying cells. You can easily do some research online of its use by DIY EV and home RE users to confirm this.  The second link I provided is a start.

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Firstly, all cells in a large series stack should be monitored and the pack shut down if any cell reaches a minimum cell voltage. Yes, you do not get the max capacity but it stops cells from being destroyed.
  With bottom balancing, you also still have a low voltage cut-off employed to prevent any cell from reaching a low SOC failure point.

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His system is using the full capacity range of the cells. In EV systems, this is unusual because EV application normally require a minimum sevice life. The main strategy to acheive long life is to limit the max and min SOC to 20% for min and 80% for max. By doing this, again it avoids bringing cells down to the point where they get reverse biased.

No he is not using the "full capacity"  He is simply defining 0% SOC as the point were the bottom balance is. You are not understanding how the concept of bottom balancing is employed. In fact it is used in a way that makes a more conservative used of the middle part of a cells capacity - specifically to ensure longer life.  In the case of home RE applications, after the cells are balanced at the "bottom" the systems charge controller utilizes it's charge voltage setting and the inverters low voltage cut-off settings to ensure this.  Ideally one utilizes a current-shunt to monitor AH in and out as well.

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Bottom balancing is bring all the cells in a series stack to the voltage that is 0% SOC
Not really that simple,  though I can understand why you would think that based on the terminology used by the articles I linked. One can choose any voltage one wants to bottom balance at.  The manufacturer's recommended "max discharge voltage" is sometimes chosen - (e.g. 2.5V for Calb LiFePO4 cells), though a more conservative approach is to chose a voltage a few tenths above that. After that bottom balance is done and the battery bank charged, the inverter's low voltage cut-off is set to ensure that cell voltages and pack SOC never drop below whatever level you desire - generally well above 0% SOC. 20& SOC would be typical.
« Last Edit: April 23, 2018, 05:15:52 am by mtdoc »
 

Online nctnico

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Re: Battery balancing - big cells
« Reply #26 on: April 23, 2018, 05:16:37 am »
It is the experience of many eBike and EV builders that charging Lithium batteries below freezing temperature causes irreversible damage. I personally damaged my eBike LiFePO4 pack that way.  This does not happen to lead acid batteries.
A BMS of any Li-ion battery pack should prevent charging the pack at too low temperatures. One of the problems however is that most of these packs are made in China and have low quality when it comes to the BMS functions.

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The fact -based on many decades of experience - is that deep cycle flooded lead acid batteries can withstand abuse in both extremes of temperature and charging/discharging without suffering irreversible damage. Period. Full stop. The same simply cannot be said of lithium batteries. Why else all the concern about cell monitoring, balancing, and use of BMS for lithium and not for LA?
Lead-Acid batteries need some care during charge and discharge. If you get the charging and discharging right then an LA battery will last very long and don't get problems due to sulphation.
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Offline MosherIV

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Re: Battery balancing - big cells
« Reply #27 on: April 23, 2018, 05:48:21 am »
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  Your are not understanding the concept of bottom balancing correctly.   

I think I understand what is meant by 'bottom balancing'.
I am just pointing out that the circumstances that drove the inventor to come up with the idea are dubious.

Basically, it sounds like he was driving around an EV without any BMS to protect the cells.
 

Online mtdoc

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Re: Battery balancing - big cells
« Reply #28 on: April 23, 2018, 05:49:47 am »
It is the experience of many eBike and EV builders that charging Lithium batteries below freezing temperature causes irreversible damage. I personally damaged my eBike LiFePO4 pack that way.  This does not happen to lead acid batteries.
A BMS of any Li-ion battery pack should prevent charging the pack at too low temperatures. One of the problems however is that most of these packs are made in China and have low quality when it comes to the BMS functions.

Yes, I agree, they should.  The problems, as we all know, is  that electronics are subject to failure and poor design.  LA batteries require no such protection.

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The fact -based on many decades of experience - is that deep cycle flooded lead acid batteries can withstand abuse in both extremes of temperature and charging/discharging without suffering irreversible damage. Period. Full stop. The same simply cannot be said of lithium batteries. Why else all the concern about cell monitoring, balancing, and use of BMS for lithium and not for LA?
Lead-Acid batteries need some care during charge and discharge. If you get the charging and discharging right then an LA battery will last very long and don't get problems due to sulphation.

Yes.  The main problem that LA suffers from is sulphation if they are left to sit at a low SOC or are not regularly brought up to 100% SOC and "equalized".  I think it's that historic "top balancing" approach used in LA that causes so many problems with people trying to implement a similar approach with lithium batteries.

In contrast one of the great benefits of lithium is the ability to let them sit at middle of the SOC curve for indefinite periods without damage.
 

Online mtdoc

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Re: Battery balancing - big cells
« Reply #29 on: April 23, 2018, 05:54:42 am »
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  Your are not understanding the concept of bottom balancing correctly.   

I think I understand what is meant by 'bottom balancing'.
I am just pointing out that the circumstances that drove the inventor to come up with the idea are dubious.

OK, I read your post to imply that your were blaming bottom balancing. I see that you were referring to his description of the failures that led him to change to a bottom balancing method.

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Basically, it sounds like he was driving around an EV without any BMS to protect the cells.

Not exactly. If I understand correctly, he was using the typical current shunting type of BMS employed by many.
 

Offline KL27x

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Re: Battery balancing - big cells
« Reply #30 on: April 23, 2018, 06:14:13 pm »
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The good news is that it turns out that contrary to conventional wisdom,  for these system a BMS is not needed and may actually make things more prone to failure. The answer is to bottom balance the cells.

Uhmm. Read the first link. I don't get it.

Correct me if I'm wrong.
Bottom balancing has become the method of choice. You can read more about the rationale and concept HERE and HERE.


Guy balances cells at the top with a conventional BMS. Claims this kills his cells because the weaker cells run low, first. And were thusly being overdischarged. (What!?)

Guy balances cells at the bottom and lets them be unbalanced at the top and claims eureka. Problem solved. (What?!)

What the heck am I missing?

If you monitor each cell and simply shut down the battery before that weakest cell reaches the critical lower limit, problem solved. By balancing the cells at the top (fully charging them), yeah, you can't run the stronger cells all the way down to 3.0V (or w/e the cutoff is). But you will get the exact same amp hour capacity (the capacity of the weakest cell) and a HIGHER energy capacity, since your cumulative series voltage is going to be slightly higher than when balancing them at the bottom. If there's a good reason to balance them at the bottom, the linked page doesn't provide the first clue.

Sound like this is a fairly intelligent car enthusiast with not so good grasp of the problem.

When I read the emphatic posts that BMS is necessary for safety and/or efficiency of high S cells, I was expecting some fancy system of taking power from good cells and giving it to lower voltage cells (or perhaps switching out the dead cell and running the battery at (X-1)S output, where applicable) so that the battery could run beyond the amp hour capacity of the weakest cell. This bottom balancing solution is just bull shit. It doesn't provide any supporting evidence that balancing is a must for safety and longevity reasons; technically, it is individual cell monitoring and shutdowns on both high and low end which is imperative. Top end balancing does qualify as individual cell monitoring on the top end.... bottom balancing should qualify as a way to handle just the lower end... because you HAVE to monitor individual cell voltage in order to balance cells, but you do not HAVE to balance cells to prevent damage to the battery. Perhaps balancing is just the wrong word to use, here.

Short: you have to monitor cell voltage at both top and bottom of the range. It can be helpful to balance the cells in order to guarantee maximum battery capacity (capacity of the weakest cell). Balancing cells at the top doesn't mean you set your low end cutoff voltage of the battery to the min for a single cell times S anymore than you could balance them at the bottom and then blindly charge the battery to 4.2V*S. 
« Last Edit: April 23, 2018, 07:31:02 pm by KL27x »
 

Offline paulca

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Re: Battery balancing - big cells
« Reply #31 on: April 23, 2018, 07:40:42 pm »
That's about balancing. Then, even the cell-level monitoring is not actually a hard requirement; definitely not a generic "must". Companies such as BOSCH do not do it on many of their 6s and smaller li-ion packs.

While I doubt this very much it might come down to terminology.

You MUST balance a lithium pack in series in order to charge it safely.  You can do that with individual cell protection with resistive dischargers per cell, but this will limit you charging current.

If you don't balance it during charge then when it reaches 12.6V for a 3S on the charger you might have cell voltages like 5V, 4V, 3.6V and if you are pushing high current into it, you will have a potential explosion.

Usually however laptops and power tool packs just let the cells unbalance, then monitor the peak cell voltage and end the charge when one hits the 4.2V.  This causes the pack to degrade over time and have a relatively short life span.  Usually you will find that the pack becomes substandard and fails while 5 out of the 6 cells are fine, but without proper balancing the pack is dead.  This is why they are so good for recycling into power walls.

I would not consider most commercial BMS boards to be much more than bare minimum to prevent fire, but are in no way the best way to manage the battery, usually the opposite.
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Offline MosherIV

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Re: Battery balancing - big cells
« Reply #32 on: April 24, 2018, 04:57:53 am »
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What the heck am I missing? 
Nothing, I came to the same conclusion.

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I was expecting some fancy system of taking power from good cells and giving it to lower voltage cells
This is known as Active Balancing.
It has been tried, but the losses in the transfer circuitry and the complexity of the circuitry made it not worth while.

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balancing is a must for safety and longevity reasons;
Balancing is not a safety issue.
Balancing (of cell voltage) is a good strategy to improve longevity.
Balancing during the usage phase is basically matching the discharge rate of stronger cells to the weak ones.
As I said before, it is best to start with closely matched cells. However, cells will age at different rates, so you need some way to catter for this.
As you pointed out, shutting off the pack when the weakest cell reaches min V is one way to do it.


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You MUST balance a lithium pack in series in order to charge it safely.
Yes.
What some people may not be aware of here is that for systems that have high S do not usually charge cell individually, they charge the stack in series. When a cell in the stack reaches man V, charging MUST be stopped. So yes, high S packs must be balanced during charging. Or be shut down when a cell reaches max V and the stack remians unbalanced, and gets more unbalanced over time.

 

Online nctnico

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Re: Battery balancing - big cells
« Reply #33 on: April 24, 2018, 06:06:15 am »
What some people may not be aware of here is that for systems that have high S do not usually charge cell individually, they charge the stack in series. When a cell in the stack reaches man V, charging MUST be stopped. So yes, high S packs must be balanced during charging. Or be shut down when a cell reaches max V and the stack remians unbalanced, and gets more unbalanced over time.
Agreed. And being involved in battery packs myself I think that balancing a fully charged cell is better than a nearly discharged cell. One of the problems is that it is much more unlikely a pack gets discharged enough to do the balancing at all. Also the voltage of a cell (usually a group of cells in parallel) may drop below the shutdown voltage which means the BMS has to shut down and cannot complete the balancing operation. On the other hand most packs will be fully charged when they are charged so basically the balancing will happen for each charge/discharge cycle.
Ofcourse the BMS has to shut down charging and discharging when the cell voltage limits are exceeded (usually these are not the absolute minimum and maximum voltage for the cells but have some margin to prolong battery life).
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline KL27x

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Re: Battery balancing - big cells
« Reply #34 on: April 24, 2018, 06:40:19 am »
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You MUST balance a lithium pack in series in order to charge it safely.  You can do that with individual cell protection with resistive dischargers per cell, but this will limit you charging current.
This is where terminology comes in. I disagree that a battery must be balanced for safety reasons. As long as you stop charge/discharge when the FIRST cell out of the series reaches the limit, there is no safety issue. There MIGHT be a capacity issue, IF the weakest cell is not getting fully charged, or if the weakest cell is not reaching the lower threshold, first. The former is the most likely problem, IME, which will reduce the capacity of the battery. A weak cell can be a little leaky to where it does not fully charge by the time the rest of the pack gets there. This is the big reason you want to balance them at the top, aside from the slightly higher energy density. On the flip side, the weak cell is pretty much always going to hit the lower threshold, first. It is almost impossible for the pack to get unbalanced in a way that this does not occur. So balancing them at the bottom is simply a bad strategy, AFAIC.

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What some people may not be aware of here is that for systems that have high S do not usually charge cell individually, they charge the stack in series.
What you do not understand is that in order to balance cells, individual cell voltage must be monitored, somehow. The resistive bleeders that kick in at X voltage (PER INDIVIDUAL CELL) is a form of individual cell voltage monitoring, IMO. Those bleed resistors kick in at a specific voltage. Whether you kick in the bleeder resistor or you stop the charging when this happens (the latter is technically safer, or at least it is simpler/easier to implement in a way that will always be safe), you have some kind of voltage detection on individual (stacks of parallel connected) cells, in either case, whether you are balancing the cells or not.

I reiterate. Inidividual cell voltage monitoring is what is necessary for safe charge/discharge. It is necessary at both the top and the bottom. Cell balancing is for maximizing the capacity of the battery, and it defacto requires individual cell voltage monitoring. And it's best done at the top.
« Last Edit: April 24, 2018, 07:22:26 am by KL27x »
 

Online nctnico

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Re: Battery balancing - big cells
« Reply #35 on: April 24, 2018, 07:03:14 am »
Sidenote: Texas Instruments has a whole range of cell monitoring / balancing chips which do all the work for you based on field proven algorithms. There is not much sense in trying to roll your own balancing system. TI is putting a lot of research into these chips but you have to configure them properly to match the cell specs ofcourse.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline C

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Re: Battery balancing - big cells
« Reply #36 on: April 24, 2018, 07:19:04 am »

I think that some are missing that battery voltage is not equal to charge.

A battery is not a simple thing.
There is a internal series resistance that must be taken into account.

Any current in or out of battery changes voltage you can monitor.

You have a choice of allowing for Max-Min resistance change in measured voltage
Or
Measuring internal resistance.

To compute internal resistance requires measuring Current & Voltage over a short time with different current load.
The internal battery charge will change little over short time, so internal voltage will change little.

But note that many things can change internal resistance.

c
 

Offline KL27x

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Re: Battery balancing - big cells
« Reply #37 on: April 24, 2018, 07:25:59 am »
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I think that some are missing that battery voltage is not equal to charge.
Not exactly. But cell voltage of a Li ion battery is a function of charge, and the amount of stored charge is a function of voltage. There is no case where a given cell can have a higher stored charge and a lower voltage. As the voltage goes up, the stored charge goes up. And vice versa, as the stored charge goes up, the voltage goes up. Period. Li ion ARE very simple compared to NiMh, where this is not the case.
« Last Edit: April 24, 2018, 07:28:23 am by KL27x »
 

Online nctnico

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Re: Battery balancing - big cells
« Reply #38 on: April 24, 2018, 07:41:49 am »
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I think that some are missing that battery voltage is not equal to charge.
Not exactly. But cell voltage of a Li ion battery is a function of charge, and the amount of stored charge is a function of voltage.
Definitely not. You can't read the charge from the voltage of a Li-ion battery. There is a huge section of the charge/discharge curve which is flat. This is the reason why you need a coulomb counter calibrated for the specific chemistry to determine the charge in a battery pack. The only thing the voltage indicates is whether you can charge or discharge a Li-ion battery any further.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline KL27x

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Re: Battery balancing - big cells
« Reply #39 on: April 24, 2018, 07:49:26 am »
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Definitely not. You can't read the charge from the voltage of a Li-ion battery. There is a huge section of the charge/discharge curve which is flat. This is the reason why you need a coulomb counter calibrated for the specific chemistry to determine the charge in a battery pack. The only thing the voltage indicates is whether you can charge or discharge a Li-ion battery any further.

Not completely flat. I maintain my statement is accurate. A hyperbola approaches flat, but it's still a function no matter how ridiculously flat it gets. Is it practical to measure the Y axis and to calculate the X? Not necessarily, but that doesn't mean it's not a function. The Fvd on a diode is a function of current, even though for many purposes we can practically consider it static (or flat).

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Any current in or out of battery changes voltage you can monitor.

You have a choice of allowing for Max-Min resistance change in measured voltage
Or
Measuring internal resistance.

To compute internal resistance requires measuring Current & Voltage over a short time with different current load.
The internal battery charge will change little over short time, so internal voltage will change little.
You have not thought this all the way through, C. This a red herring. Computing the internal resistance allows you to calculate the energy available at a given load. But it is not necessary to include that into cutoff limits. On the low end, the cell with higher internal resistance will sag in output more than the others. And you will still safely cut off the output when it reaches the lower limit. Maybe it cuts out a little earlier than it could have, but at least it's not a safety issue. On the charging top end, the current is very low and not a factor in fully charging a cell. It reached the proper voltage or it does not. It also is not a safety issue. On a battery charger it is a nice feature to have. It is nice to know. In practice, when the battery doesn't last long as it needs to, this is all you need to know and you will replace the high internal resistance cells. Increased internal resistance is one of the main things that goes south in properly used cells. In very low current applications compared to battery capacity, the drop is going to be neglible, and the battery works fine and doesn't drop significantly in capacity due to cutout; so you never see the problem. In higher current:C application, this problem largely takes care of itself, as stated, but you could include compensation if you want to run the batteries to the ragged edge (provided the decreased output voltage under load is not the limiting factor). Well, I suppose this brings temp sensors into light. Higher internal resistance also increases the battery temp of that cell under given draw. Yes, temp sensors are part and parcel if you are talking about large batteries and large currents and duty cycles.
« Last Edit: April 24, 2018, 09:01:03 am by KL27x »
 

Online mtdoc

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Re: Battery balancing - big cells
« Reply #40 on: April 24, 2018, 09:30:20 am »
What is missing in the above discussion is that the OP was asking about balancing very big lithium cells (most certainly LiFePO4). Many here are extrapolating their experience with small Lipo packs.

Of course a high quality BMS which monitors individual cell voltages at the top and bottom can be a good thing - as long as it is reliable.

It is not easy to purchase or build an adequate, safe, reliable and effective BMS for a 20 kWh bank of prismatic LiFePO4 cells.

What many in the DIY EV and RE world have found from first hand experience with large, expensive LiFePO4 banks is that bottom balancing alone is sufficient and probably better than the available BMS options.  These are people who have spent many thousands of dollars on their battery packs. IOW they have skin in the game and are not simply opining on an internet forum.

When the idea of bottom balancing large LiFePO4 packs was first thrown around on the internet 9 years ago, I was skeptical too - as I believe were most people who follow this stuff in the EV and RE world were.  But after following the debate for years and watching as people whose opinion I trust shared their experience and seeing that this is now replacing the conventional wisdom for these applications, I'm convinced.  Many people have destroyed their expensive LiFePO4 battery banks relying on traditional top balancing methods.  I have yet to hear of anyone not employing the bottom balance method successfully. Of course it does take more knowledge and understanding by the end user than the brainless "plug and play" BMS approach that most consumers expect.

Feel free to be skeptical -but before writing the idea off and extrapolating your experience with small Lipo packs (i.e. not very large LiFePO4 battery banks) spend more than 5 minutes reading one guys early take on it. 

And if you have a better concrete answer for the OP as to were to find a high quality, affordable BMS system for very large cells, please share.  The OP mentioned 58Ah cells which are almost certainly LiFePO4 prismatic cells.
 

Offline KL27x

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Re: Battery balancing - big cells
« Reply #41 on: April 24, 2018, 12:09:00 pm »
^This is the take you linked, dude.

If it is better, this guy didn't give any clue why it is better to balance at the bottom, nor how you even do it. When the lowest cell hits the floor, you cutoff the battery and you drain the others? Then you charge until the first cell hits the max voltage and stop charging?

You say experience and investment and blah. But I don't care how much money someone want so spend and burn. If they can't explain what/why/how, and they are the dudes linked to for information, this is the reason people are burning up batteries. There is a clear lack of understanding compounded by dumbasses.

In every field there are dumbasses who are deemed experts. They learn one way to do something, lacking clear grasp of physics and science and engineering and abstract thought. And they create a new language based around faulty conclusions and incomplete understanding of the big picture.

Being the first person to stumble backwards onto a practical working solution or improvement through thousands of dollars of trial and error doesn't make you not an idiot. Accepting incorrect explanations and faulty terminology just ensures that people continue to blow up batteries.

Creating new language and vague explanations for things we already have words for is the hallmark of the charlatanism that accompanies every major industry. We don't need to accept that, here.
« Last Edit: April 24, 2018, 12:30:46 pm by KL27x »
 

Online mtdoc

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Re: Battery balancing - big cells
« Reply #42 on: April 24, 2018, 12:39:43 pm »
So you have no solution the OP, then?.  And you have no experience with daily cycling of large LiFePO4 battery banks? 

Hmm, who’s opinion to trust, yours or the many people, including several EEs who do have that experience over several years.?

It’s always revealing when someone exhibits such strong emotional responses to something new to them.

There is a clear lack of understanding compounded by dumbasses.

In every field there are dumbasses who are deemed experts.

Or deem themselves experts.  Yes, clearly true.
 

Offline KL27x

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Re: Battery balancing - big cells
« Reply #43 on: April 24, 2018, 12:43:37 pm »
Your link explains nothing. Explanations that are incomplete.

2+ bangledesh = chocolate chip icecream.

We are supposed to accept this as the holy grail of battery management.

I'm not an expert at using batteries. And no one has to be an expert in anything to understand this link is complete bull shit. W/e bottom balancing is, it might be sliced bread, but this web page is bull shit.
 

Online mtdoc

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Re: Battery balancing - big cells
« Reply #44 on: April 24, 2018, 12:45:47 pm »
You can't read the charge from the voltage of a Li-ion battery. There is a huge section of the charge/discharge curve which is flat. This is the reason why you need a coulomb counter calibrated for the specific chemistry to determine the charge in a battery pack. The only thing the voltage indicates is whether you can charge or discharge a Li-ion battery any further.

Yes, absolutely right. I believe this is part of the reason why traditional “ top balancing” of series strings of large LiFePO4 so often fails.
 

Online mtdoc

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Re: Battery balancing - big cells
« Reply #45 on: April 24, 2018, 01:02:41 pm »
Your link explains nothing. Explanations that are incomplete.

Why the obsession with that one link? I never claimed it was the be all end all of the bottom balancing concept. I only posted it because it was the first one that I saw in 2009 that introduced the concept. It has since been flushed our as a valid method of cycling large LiFePO4 battery banks (where no good BMS solution exists). As I’ve said,  there are now many people (some EEs) who have been successfully using this method over several years now. I know one personally and have seen his system in action.

This method has proven sound. If it offends your delicate lithium battery care sensibility, fine. Why the rage?

You seem to admit you have no experience with it or other methods of daily cycling of large LiFePO4 battery banks. What solution do you have to offer the OP?
 

Offline KL27x

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Re: Battery balancing - big cells
« Reply #46 on: April 24, 2018, 01:06:07 pm »
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It’s always revealing when someone exhibits such strong emotional responses to something new to them.
It's almost as revealing as people linking webpages that make no sense but look sorta official.

This web page is nothing but misinformation. It takes something which is actually very simple to discuss and understand and explain. Yet it manages to obfuscate the subject while hinting at enlightenment.

I've said it before. There is a reason we watch li ion batteries explode in the news, so much. It's the same reason there's so much intentional misinformation and anti-education. It's because batteries are a billion dollar a year industry, and the less we know the more they make.

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Why the rage?
The rage is at you, specifically, repeatedly stating things without backing them up. You have used this link as your support, but this emporer is wearing no clothes. Please explain what the hell you are claiming as facts in things called words, and we will gladly listen.

I fully respect your right to feed your family. You have disclosed your work in the industry. If you are a knowing shill, I respect that. If you're an unwitting shill, I feel sorry for you. There are many people on this forum who understand how batteries work, and your attempt at obfuscation is offensive. Can you explain this bottom balancing thing, technically? Or do you just sell something and point to experience and money invested as why I should buy it? What department do you work in, anyway? In what capacity are you involved in designing BMS? Why you talk around it instead of talking about it?
« Last Edit: April 24, 2018, 01:29:56 pm by KL27x »
 

Online mtdoc

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Re: Battery balancing - big cells
« Reply #47 on: April 24, 2018, 01:54:04 pm »
The rage is at you, specifically, repeatedly stating things without backing them up.

Example? I suggested a solution for the OPs question with links to others who agree. That’s all.  You’ve offered the OP nothing (while backing up nothing you’ve said).

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You have used this link as your support, but this emporer is wearing no clothes.
Nonsense. It was one of two links i offered to explain the concept of bottom balancing. It was not meant to prove anything.

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Please explain what the hell you are claiming as facts
The only “fact” that I’ve claimed is the fact that many people have been successfully using bottom balancing to cycle and maintain large LiFePO4 battery banks.  A simple internet search will confirm that.

You’ve offerred only your opinion about something you'd never heard of until this thread.

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You have disclosed your work in the industry.
  :wtf:  Many on this forum know my profession and it has nothing to do with this topic.  I do have several years experience maintaining large LA battery banks for solar PV systems (as a non-professional) and LiFePO4 for ebikes.  I am only an electronics hobbyist with a strong  interest in Home RE systems, ebikes and EVs.  I’ve never claimed otherwise. You need to get a grip..

Once again since you seem to have completely misread my posts:

I suggested to the OP that a BMS system is not required to maintain large LiFePO4 battery bank - that there was a method called bottom balancing that many are using successfully. (Easily verified)

There is nothing inherently wrong with BMS for large capacity LiFePO4 battery banks- it’s just that affordable and reliable ones do not seem to exist. The top balancing current shunting systems that many have used have often failed.(also easily verified).

Bottom balancing is just an alternative method that has proven valid.

You don’t have to agree (neither did I when I first heard about it) but demanding proof and spewing nonsense about me and accusations of “shilling” ( again WTF? Selling what exactly?) while offering no answers to the OP youself does not look so good..
« Last Edit: April 24, 2018, 02:02:14 pm by mtdoc »
 

Offline KL27x

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Re: Battery balancing - big cells
« Reply #48 on: April 24, 2018, 02:19:29 pm »
I'm sorry. I was wrong. I should have just walked away after my first response to this link. Please accept my apology. I was confusing balancing (initial balancing) and active balancing. And confusing posters. And I had poor impulse control.


This explains another guys reason why bottom balancing is better. Still don't get his explanation.
A bunch of people following the leader in something that works doesn't make any of them understand why it works or able to explain it.

As he says, you have to avoid over charging, as well as over discharging. And completely removing any cell monitoring from the bottom would just shift that burden to the top end, would it not?

Perhaps it's a simple matter of LiFePO4 are harder to damage by overcharging and easier to damage by overdischarging than Li ion. Or maybe it's harder to detect the lower threshold by voltage monitoring. (Pretty stupid easy with Li ion cells).  But this is just speculation. Perhaps there is someone out there that can explain why the hell they are doing it this way. If not, just say it works better and I don't know why. Instead of showing fancy graphs and using circular logic.

Another speculation, despite all the arguments that coulomb counting is the only way to do it, is that by bottom balancing it might be possible to detect at an earlier point in time when the battery is getting low, and in an EEV you don't want to be suddenly stranded in traffic.

I an do a lot of speculation. The guys that are driving these vehicles and playing with these batteries could actually give reasons. If they could only articulate them.

« Last Edit: April 24, 2018, 02:33:58 pm by KL27x »
 

Offline C

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Re: Battery balancing - big cells
« Reply #49 on: April 24, 2018, 03:06:18 pm »
KL27x's quote
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You have not thought this all the way through, C. This a red herring. Computing the internal resistance allows you to calculate the energy available at a given load. But it is not necessary to include that into cutoff limits. On the low end, the cell with higher internal resistance will sag in output more than the others. And you will still safely cut off the output when it reaches the lower limit. Maybe it cuts out a little earlier than it could have, but at least it's not a safety issue.

Looks to me that "You have not thought this all the way through"

Yes a low voltage cutoff works for safety, but what is the result?

Simple example
panasonic ncr18650b
https://na.industrial.panasonic.com/sites/default/pidsa/files/ncr18650b.pdf

If you pick a protect the cell from low voltage at 3 volts,
you are stopping the output at around
2250 mAh for higher current case for use of about 69% of capacity.
3150 mAh for a lower current case for a use of about 96% capacity.

And this is from a graph that is not complete, this is just different discharge rates.

If you pick a lower voltage and then you have to hope that your voltage sense is up to task and some other variable does not go out of range.

You can't read the charge from the voltage of a Li-ion battery. There is a huge section of the charge/discharge curve which is flat. This is the reason why you need a coulomb counter calibrated for the specific chemistry to determine the charge in a battery pack. The only thing the voltage indicates is whether you can charge or discharge a Li-ion battery any further.

Yes, absolutely right. I believe this is part of the reason why traditional “ top balancing” of series strings of large LiFePO4 so often fails.

Even this is not complete.
To use a coulomb counter you need it calibrated to the specific pack & it needs to be re-calibrated as the pack changes.

One way to do this calibration is to sense the rate of voltage change with a constant current as you get close to lower limit.
You are sensing the curve.

But again if you have a series pack, each battery in series will have a different capacity and change over time.

So use little of battery an pick very safe top and bottom voltages where any other change can not take you out of safe zone.

Or get more facts by measuring them an making a better decision allowing more use of packs capacity.

C






 


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