# Question regarding BMS and bottom balancing



## wessss77 (Jan 4, 2013)

Hey all...I regard all of your opinions as you are all very active on the threads and my question is quasi related...if bottom balancing is used and only since $ is a factor since I too will never use the full range on a normal day to day, is there a reason to use a digital over a analog BMS? Or do you think it would okay to go without a BMS for a short period of time while the funding came available? Thanks, Wes


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## dougingraham (Jul 26, 2011)

wessss77 said:


> Hey all...I regard all of your opinions as you are all very active on the threads and my question is quasi related...if bottom balancing is used and only since $ is a factor since I too will never use the full range on a normal day to day, is there a reason to use a digital over a analog BMS? Or do you think it would okay to go without a BMS for a short period of time while the funding came available? Thanks, Wes


I've been driving for a little over 4 months now with just a bottom balance. I checked balance just a few minutes ago after a drive that used about half the charge and an hours rest. Still all within 0.001 volt of each other. Doesn't look like you need a BMS to me. Others have a lot more miles and months on their cars without BMS than I do. I am sure some of them will say something.

With bottom balance and and a small amount of undercharging there is no real reason to monitor the individual cells. All it will tell you is that a cell has gone bad. It wont prevent you from losing one. Save the money you would spend on a BMS and buy a few extra batteries.


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## Elithion (Oct 6, 2009)

wessss77 said:


> is there a reason to use a digital over a analog BMS?


The choice of analog vs digital BMS is not affected by your other considerations. Here I'd like to address just your core question, about digital over analog BMS.

Both analog and digital BMSs will top balance.

If designed and installed properly, both analog and digital BMSs will protect your pack from undercharge and overcharge, by keeping each and every cell operating within its SOA (Safe Operating Area). With a BMS, cells will not be abused so you won't need to keep on replacing cells.

If you never need to troubleshoot, and you don't care to see how your pack is doing, and you don't need a "gas gauge", both analog and digital BMSs will work fine.

So, then, what's the difference between analog and digital BMSs?


An analog BMS is easiest to understand, set-up and install
A digital BMS provides feedback on your pack (State Of Charge, individual cell voltages, temperatures, etc.), has over-temperature protection, and, if a cell has an issue, it can tell you which one, what the issue is, and how bad it is
The prices of the best value analog and digital BMSs are pretty much awash: about $ 700 for 48 cells in series.



Use this utility to select a BMS that best meets your needs.


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## Ziggythewiz (May 16, 2010)

BMS flame war #6201

Why not just go no BMS period? Ask Boeing how much their BMS helped them.


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## wessss77 (Jan 4, 2013)

Thanks guys. As much as I have read this same information in other threads, it actually finally makes sense. Much appreciated!


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## corbin (Apr 6, 2010)

dougingraham said:


> I checked balance just a few minutes ago after a drive that used about half the charge and an hours rest. Still all within 0.001 volt of each other. Doesn't look like you need a BMS to me


Whoa! I'd be careful of the confidence that gives you. Consider the discharge curve for LiFePo cells; they are usually sitting at around 3.2v when at 20%-80% charged, and will all appear well balanced within that range. It is when things get below or above that range that you really need the BMS.

So, driving for a while, and measuring the voltage is a very poor indicator for determining if you need a BMS or not. However, if you drove your car to say 5% or 10% state of charge, and then measured the voltages, then you would have a better indication of how balanced your pack is.

I highly recommend a BMS.

-corbin


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## corbin (Apr 6, 2010)

wessss77 said:


> Or do you think it would okay to go without a BMS for a short period of time while the funding came available? Thanks, Wes


Hi Wes,
The BMS is designed to protect your pack in two ways: overcharging and over discharging. You would have to be very careful when driving to ensure you don't over discharge any cells, and be very careful when charging to verify you don't overcharge a cell. This can be difficult to do, and very time consuming to do properly. So, it is up to you about how much you like your battery pack 

-corbin


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## ga2500ev (Apr 20, 2008)

corbin said:


> Hi Wes,
> The BMS is designed to protect your pack in two ways: overcharging and over discharging. You would have to be very careful when driving to ensure you don't over discharge any cells, and be very careful when charging to verify you don't overcharge a cell. This can be difficult to do, and very time consuming to do properly. So, it is up to you about how much you like your battery pack
> 
> -corbin


Everything I've seen about balancing discusses potential failure modes. With top balancing, both the overcharging of a cell and the over discharging of a cell are potential failure modes because top balanced cells do not contain the same energy content at a 100% SOC. So if you have two nominal 100 Ahr cells where one cell is actually 99.5 Ahr and the other is 100.2 Ahr, then it's pretty easy to see that it takes different amounts of energy to charge the cells (which requires monitoring) and different amounts of time to discharge the cells under the same load (which means monitoring to verify that the weakest cell has not emptied).

Bottom balancing on the other hand works on the premise that each cell will contain exactly the same amount of energy. Each cell is "emptied" to a voltage that represents a 0% SOC. Then all cells are charged with the same amount of energy. Note that each cell is at a different SOC (because their capacities are different). However, as long as all the cells are discharged together, they will (in theory) empty to a 0% SOC all at the same time. So the failure modes are still overcharge, which can be mitigated by undercharging the pack so that no cell is at 100% SOC, or by only charging the pack until the weakest cell is at 100% SOC. The over discharge failure mode (again in theory) cannot happen because all the cells empty together. So a simple low voltage cutoff, say at 2.9V/cell would be sufficient to protect the pack because all the cells will drop to 2.9V together instead of a single cell (which is empty) dropping to 2.9V while all the others maintain their mainline 3.2V because they have not yet reached the falloff curve.

So the analysis is that bottom balancing requires less monitoring, and not at the cell level, in order to maintain the integrity of the pack. A charger that deliberately undercharges the pack (say the weakest cell to a 95% SOC) and has a simple LVC at or below 3.0V should be sufficient to protect all the cells from catastrophic failure. OTOH top balancing requires cell level monitoring at both the charging end, because each cell needs to be charged to 100% SOC each with a different amount of energy, and during discharge, because each cell can reach a 0% SOC and fall off the curve at different times.

BTW the advantage of top balancing, which makes it worthwhile, is the fact that the total energy of the pack is higher. But it does take more work to execute it.

ga2500ev


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## Ziggythewiz (May 16, 2010)

ga2500ev said:


> OTOH top balancing requires cell level monitoring at both the charging end, because each cell needs to be charged to 100% SOC each with a different amount of energy, and during discharge, because each cell can reach a 0% SOC and fall off the curve at different times.


No it doesn't. If you balance them near 100% full then only take them up to 95% in use you don't need much monitoring at the top; if anything it's less than for a bottom balanced pack. You just have to stay away from the bottom as it would be more prone to damage than a bottom balanced pack, but most people will want to limit it to 70 or 80% DOD anyway to prolong cell life, so there's really no risk there.


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## Elithion (Oct 6, 2009)

Videos:


Reasons for top balancing: intro, part 1, part 2
The case for bottom balancing


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## EVfun (Mar 14, 2010)

ga2500ev said:


> Everything I've seen about balancing discusses potential failure modes. With top balancing, both the overcharging of a cell and the over discharging of a cell are potential failure modes because top balanced cells do not contain the same energy content at a 100% SOC. So if you have two nominal 100 Ahr cells where one cell is actually 99.5 Ahr and the other is 100.2 Ahr, then it's pretty easy to see that it takes different amounts of energy to charge the cells (which requires monitoring) and different amounts of time to discharge the cells under the same load (which means monitoring to verify that the weakest cell has not emptied).


If they are top balanced it will take the same number of amp hours to charge all the cells. You have to stop the discharge before the first cell is empty, but your smallest cell will always limit your range. An amp hour counter would be a good addition to an EV with a top balanced pack and without a BMS. If you stay between 15% SOC and 95% SOC it seems to work quite well.

With a top balanced pack shutting off the charger is not very critical. I had my charger timer fail once. The pack sat at 135 volts for about 5 extra hours before I noticed. None of the 38 cells exceeded 3.75 volts. I guess the only real difference between top balanced and bottom balanced non-BMS use is whether you trust the charger more, or the driver more. 

If you choose bottom balancing I'm not aware of any BMS system available that is designed to manage that. If the BMS has a shunt that comes on at some maximum voltage then it is top balancing by design. If each cell module doesn't use exactly the same current then it is destroying a bottom balance by being present. Even a cell module current difference of 1/10th of one milliamp will unbalance a pack about 7/8th amp hour per year.


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## ga2500ev (Apr 20, 2008)

I said:

OTOH top balancing requires cell level monitoring at both the charging end, because each cell needs to be charged to 100% SOC each with a different amount of energy, and during discharge, because each cell can reach a 0% SOC and fall off the curve at different times.



Ziggythewiz said:


> No it doesn't. If you balance them near 100% full then only take them up to 95% in use you don't need much monitoring at the top; if anything it's less than for a bottom balanced pack. You just have to stay away from the bottom as it would be more prone to damage than a bottom balanced pack, but most people will want to limit it to 70 or 80% DOD anyway to prolong cell life, so there's really no risk there.


Now I'm confused. My understanding of top balancing is that each cell is charged to 100% of capacity. Since each cell has differing capacities, it means that each cell contains different amounts of energy. Bottom balancing drains each cell to a 0% SOC then fills each with the same amount of energy. "take them up to 95%" makes no sense because then they neither have the same SOC, nor the same amount of energy. So then by definition, then are not the cells unbalanced?

If you stay between 70% and %90 SOC of the weakest cell, then absolutely none of this discussion matters as long as all the cells are in the same ballpark in terms of the total energy they contain. The BMS is all about pack protection at the extremes of overcharging or over discharging. If all the cells are about even in terms of SOC, and the pack never gets to either boundary, then it will be perfectly safe.

The question is the OP concerned about catastrophic pack failure if for any reason the pack is subjected to either extreme?

ga2500ev


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## Ziggythewiz (May 16, 2010)

ga2500ev said:


> "take them up to 95%" makes no sense because then they neither have the same SOC, nor the same amount of energy. So then by definition, then are not the cells unbalanced?


By 95% I mean of the pack, not each individual cell. If they're 100AH nominal, one might be 105, another 101, and another 110. You fill them all up, call that 100, don't take more than 70-80AH out, then when filling you go to 95, and consider it full. Technically the SOCs are all unbalanced at anything under 100, but this way they're closest at the peak of the charge.

With simple voltage protection nothing catastrophic can happen without operator incompetence, which is a bad thing for any car. The pack will never be subjected to either extreme. The closest it gets is when you do the initial balance.


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## wessss77 (Jan 4, 2013)

ga2500ev said:


> The question is the OP concerned about catastrophic pack failure if for any reason the pack is subjected to either extreme?
> 
> ga2500ev


My original questions were answered now...First being if I bottom balanced would i still need a BMS and secondly if so, due to cost differences what is the differences between analog and digital and can it wait for a period of time for funding. My take away is also two fold. 
1. Bottom balancing will protect me temporarily as long as i stay in the "safe zone" from a catastrophic mishap but is not a good idea when talking about an expensive traction pack. So a BMS is in my eyes a definite necessity. 
2. I can hold off for 2 months with my current needs for the daily driver and the pack I am installing. When I do install it I will use a digital because of the "features" associated. I like features.


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## IamIan (Mar 29, 2009)

ga2500ev said:


> Now I'm confused. My understanding of top balancing is that each cell is charged to 100% of capacity.


AFAIK Balanced is just to the same %SoC ... it could be any %SoC .... 73.5% ... 26.8% ... etc... as long as all cells are at the same %SoC then the pack is SoC Balanced.

AFAIK it is common for people to refer to top or bottom as a range ... 90% or above might be considered the 10% range at the top ... but would just be called top ... while 15% or less might be considered the 15% range at the bottom... but would only be called the bottom.

The aspect being exploited in either is the change in terminal voltage of the cell at the top and bottom of the SoC curve ... even if one doesn't know all the details of a battery ... one can know it is in the near top or near bottom range once the terminal voltage reaches certain markers.

So the person doing a 'top' balance doesn't have to go all the way to 100% ... they just have to go far enough into the top range to reach certain terminal voltage markers ... and likewise the 'bottom' balance doesn't have to go all the way to 0% ... they just have to go far enough in the bottom range to reach certain terminal voltage markers.



ga2500ev said:


> Bottom balancing drains each cell to a 0% SOC then fills each with the same amount of energy. "take them up to 95%" makes no sense because then they neither have the same SOC, nor the same amount of energy. So then by definition, then are not the cells unbalanced?


I agree were balanced at the bottom ... once they leave the bottom you are correct they are no longer balanced at all other points on the SoC curve.

Like wise if balanced at the top ... once they leave the top they are also no longer balanced at all the other points on the SoC curve.

Because we never get 100% identical cells ... they are never treated 100% exactly the same ... and no BMS can actively and accurately keep up with 100% of real time loads ... and because bad things happen to either over charged cells or over discharged cells ( or cell reversals ) ... so we have to pick one or the other ... and thus people will disagree.

- - - - - - - 
For the OP:
I think ultimately top , bottom, BMS, or no-BMS ... can all be done effectively and safely ... but they are different things , that need to be done differently ... anybody who wants to do any combination , whatever it is ... should do their homework to understand the pros and cons ... and then follow through to do it right... in the build itself as well as the usage.


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## wessss77 (Jan 4, 2013)

IamIan said:


> For the OP:
> I think ultimately top , bottom, BMS, or no-BMS ... can all be done effectively and safely ... but they are different things , that need to be done differently ... anybody who wants to do any combination , whatever it is ... should do their homework to understand the pros and cons ... and then follow through to do it right... in the build itself as well as the usage.


I agree as the OP and this discussion was for my homework and once again is very much appreciated. (Now I just gotta choose a charger...my brain is beginning to hurt. ) New questions...new thread maybe...still seeing what is out here.


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## Ziggythewiz (May 16, 2010)

Elithion said:


> Videos:
> 
> 
> 
> ...


I'm no fan of bottom balancing, but your description of it represents a complete lack of understanding of the process. 

Unless you're top balancing every charge with a BMS (which I'm even less of a fan of), balancing is a one time occurence. You would not drain your cells and rebalance them on the bottom before each charge. That would put tons of extra wear on the cells and likely cut their life in half.


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## dtbaker (Jan 5, 2008)

wessss77 said:


> My original questions were answered now...First being if I bottom balanced would i still need a BMS


If you bottom balance, you MUST have a cell-level sensing system that can shut down the charger each charge cycle via DIN connection when the first cell hits upper limit. you actually don't want an active shunt balancing BMS as they are designed to maintain top balance, all you need is a monitor/sense system that can end charge when the first cell hits upper voltage.

This is kinda why I am in the top-balance, BMS-less camp. Once you've got the pack initially top balanced, the cells really don't drift much relative to each other, so you really don't need a BMS.... just trust a simple CCCV charger to do its job and check the end of charge balance every 5000 miles or so.


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## Elithion (Oct 6, 2009)

Ziggythewiz said:


> bottom ... balancing is a one time occurence. You would not drain your cells and rebalance them on the bottom before each charge.


No, I didn't say that in the video, not at all. 
I said, quote "if you're going to hand balance the pack, you can get away with doing it maybe every couple of months".

As far as doing it only once in the life of the pack, I think that a bottom balance proponents themselves will tell you that they repeat the procedure once in a while.

By all means, balancing is not required for safety (cell level protection takes care of safety). Balancing is mostly a matter of convenience, to maximize the energy that the pack can deliver.



Ziggythewiz said:


> your description of it represents a complete lack of understanding of the process.


Thank you for pointing that out. If you could please tell me in what respect, I would like to add corrections to the video.


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## Ziggythewiz (May 16, 2010)

Elithion said:


> No, I didn't say that in the video, not at all.
> I said, quote "if you're going to hand balance the pack, you can get away with doing it maybe every couple of months".


You said that bottom balancing was not compatible with on demand charging/driving because the charger would first drain the batteries, then balance them, then recharge them, so if you had to go somewhere you'd be worse off than before you started charging.




Elithion said:


> As far as doing it only once in the life of the pack, I think that a bottom balance proponents themselves will tell you that they repeat the procedure once in a while.


I've heard quite the opposite. The only ones I've heard needing to re-balance is because they've added cells or something. I've heard recommendations that people should re-balance if they have something causing milliAH deltas per year, which is just silly.




Elithion said:


> By all means, balancing is not required for safety (cell level protection takes care of safety). Balancing is mostly a matter of convenience, to maximize the energy that the pack can deliver.


Who needs cell level protection? Balance once and don't mess it up with regulators or other taps and you're set for life.


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## Elithion (Oct 6, 2009)

Ziggythewiz said:


> You said that bottom balancing was not compatible with on demand charging/driving because the charger would first drain the batteries, then balance them, then recharge them, so .


Thank you for getting back to me.

I said, quote (at 5:42 into the video) "Well, if you had something like a Coulomb counter that measures the current going through the battery, yeah, that would work. But it assumes that you start from completely empty. What it you used them only part way and then you're charging? How much charge are you going to put back into the battery? You don't know. Then you have to use the same Coulomb counter to measure the current going out of the battery, so you can put it back into the battery. And, because of tolerances, if you do that a few times, your Coulomb count will be off, and you are going to be either undercharging or overcharging. But let's say that you are able to always discharge the batteries completely, so you always start from a known point: zero State Of Charge. In that case, instead of a Coulomb meter, you can use a stop watch, and knowledge of haw much current is coming from the charger.".

I didn't say "that bottom balancing was not compatible with on demand charging/driving because the charger would first drain the batteries, then balance them, then recharge them". 
I said than neither Coulomb Counting nor Charge Timing give you a way to charge a bottom balanced pack and stay away from overcharging the lowest capacitance cell. I did not say that convenience charging is incompatible with bottom balancing.

Nowhere in the video did I say anything close to "so if you had to go somewhere you'd be worse off than before you started charging".

I am beginning to wonder if the problem is my accent. If so, I am sorry that did not speak more clearly.

So, my request to you remains: could you please point out any errors or misunderstanding I expressed in that video, so that I can correct them? I truly, truly would appreciate that.


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## Ziggythewiz (May 16, 2010)

Elithion said:


> I didn't say "that bottom balancing was not compatible with on demand charging/driving because the charger would first drain the batteries, then balance them, then recharge them".
> ...
> 
> I did not say that convenience charging is incompatible with bottom balancing.
> ...


Please watch your video, part 2, you don't seem to remember what you've said.



Elithion said:


> 7: Top balancing is compatible with unscheduled use
> 
> "Lets look at the bottom balancing solution...The charger, first thing it does is discharge the cells down to zero, balance them at zero, then bring them up until one cell hits the top because it's the lowest capacity cell.
> 
> ...


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## Elithion (Oct 6, 2009)

Ziggythewiz said:


> Please watch your video, part 2, you don't seem to remember what you've said.


No, I didn't remember; you're right: I did that video in 2011. 
Today I was watching the "case for bottom balancing" video, not this video, so that's why I couldn't find that part. Thank you.

I have made that correction to the video. If you go to 4:47, you will see that not only I retracted what I said, but I also credited you personally for correcting me. Thank you!

Anything else I can correct?


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## Ziggythewiz (May 16, 2010)

Elithion said:


> Anything else I can correct?


Just the basic premise that balancing, either top or bottom, needs to be repeated often. Without adding unbalancing taps (like balancers) there's no mechanism to unbalance the cells. What you take from one cell, you take from all. What you put into one cell, you put into all.

I agree that an AH counter and the charging process in general may not be accurate enough to run untouched for months, but all you have to do is reset the meter when charged. The point you treat as full (which should be at least 3-5% below your weakest cell's actual full) will always be your full and you can safely take out anything less than your limit and recharge back to that point and call it full, with no additional balancing needed. The key is to make your full point (or empty if bottom balancing) closer to nominal than the point where you balanced at.

The only instrumentation that is really necessary for even an OEM level of hands-off battery care is a volt meter, an AH counter, and some form of half pack comparator that can trigger a CEL to have the pack checked and eventually disable the car/charger if the imbalance becomes too great.

You also talked about what happens to the weakest cell when you charge above the pack's full voltage. Why would you do that? Stop charging before any of the cells are full and there's no issue.

I haven't heard about TS/Winston's 4.0V magic previously. Do you have any links to manuals or manufacturers discussing this? I would be quite surprised to find that they are so different from other LiFePO4 that are full under 3.4V.


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## Elithion (Oct 6, 2009)

Ziggythewiz said:


> Without adding unbalancing taps (like balancers) there's no mechanism to unbalance the cells.


That mechanism does exist, and is the variation in self-discharge current (leakage). If all cells had the same leakage, then you'd be right. But all cells have different leakage. More importantly, when cells are hotter, they self-discharge more than cold cells. Cell get hotter because of location in a vehicle, or during heavy use because they have higher internal resistance.

Therefore, balanced packs do get out of balance, whether they are used or not.


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## palmer_md (Jul 22, 2011)

Elithion said:


> That mechanism does exist, and is the variation in self-discharge current (leakage).


wait...where is the mechanism. The linked document just uses the assumption that there is a self discharge leakage current. My understanding of LiFePO4 is that there is no mechanism to discharge without a circuit, so a cell left open will hold its SOC and does not need to be balanced.


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## Siwastaja (Aug 1, 2012)

palmer_md said:


> wait...where is the mechanism. The linked document just uses the assumption that there is a self discharge leakage current. My understanding of LiFePO4 is that there is no mechanism to discharge without a circuit, so a cell left open will hold its SOC and does not need to be balanced.


The idea that there was no self discharge at all is physically impossible and ridiculous, and not supported by any practical evidence.

It is just that li-ion self discharge is very very small, almost negligible especially when compared to other battery chemistries.

I've seen no-bms people on this forum say that they need a rebalance every few years. So you guys just seem to disagree on its magnitude for about one order of magnitude. It's not that much. It is well known that abused or just low-quality cells can have very high self discharge, so I would guess there could be some cell brands that would need rebalance every few months in some certain situations (like high temperature differential between the cells), while others could be just fine for their whole lifetime.

So I can very well see that Davide as a professional cannot guarantee that any and every li-ion battery pack would stay in balance without balancing for longer than a few months, even though there may be a few cases where it just works.


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## dtbaker (Jan 5, 2008)

I have noted no *significant* relative cell drift in my LiFePO4 pack over the first 10k miles. My pack is split between motor bay, and under rear cargo area in the Swift, so sections are at slightly different temperatures, but close since they are fully ineclosed and have similar materials and insulation.

While warmer cells may be have slightly differently than colder, the variations during charging and driving pretty much seem to cancel out. LiFePO4 cells seem to have *very little* self-discharge, certainly negligible in a daily or almost daily use situation.

Potential parasitic drains are the dc-dc, the car's 12v background drain from clock, radio, etc, and some BMS systems have significant load, and unless the system is on every cell they can unbalance the system.

...my only point is that the LiFePO4 packs seem to do a remarkable job staying balanced if there are no external loads on partial pack, and very little self-discharge at all.


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## Elithion (Oct 6, 2009)

I can confirm that LiFePO4 self discharge is very minimal (though not 0).

One of the test set-ups we use at Elithion to test a BMS uses 26650 LiFePO4 cells from K2, with a cell board across each. I haven't charged them in 4 years. They were fully charged and balanced 4 years ago. Today they are unbalanced, for sure, but they are still partially charged. So, yes, the LiFePO4 self discharge is very minimal.

But there is a self discharge.

Note that the test set-up is in an office environment, with little excursion in temperature. In a EV environment, cells will get warmer than in an office, and therefore self-discharge more.

After your statement last night that LiFePO4 has no discharge, I asked my helpful, bright chemist at a well known cell manufacturer; he just replied: 

"LiFePO4 is just another variant of Li-Ion, in additional to LiCo, LiMn, etc. If you are reading articles about general Li-Ion mechanisms, then they should generally apply to LFP, as well. The basic chemistry is simply the movement of Li ions, regardless of the particular formulation. LFP has self-discharge."

You asked about what the self discharge mechanism is; I searched for it and got a plethora of scientific papers about it; have at it.


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## Ziggythewiz (May 16, 2010)

To say "just another variant of Li-Ion" is a gross generalization. It's well known that laptop batteries (and Teslas) can brick themselves just sitting around for months. LiFePO4 cells have been left sitting around, and unless previously damaged, not only do not become bricked but they show no significant leakage, even after years.

It really doesn't matter if the mechanism is the same. What matters is the magnitude. If cells can become unbalanced by say 1% / year, then if I have a 5%cushion I need to re-balance every 5 years. If they unbalance by .1% /year I have to re-balance every 50 years. So far that magnitude of imbalance is unknown because no one has had cells become unbalanced enough to be measurable.


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## palmer_md (Jul 22, 2011)

Elithion said:


> I can confirm that LiFePO4 self discharge is very minimal (though not 0).
> 
> One of the test set-ups we use at Elithion to test a BMS uses 26650 LiFePO4 cells from K2, with a cell board across each. I haven't charged them in 4 years. They were fully charged and balanced 4 years ago. Today they are unbalanced, for sure, but they are still partially charged. So, yes, the LiFePO4 self discharge is very minimal.


Perhaps I'm misreading your post, but I think you are saying that you have a board installed on each cell that is monitoring the SOC of the cell. If that is the case, then you've invalidated the test. Each cell must be kept static and linked in a string such that any current in one must also be in every other one, and this entire string of cells must remain open circuit. Once per test interval you would then take a reading of each cell over the course of the testing period. For example once a month over the past 4 years.

If you have something monitoring continuously there could be small differences in the load for each cell.

Did I misunderstand? Do you follow my logic?


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## Elithion (Oct 6, 2009)

Sorry, I wasn't clear. I was confirming that the LiFePO4 self leakage is very small. So small that after 4 years the cells are not discharged. The fact that cells boards are installed on the 2 Ah cells, and the cells are not discharged after 4 years, indicates that the Elithion cell boards draw extremely low current from the cells they are mounted on. If either K2 LiFePO4 cells had high discharge, OR Elithion cell boards drew a lot of current, the cells would be discharged. The fact that they aren't, show that K2 LiFePO4 cells have very low discharge, AND Elithion cell boards draw very little current


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## Ziggythewiz (May 16, 2010)

I'd still like to hear more about TS/Winston's 4.0V magic, unless that was just a 2011 misconception.


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## palmer_md (Jul 22, 2011)

Elithion said:


> Sorry, I wasn't clear. I was confirming that the LiFePO4 self leakage is very small. So small that after 4 years the cells are not discharged. The fact that cells boards are installed on the 2 Ah cells, and the cells are not discharged after 4 years, indicates that the Elithion cell boards draw extremely low current from the cells they are mounted on. If either K2 LiFePO4 cells had high discharge, OR Elithion cell boards drew a lot of current, the cells would be discharged. The fact that they aren't, show that K2 LiFePO4 cells have very low discharge, AND Elithion cell boards draw very little current


Thank you. That makes sense.


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## IamIan (Mar 29, 2009)

Siwastaja said:


> The idea that there was no self discharge at all is physically impossible and ridiculous, and not supported by any practical evidence.
> 
> It is just that li-ion self discharge is very very small, almost negligible especially when compared to other battery chemistries.
> 
> ...


For example ... my own testing of 55 A123 cells I got showed significant variation in self discharge rates ... but someone else could easily have got a bunch of cells with a smaller margin ... and also potentially contributing to balance drift over time my cells have different Ah and Wh cycle efficiencies ... again a different batch might be much smaller differences.

And if the differences are small enough ... the drift would be small enough and slow enough to take a significant amount of time to develop into a functional problem ... on the other hand some unlucky person could have very poorly matches batch of cells , and they could see the effects faster.

( Note: the graphs have the cells in order by graph topic ... for example: cell 1 in graph 1 may not be the same cell 1 in graph 2 )


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## Elithion (Oct 6, 2009)

IamIan said:


> my own testing of 55 A123 cells...


Fascinating, IamIan! I sent you a PM about this. Can you please read it and write me? Thanks.


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## dougingraham (Jul 26, 2011)

IamIan,

Am I correct in reading that chart that you saw a range of 12% to 74% self discharge over a 2 month span of time on A123 pouch cells? Or were those the 26650 cells? How were the cells stored? Were they connected to any kind of monitor device during the storage interval?

I like your AH WH efficiency chart. What C rates for charge and discharge did you use to get those numbers? What device did you use to take the measurements.

In my own tests of 100AH GBS cells at a 30A rate I cant get a good number for AH efficiency. The amount the charger/discharger puts in and then takes back out is masked by some inaccuracy of the measurement. About half the time the device would show over unity by a small amount. Over 60 cycles across 4 different cells (my worst cells by capacity) gave an AH inefficiency of 0.03%. I expect this might be a bias in the charger where it reads slightly lower current during charge or slightly higher during discharge. Or they could be doing some odd sort of rounding when they accumulate the amp hours. And it is this kind of instrumentation issue that you have to be aware of. It wasn't until I started analyzing the data that I noticed this issue. This was using an iCharger 3010b.


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## IamIan (Mar 29, 2009)

Elithion said:


> Fascinating, IamIan! I sent you a PM about this. Can you please read it and write me? Thanks.


reply sent



dougingraham said:


> IamIan,
> 
> Am I correct in reading that chart that you saw a range of 12% to 74% self discharge over a 2 month span of time on A123 pouch cells?


No.
The left side is average mWh/day ... not %.

12mwh/day for 60 days is all of 720 mwh ... or 0.72 wh ... or an average hourly rate of just ~0.008mW ... a tiny % from about ~60wh cells ... and The other thing to keep in mind is Self discharge rate is a logarithmic decay ... it is not linear ... ie 30 days in month 6 will have less self discharge than 30 days in month 1.

I have not yet finished testing to see a more detailed shape of the A123 yearly self discharge graph ... but it is on my list to do eventually.



dougingraham said:


> Or were those the 26650 cells?


stamped as 20Ah ... tested as less than that at a 10Amp rate.



dougingraham said:


> How were the cells stored?


room temperature about ~70F ... not in any expensive climate control room.



dougingraham said:


> Were they connected to any kind of monitor device during the storage interval?


No ... Disconnected between test cycles.



dougingraham said:


> I like your AH WH efficiency chart. What C rates for charge and discharge did you use to get those numbers? What device did you use to take the measurements.


I used a roughly C/2 rate ... 10Amps for 20Ah rated cells.

PL8 ... exported to a spreadsheet for data analysis.



dougingraham said:


> In my own tests of 100AH GBS cells at a 30A rate I cant get a good number for AH efficiency. The amount the charger/discharger puts in and then takes back out is masked by some inaccuracy of the measurement.


yup ... no measurement is perfect.
the PL8 has a fairly good calibration... and there are ways to fine tune it as well if one wants to narrow the margin even more.

- - - - - 

More detail is in the thread I started for my testing ... I am not complete yet ... there are other factors that need to be tested and such... I don't want to hi-jack this thread ... I was just pointing out that there are differences between cells ... and it is entirely possible for different batches to have different amounts of variation between cells.

http://www.diyelectriccar.com/forums/showthread.php/quantified-lot-a123-20ah-cells-phev-79846.html


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