# BMS and Care For LiFePo4



## Ziggythewiz (May 16, 2010)

Balancing makes all the voltages equal at that point. Once you start charging or discharging the voltages will vary. If you were to return to the point of balance they should all be equal again (assuming no drift has occured) but you should never return to that point operationally. If bottom balanced don't go that low again, if top balanced don't go that high again, and your cells will be safe on that end.

BMS work with top balanced systems, and most just alert or try to prevent a cell from going too high voltage. I'm no expert on the details though as I don't use a BMS and never intend to.


----------



## GE11 (Oct 24, 2011)

OK so lets say we balance at 2.5V at the bottom end. Then you say do not ever co below 2.5V when discharging them? So then that means the lowest case of dicharge should be around 2.7V correct? Then how do you know at a 2.7V discharge you don't have a cell or two below that? How do you know that cels have not driffted?

The same with charging... How do you know one cell does not have a higher voltage than another if your only measuring the voltage on the whole pack?


----------



## dougingraham (Jul 26, 2011)

GE11 said:


> A Balanced Pack,
> Means that all the cells have the excat same voltage. Right???


If bottom balanced then they will be the same when the pack is discharged the idea being if you try to keep driving when it is empty all the cells reach empty at the same time and no cell is destroyed by going into cell reversal.

If top balanced then they will be the same when the pack is fully charged and allowed to return to rest. You can't really tell anything once they get above about 3.45 volts during charge because of diffusion delay voltage variance. The idea behind top balance is that all the cells will become full at the same time during charge so no cell will be damaged by overcharge.




GE11 said:


> Lets take the bottom balance methode which I understand. So we take some sort of Load and we drain one cell battery down to around 2.5V OK How ever methode you would like. Keeping in mind that after you initally drain it down and you discontect it, it will recover alittle bit so you have to drain it back down to 2.5 Volts again untill you get a rest voltage of 2.500 volts! Right??? Then you connect your pack together in series and Charge the pack. Until one of the Batteries gets to 3.5V first.


In essence that is correct. My own approach was to use an RC hobby charger to discharge the cells at 30 amps down to 2.5 volts. I don't bother to fine tune the state of charge since the only thing that matters is that they all dump at the same time. And any cell that hits 2.5 volts at 30 amps is for all practical purposes dead. Pulling a couple of mah more out of a cell to make the resting voltage the same as its neighbors doesn't add much. 



GE11 said:


> Ok how do you know while your charging that one of the Batteries doesn't do a good job of taking the Charge current and converting it to Charge but heat? So when you get your first cell to the 3.5V some of them might be at 3.3V or 3.4V or 3.2V...becuase they were less effecient. So then you go to dischargeing the pack and your going to have some go way below others??


This is easy. The cell that arrives at 3.5V first is the weakest cell. If it wasn't it wouldn't be the first to get there. With these cells AH in is pretty much equal to AH out. So when bottom balancing the cell with the lowest resting voltage at full charge is actually the cell with the highest capacity. You could put more in that cell but that would blow the bottom balance. When bottom balanced and a full pack the resting voltage of the cells will not be the same. But when discharged to empty they will all get there at the same time.



GE11 said:


> I just don't understand how you Cycle through Cycle know or not know which cell is too low or too high?? Some Cells are worst than others.


Yes they are. But it turns out it doesn't matter as long as you stop charging a bit early so the weakest cell never gets overcharged.



GE11 said:


> Then the BMS circuit that is connected to the terminals, I am not sure how these work. Some how they are supposed to have some sort of resistor that "Kicks in" and drains a cell when it is higher than all the others???
> Well, how does that one circuit know what state of charge the other ones are? I mean it could be in the middel of charging.
> Does it "kick in at some Top Voltage?? If so, lets say it "kicked in" the parallel resistor at 3.5V there is still current going through the Battery it is not a dead short accross the Battery. This cell is still charging but just slower waiting on the other Cells to charge.


In the simplest case the shunt regulator BMS does nothing except turn some of the current that would be charging a particular cell into heat. It does this when the voltage at the terminals of the cell it is attached to go over a certain voltage threshold. If the shunting current equals the charging current then the battery stays at its current state of charge. There are several problems with this but the biggest is thinking that the voltage of the cell when charging is above about 3.45 volts actually is representative of state of charge. Another problem is making the boards all have a reference voltage that is the same. A precision reference would be required on each board or you can have a voltage imbalance of a couple of percent. These boards power themselves off the cell they are connected to so unless they all draw the same current they will be the parasitic load that imbalances the pack. Most of the high price units also send back information to a central device which can display cell voltage and sometimes temperature. Useful information to be sure but it does not help you drive the car or do much more than tell you a cell is going bad and which cell it is.


----------



## GE11 (Oct 24, 2011)

It is absolutly amazing that in this Bottom Balanceing procedure that when a Pack is cycled through and back empty that all the voltages are back at the same... How is there not any, what do I say, Loss, or some other battery holding more..know what I mean? There is no drift during a cycle where one batter lost a bit more charge than another?

Then you say prttey much A hours in equals Amp hours out? Here again does the Pack get warm durning chargeing then to me there is some loss durning charging.
Is this how all Batterys work whether they are Lead acid or LifePo4? Or is this just LifePo4?


----------



## dougingraham (Jul 26, 2011)

GE11 said:


> It is absolutly amazing that in this Bottom Balanceing procedure that when a Pack is cycled through and back empty that all the voltages are back at the same... How is there not any, what do I say, Loss, or some other battery holding more..know what I mean? There is no drift during a cycle where one batter lost a bit more charge than another?


It is one of the areas that is so different from other battery types. If you take out 80 AH you put back in 80 AH. I see less than a percent difference and this could be completely from measurement error. This is across the battery as a whole. The 51 cells in my pack were bottom balanced once in the fall of 2011 and I have looked at the relative voltages only three times since then. There is no significant drift or I would see it by now. The cells in my pack do vary in capacity. The worst one was just over 100AH and the best one was a little over 105AH. WIth bottom balance the 100AH cell is filled to the brim. The 105AH cell is only 95.2% full. Since there is no internal imbalance mechanism and all cells are in series they all see exactly the same numbers of coulombs passing in and out and thus stay in balance.



GE11 said:


> Then you say prttey much A hours in equals Amp hours out? Here again does the Pack get warm durning chargeing then to me there is some loss durning charging.
> Is this how all Batterys work whether they are Lead acid or LifePo4? Or is this just LifePo4?


They don't get warm during charge. Most of the LiFePO4 cell types can do 2C or 3C charge. For me that would be 3C or 300 amps. My charger from a 110vac outlet does 8 battery amps. Not going to get hot at 8 amps. Guessing that if I could pump 300 amps into one it might warm up a little. Jack at EVTV has done this and measured the temp increase. With LiFePO4 cells there is loss. It just isn't found in AH in vs AH out. If you measure WH in vs WH out you see the losses. All other types I have played with Lead Acid, NiCd, and NiMH all display a disparity in AH in vs AH out. I am guessing you might start to see some with LiFePO4 if you did a full cycle at the max continuous rate both charge and discharge. In that case you would see some heat in both directions.


----------



## GE11 (Oct 24, 2011)

What is Cell reversal?

I guess instead of a BMS on each cell you could just have every Cell connected to some sort of computer to moniter everything for you with out getting a Volt Ohm Meter on every Cell.

Are ther systems that do this?


----------



## ga2500ev (Apr 20, 2008)

GE11 said:


> It is absolutly amazing that in this Bottom Balanceing procedure that when a Pack is cycled through and back empty that all the voltages are back at the same... How is there not any, what do I say, Loss, or some other battery holding more..know what I mean? There is no drift during a cycle where one batter lost a bit more charge than another?


It's not a function of voltage. It's a function of energy. As Doug pointed out above a lithium cell at 2.5V is pretty much completely empty in terms of energy. So if you charge all the cells together, and you do not overcharge any of the cells, then you put the same amount of energy into each of them. When you discharge them all together, they will all run out of energy at the same time.

LiFePo4 discharge curve is very flat as long as a cell has energy. You can pretty much ride it at about 3.2V until the very end where the voltage falls off a cliff. Bottom balancing ensure that each cell has the same amount of energy. So they will fall of the cliff together as all the cells run out of energy at the same time.

The problem comes with being out of balance. A cell at a 20% state of charge (SOC) and a cell at a 60% SOC will measure the same voltage upon discharge. However, the 20% SOC cell will run out of energy first while the 60% SOC cell is still going strong. This state is what tears up Lithium packs.



> Then you say pretty much A hours in equals Amp hours out? Here again does the Pack get warm durning chargeing then to me there is some loss durning charging.


A cell only really warms significantly when being overcharged. By cutting off the charge when the first cell gets to 3.5V, you can guarantee that overcharging does not occur.

Again as Doug pointed out, a few maH of energy difference isn't going to matter much. It's the situation where some cells are completely empty while others are half full that creates the major issues.



> Is this how all Batterys work whether they are Lead acid or LifePo4? Or is this just LifePo4?


Just lithium. Lead Acid has significant self discharge. In addition FLA can be overcharged (gassed) without too much harm. The final item is that LA's discharge curve has a voltage proportional to SOC. In short it's a completely different animal.

ga2500ev


----------



## ga2500ev (Apr 20, 2008)

GE11 said:


> What is Cell reversal?


Cell death. A cell empty of energy in a pack that is still providing power. The cell destroys itself trying to provide energy that it doesn't have to the system.



> I guess instead of a BMS on each cell you could just have every Cell connected to some sort of computer to moniter everything for you with out getting a Volt Ohm Meter on every Cell.
> 
> Are ther systems that do this?


I think you are missing the point. If you follow three simple rules, you'll find that you don't need all of cell monitoring equipment:

1. Never overcharge any cell.
2. Never overdischarge any cell.
3. Make sure that each and every cell has the same amount of energy.

Bottom balancing is #3. Low voltage cutoff (LVC) on the cart is #2. Voltage cutoff on your charger is #3.

You say you need 48V. So you will have 15 cells. 3.0V per cell is plenty enough headroom to ensure that you do not over discharge. So if your pack is at 15*3.0V = 45.0V then you are out of energy. Set your LVC to 45.0 volts so that the cart gets no power when the pack is at 45.0V. This takes care of #2.

For #3 you bottom balance all 15 cells to 2.5V. They are now all empty. You then attach them to the charger in series (monitoring each cell) and charge until the first cell gets to 3.5V. Now since it's the first cell you pack voltage will not be 52.5V (15 * 3.5V) but something a bit less, say 52.15V. You set your charger to cut off at 52.15V. This will ensure #1.

Simply put as long as your pack stays between 45.0V and 52.15V, you never need to worry about it. If you hit your LVC, you will see that each of the 15 cells will be right at 3.0V. When you hit 52.15V on your charger, the first cell you identified in the initial charge (which has the least capacity) will be right at 3.5V. Each and every cell will have the exact same amount of energy.

Now if you overcharge (53V for example) or overdischarge (below 37.5V) then you need to ensure that you didn't damage any of the cells and rebalance. Once you violate rule #1 or #2, then you can no longer guarantee that each cell has the same amount of energy anymore. Then you need to rebalance.

I hope this helps,

ga2500ev


----------



## GE11 (Oct 24, 2011)

ga2500ev said:


> Cell death. A cell empty of energy in a pack that is still providing power. The cell destroys itself trying to provide energy that it doesn't have to the system.
> 
> 
> 
> ...


Thank you my friend this helps A WHOLE lot!!


----------



## GE11 (Oct 24, 2011)

I greatly appreciate all the responses!!


----------



## Moltenmetal (Mar 20, 2014)

Thank you: this makes it ALL make sense! Still need to read a bit more to understand how taking all cells to 2.5 V during bottom balancing isn't going to result in over-discharge. Are you saying that a cell may be discharged to 2.5 V without damage if it is done under slow and controlled conditions, but because the discharge voltage curve is so steep that 3.0 V is a safer low voltage cut-off point?


----------



## ga2500ev (Apr 20, 2008)

Moltenmetal said:


> Thank you: this makes it ALL make sense! Still need to read a bit more to understand how taking all cells to 2.5 V during bottom balancing isn't going to result in over-discharge. Are you saying that a cell may be discharged to 2.5 V without damage if it is done under slow and controlled conditions, but because the discharge voltage curve is so steep that 3.0 V is a safer low voltage cut-off point?


Not exactly. We need to differentiate between a cell and a battery. A cell for LiFePo4 is a unit whose nominal voltage is 3.2-3.3V. This can be a single item, or a collection that is paralleled together so that all the positive terminals and all the negative terminals are tied together. For the sake of discussion let's call this group of units a "total cell" 

A total cell as defined above (whether 1 unit, or 5, or 12) can be discharged down to 2.5V without damage. Even at high current rates. The individual units will share energy and work to keep their shared terminal voltage equal. So if two units have different states of charge and you are discharging the entire total cell, as the unit with less energy starts to run out, its terminal voltage will drop relative to the other unit. At some point the unit with more energy will start charging the unit with less energy and the total cell will retain the nominal 3.3V terminal voltage. Only when the energy of all the units are completely drained with the voltage of the total cell start to drop. The units will continue to share energy to keep the voltage of the total cell level. At 2.5V the energy of the total cell is virtually empty with all of the individual units of the total cell also empty.

Now on the other hand when total cells are linked together in series you get a battery whose overall voltage is 3.3V * number of total cells linked together.
Now draining the battery when the total cells that make it up is dangerous if the individual total cells that make up the battery have different amounts of energy. This happens because unlike the units in a total cell, the total cells that make up the battery cannot share energy with each other. So if one of the total cells in a battery completely drains while the others are still going strong, the other total cells will actually force enough energy into the drained cell that the voltages on the terminals on that drained total cell reverses. This cell reversal causes a catastrophic destruction of the drained total cell.

So the question of how far down can you drive a total cell depends on how it's connected. In isolation, since there is no outside energy to force cell reversal, you can drive the total cell to 2.5V without harm. However, it a total cell is linked into a battery, that 2.5V can be a problem because the rest of the system can still be driving energy even though that individual total cell is virtually empty.

So this is where bottom balancing helps in a couple of ways. Again the idea is to ensure that all the total cells of a battery have equal energy. The three positive outcomes from this is 1) that all the total cells will empty their energy at the same time. So when a single total cell gets to a low energy state, all the other total cells of the battery are also low energy. So there's no additional energy to drive the cell reversal process. 2) As a single total cell loses energy, its terminal voltage drops. Again if all the total cells of a battery have the same energy, then all their terminal voltages will drop together. This is excellent because as the battery (all the total cells) drains, then the overall voltage will drop significantly as all the total cells lose their energy. 3) On the charging side with equal energy, once a single total cell is full, all the others are nearly full too since they all have the same energy.

Now just to be fair, top balancing, where each of the total cells are stuffed with as much energy as they can hold does have an advantage. Since each total cells has the maximum amount of energy, so does the overall battery. However, that energy isn't equal among the cells so you lose the benefit of 1) and 2) above from bottom balancing.

But the conservative approach to bottom balancing makes it a no brainer if you do not want to destroy your cells. Discharge as a single total cell until it's empty, reconfigure as a battery, charge until one of the total cells tops off.

BTW the 3.0V/total cell low cutoff is a safety measure for a bottom balanced pack. If the total cells are anywhere near the same amount of energy, then once the battery reaches 3.0V/total cell, then all the total cells in the battery have drained significantly, but not so significantly that any individual total cell has a chance to completely drain and reverse. You could set it a bit lower if you really wanted to squeeze out a few last drops of energy. But remember that bottom balancing is a conservative approach to battery management. It's always better to be safer than sorry.

Hope this helps,

ga2500ev


----------



## Moltenmetal (Mar 20, 2014)

Ok, I understand the difference between a pack in series and a cell or group of cells in parallel. I understand how the lowest capacity cell will go over voltage first during charging or drop below tolerable voltage and risk reversing in voltage during discharge. If the BMS is acting as a voltage monitor on each cell, acting as an interlock on charging or discharging in the whole pack based on the 1st cell out, I get it's purpose. But surely shunting a tiny fraction of the current on each cell during charge or discharge doesn't do much? 2A is such a small fraction of the current I'm wondering what practical value those shunts provide.


----------



## Moltenmetal (Mar 20, 2014)

I also understand the benefit of balancing as the energy content at the outset of each cell matters too, even if all the cells have the exact same ultimate capacity. Just having a hard time understanding how these tiny currents do useful balancing.


----------



## Old.DSMer (May 18, 2012)

Moltenmetal said:


> Just having a hard time understanding how these tiny currents do useful balancing.


Me too. Take the EMW 12 kW charger for example. In my 86s pack, it would be capable of around 40A charging.

So how can a small shunt current possibly handle this excess when that first cell reaches its capacity? Wouldn't the BMS have to shunt a significant portion? And then, wouldn't that end up SLOWING the charge rate of the cells above it?


----------



## corado (Feb 6, 2011)

No, that's realy no Problem.
I charge often with about 100 to 150A and the BMS can only shunt with 1,5A.
Because it works every day, when I charge, there are realy small different between the Cells.
That it the reason, why this works well.
Ab good balanced Battery can work week or month without Problems without Balancing.But then you need longer time If you use the BMS then.
Because then the cell difference is much more then only a few µAh or mAh-
You shouldn'T forget, the real BMS Systems starting with Balancing at 3,4V and have more then a hour time to balance, till the cells got about 3,6V..
Only the cheap Systems, that only use passive Shunting at 3,65V have often problems, because they have much less time.If you charge them after every drive full it works well..but if you often charge only a little bit or much under 3,6v the difference will be bigger and bigger, if you then charge realy full, it can be critical...
That's the reason, why a real BMS is the better investion.


----------



## mora (Nov 11, 2009)

Shunting is pretty useless if your pack is too much out of balance. One should keep shunting at minimum. It should happen only when charger is at CV (constant voltage) phase of charge and nearing the finishing point. There charge current is usually pretty small and 2A shunting will help highest voltage cells from reaching high voltage cutoff point. In my opinion shunting voltage should always be higher than charger cutoff per cell. If pack is properly top balanced charger cuts off before any cell needs shunt resistor activity. No energy wasted as heat.


----------



## ga2500ev (Apr 20, 2008)

Moltenmetal said:


> Ok, I understand the difference between a pack in series and a cell or group of cells in parallel. I understand how the lowest capacity cell will go over voltage first during charging or drop below tolerable voltage and risk reversing in voltage during discharge. If the BMS is acting as a voltage monitor on each cell, acting as an interlock on charging or discharging in the whole pack based on the 1st cell out, I get it's purpose. But surely shunting a tiny fraction of the current on each cell during charge or discharge doesn't do much? 2A is such a small fraction of the current I'm wondering what practical value those shunts provide.


A BMS is used for top balancing, not bottom balancing. For bottom balancing all that's required is a low voltage cutoff for discharge, a high voltage cutoff for charging, and an occasional maintenance check to ensure that all the total cells are still in the same ballpark when discharged and that the original low capacity total cell is still the low capacity one.

A shunting BMS attempts to shuttle current from total cells that are already full to ones that are not yet completely full. Since it happens at the end of the charging cycle, these are low current transfers. It doesn't take much current to bring a total cell that's 97% full to 100% full.

But it's fraught with issues. You've already alluded to the first, which is per total cell monitoring. The next is that BMS deliberately charge each total cell to a 100% SOC, the definition of top balancing. The problem is that even with the most evenly matched total cells, that the amount of energy each contains will be different. So when discharging, the dropoff curves of each total cell will be skewed. 

I never got a clear indication of what to do under those conditions as the battery is being discharged under full current. So you cannot shuffle drops of energy between total cells. And if the difference is enough you could drive
one of the total cells into reversal. So the other option is to cut off once a single total cell starts to drop off. But then it seems to me that defeats the purpose of top balancing to begin with because top balancing gives more total energy. But if you cannot pull that energy out at the bottom end, when you really need it, then what exactly have you gained?

It seems to me that virtually every pack would be better off adding an additional total cell, which raises the voltage by 3.3V, and bottom balancing than to add a BMS and top balancing. Bottom balancing gives the opportunity to use virtually all the energy in the battery without the fear of destroying a total cell in the process.

ga2500ev


----------



## Moltenmetal (Mar 20, 2014)

ga2500ev: you've convinced me that bottom balancing without a BMS is the way to go. A couple extra cells are worth more than the extra cost of the BMS in my opinion. I don't mind going around the pack from time to time with a DVM- frankly I'm probably going to do that anyway.

My puzzle at the moment is the cost: at $125/100Ah which is what I'm seeing advertised, the 12-16 kWh pack I'll need for my range at 80% cell capacity is going to cost a mint. A Volt battery from the wreckers is looking very tempting- assuming it can be charged and discharged safely. Much less happy to work with the LiCoO chemistry without a BMS...something tells me they're quite a bit easier to kill. Not all that happy to work with 288 cells either, especially since getting at them cell by cell is pretty much prevented by design on those packs from what I can tell from the photos of people hacking them cell by cell...


----------



## Old.DSMer (May 18, 2012)

mora said:


> Shunting is pretty useless if your pack is too much out of balance. One should keep shunting at minimum. It should happen only when charger is at CV (constant voltage) phase of charge and nearing the finishing point. There charge current is usually pretty small and 2A shunting will help highest voltage cells from reaching high voltage cutoff point. In my opinion shunting voltage should always be higher than charger cutoff per cell. If pack is properly top balanced charger cuts off before any cell needs shunt resistor activity. No energy wasted as heat.


That clears it up (only active in CV). Small adjustments every charge, so no bulk current requirements. Thanks Mora!


----------



## corado (Feb 6, 2011)

no, you don'T need CV for Balancing.
It is not good for the cells, if they stay long time at high Voltage.
Better equalicing!! 
Then you don'T need CV.
Every time you cahrge, the BMS can correct the Voltage Level a little bit.
Aber few cycles all are identical.

This is the reason, why you should do a initialcharge if you use the cells the first time.
Because, if there are too much difference, it need much to long till ale cells are in Balance.


----------



## EVfun (Mar 14, 2010)

ga2500ev said:


> Now just to be fair, top balancing, where each of the total cells are stuffed with as much energy as they can hold does have an advantage.


The advantages I see to top balancing:
1. The available BMS systems with shunting ability are designed around top balancing.
2. When not using a BMS the voltage rise at the end of charge is very sharp and large, as ALL the cells are in the end of charge phase. It is easy for a charger to detect and use to shut off. 
3. There are several different amp hour counter (fuel gauge) meters available. They can be used to shut the pack off when a predetermined number of amp hours have been removed. You can even add some warning accurately, in seconds or amp-minutes, to advise of the impending shut down. I don't see opening the main contactor as unsafe, gas cars have been abruptly running out of gas for 100+ years -- often without an accurate fuel gauge.


----------



## ga2500ev (Apr 20, 2008)

EVfun said:


> The advantages I see to top balancing:
> 1. The available BMS systems with shunting ability are designed around top balancing.


True. But the BMS of course introduces another potential failure point. We've all seen stories here where the BMS failed to do its job. And this happens during the charging cycle, one of the ends where the damage potential is great.


> 2. When not using a BMS the voltage rise at the end of charge is very sharp and large, as ALL the cells are in the end of charge phase. It is easy for a charger to detect and use to shut off.


But there's a huge catch-22 here. This operates off the presumption that the total cells that make up the battery are in fact balanced. But in fact they are not when top balanced because each total cell has a different capacity. This can create situations where by the time the main group of cells are charge and their voltages start to rise, the lowest capacity cell could be at 3.8-4.0V already. And this overcharging would happen each and every charge cycle. It's another potential failure mode.


> 3. There are several different amp hour counter (fuel gauge) meters available. They can be used to shut the pack off when a predetermined number of amp hours have been removed. You can even add some warning accurately, in seconds or amp-minutes, to advise of the impending shut down. I don't see opening the main contactor as unsafe, gas cars have been abruptly running out of gas for 100+ years -- often without an accurate fuel gauge.


But again it adds complexity where simplicity will do just as well. If all the total cells have the same energy, then their voltage will drop off the cliff together. So a simple low voltage cutoff of the entire pack is all that's required. As for running out of "gas" if this happens when there's no energy left, then nothing gets damaged and it's just an inconvenience.

I just feel that top balancing is a misnomer. There is no point in time during a top balanced system's cycle where anything is equal other than the state of charge at the end of the charging cycle. When the voltage is essentially the same at 80% SOC and 20% SOC, nothing about the voltage of a cell helps in determining the remaining power available.

But bottom balancing has a simple rule: keep all cells with equal energy. This makes the voltage significant.

ga2500ev


----------



## EVfun (Mar 14, 2010)

ga2500ev said:


> But there's a huge catch-22 here. This operates off the presumption that the total cells that make up the battery are in fact balanced. But in fact they are not when top balanced because each total cell has a different capacity. This can create situations where by the time the main group of cells are charge and their voltages start to rise, the lowest capacity cell could be at 3.8-4.0V already. And this overcharging would happen each and every charge cycle. It's another potential failure mode.


You are trying to invent a new definition of balanced. Balanced, top or bottom, only means that they are at the same state of charge at that point. There is no pack where all the cells have the exact same capacity, no pack will be balanced at both 2.7 and 3.6 volts per cell. If the cell impedance is low enough to allow regular 5C peak discharge rates then the effect it is having at 0.1C is not going to result in any significant imbalance.

Top balancing *prevents* any one cell from rising significantly above the others at the end of charge. All the cells are one "main group" at the 3.6+ volt range. You have described a potential weakness of bottom balancing. It is easy to check periodically that cells are remaining top balanced too, just charge the pack a little extra until the cells are around 3.6 volts and check their voltages. 

How many people have checked to see if their bottom balanced pack is still bottom balanced after 1000 cycles? We are working on the assumption all cells change the same over time. That seems true enough for young cells that haven't been abused. I'm not terribly comfortable with that long term until a few packs run BMSless for 10 years or 2000 cycle and are then checked for balance and capacity.


----------



## ga2500ev (Apr 20, 2008)

EVfun said:


> You are trying to invent a new definition of balanced. Balanced, top or bottom, only means that they are at the same state of charge at that point.


Cells can only have the same state of charge if they have the same capacity.


> There is no pack where all the cells have the exact same capacity,


Exactly.



> no pack will be balanced at both 2.7 and 3.6 volts per cell. If the cell impedance is low enough to allow regular 5C peak discharge rates then the effect it is having at 0.1C is not going to result in any significant imbalance.


Here's where I disagree with you. While I agree that no two cells will have the same state of charge at 2.7 and 3.6V, with bottom balancing, they can have exactly the same amount of energy despite the differences in the the SOC.

The key item with bottom balancing is that the cells all run out of energy at the same time. Because the capacities are different, they will be at different states of charge the entire time. But the energy contained will be the same.



> Top balancing *prevents* any one cell from rising significantly above the others at the end of charge. All the cells are one "main group" at the 3.6+ volt range. You have described a potential weakness of bottom balancing. It is easy to check periodically that cells are remaining top balanced too, just charge the pack a little extra until the cells are around 3.6 volts and check their voltages.


It's not a weakness as I see it. For top balancing, it's not a periodic check. It's a check that must be done each and every time the battery is charged, and must be done for each and every cell of the battery each and every time. The whole point of the BMS is to do this monitoring. With top balancing each charge puts a different amount of energy into each of the cells. Each will start off with a 100% SOC. However, because they each have different capacities, as soon as you start discharging, the SOC's of each cell goes ragged and is different from the SOC of each of the other cells. So now you're in a situation where both the SOC and the amount of energy for each of the cells is different. And this happens in a discharging profile where the voltage is virtually constant all the way through the discharge.

I see that when discussing top balancing, the discussion is always on the charging end. That's not where the problem arises. The problem arises at the end of the discharge cycle. Honestly, if there is never a situation where the pack will be deeply discharged, the top balancing is the way to go. The problem is that if any cell is ever fully discharged in this situation, it will be destroyed by the other cells in the battery.



> How many people have checked to see if their bottom balanced pack is still bottom balanced after 1000 cycles? We are working on the assumption all cells change the same over time. That seems true enough for young cells that haven't been abused. I'm not terribly comfortable with that long term until a few packs run BMSless for 10 years or 2000 cycle and are then checked for balance and capacity.


Bottom balanced packs have to have regular maintenance checks for precisely the reasons that you outline above. But they are fairly simple. If the pack has never been overcharged or overdischarged, then simply do a normal charge cycle and make sure that the smallest capacity cell reaches its target voltage first. As long as that's the case, then all the cells still are holding the same energy, and simply drive along. If not, then you have to bottom balance again (A pain without a doubt), and locate the new lowest capacity cell and reset the charger to cut off at the pack voltage when this cell reaches the target.

Nothing is a set and forget situation. Running blind without a BMS is always dangerous territory. Bottom balancing makes it simpler to do.

ga2500ev


----------



## mora (Nov 11, 2009)

ga2500ev said:


> With top balancing each charge puts a different amount of energy into each of the cells. Each will start off with a 100% SOC. However, because they each have different capacities, as soon as you start discharging, the SOC's of each cell goes ragged and is different from the SOC of each of the other cells. So now you're in a situation where both the SOC and the amount of energy for each of the cells is different. And this happens in a discharging profile where the voltage is virtually constant all the way through the discharge.


I understand that every cell is different, even though it might be very minor difference, but how come charger puts different amount of energy into each cell if they are in series? I see this is possible if shunt regulators burn some energy as heat.

Even in top balance scheme every single cell is not at 100% SOC. Not if it is done by setting every cell to same voltage.

Amount of energy taken off and put back into pack remains the same unless there are shunt regulators or something that drains cells unevenly. I assume that all monitoring circuits draw exactly same amount of power. Only SOC differs between cells then.


----------



## EVfun (Mar 14, 2010)

EVfun said:


> Top balancing *prevents* any one cell from rising significantly above the others at the end of charge. All the cells are one "main group" at the 3.6+ volt range. You have described a potential weakness of bottom balancing. It is easy to check periodically that cells are remaining top balanced too, just charge the pack a little extra until the cells are around 3.6 volts and check their voltages.





ga2500ev said:


> It's not a weakness as I see it. For top balancing, it's not a periodic check. It's a check that must be done each and every time the battery is charged, and must be done for each and every cell of the battery each and every time. The whole point of the BMS is to do this monitoring. With top balancing each charge puts a different amount of energy into each of the cells. Each will start off with a 100% SOC. However, because they each have different capacities, as soon as you start discharging, the SOC's of each cell goes ragged and is different from the SOC of each of the other cells. So now you're in a situation where both the SOC and the amount of energy for each of the cells is different. And this happens in a discharging profile where the voltage is virtually constant all the way through the discharge.
> 
> I see that when discussing top balancing, the discussion is always on the charging end. That's not where the problem arises. The problem arises at the end of the discharge cycle. Honestly, if there is never a situation where the pack will be deeply discharged, the top balancing is the way to go. The problem is that if any cell is ever fully discharged in this situation, it will be destroyed by the other cells in the battery.


You don't have to check each and every time the battery is charged with a top balanced pack. If you do not have a BMS on the pack the cells receive the give the same amount of charge every cycle. You charge them all full once and they will return there each charge. Generally you want to fully charge them individually for the top balance (3.60 volts at 0.05 C) and then you set a somewhat lower point for cyclic use (3.5 volts at 0.1C is what I was using.) Occasionally you can check the cell voltages near the end of charge to verify they are staying in line. 

Currently my pack is nearly inaccessible, so I am running shunt regulators with a red LED that lights at 3.6 volts (when the shunt turns on.) I can look at the LEDs with a mirror. I have set up the charger so it barely reaches a high enough voltage to do that. If some LEDs where to fail to light then other cells would be higher in voltage. 

One thing about top balancing is that you could loose a cell or two to an internal short and none of the rest of the cells would get too high on the charge cycle. This is because at the top is where the voltage is in line. I trust my right foot more than my charger or my cells. My right foot is only about 3 feet down from my wallet.


----------



## Jerry Liebler (Feb 1, 2011)

I see in reading through this thread repeated assertions that a pack that is "bottom balanced" behaves somehow differently than a pack that is "top balanced. this is simple BS! The only thing balanced by either balancing method is the cell voltage! Each and every cell has a unique SOC vs cell voltage and it is constant over time (or individual cell monitoring would ALWAYS be required). Charge is amp hours or coulombs, it is NOT energy! Cells charged or discharged in series connection receive or loose IDENTICAL amounts of charge NOT ENERGY! The energy stored in each cell is the area under it's state of charge versus cell voltage graph! With either balancing technique TWO voltage endpoints are necessary to keep each an every cell in the pack in it's "safe" range. For bottom balanced packs it is well understood that an upper limit on pack voltage (determined by identifying the first cell to hit the upper limit of it's "safe operating range) needs to be set. What is not understood is that a similar limit, on total pack voltage must be established and observed for the top balanced pack (by a discharge test to identify the first cell to hit the lower limit of it's "safe" operating range and establishing the corresponding pack voltage as the ABSOLUTE minimum. A shunting BMS repeats the top balancing with every charge cycle. Running without individual cell monitoring depends on the fact that cell characteristics are stable and proper limits on pack voltage excursions are observed by TWO tests during "commissioning". Balancing the cells at either end REQUIRES a test at the other end. Either balancing technique will identify the SAME lowest capacity cell! A shorted cell failure will have the SAME" impact on a pack "commissioned" with either top or bottom balancing in that the limit at the other extreme will cause the remaining cells to "share" the dead cells normal voltage.


----------



## EVfun (Mar 14, 2010)

Jerry Liebler said:


> A shorted cell failure will have the SAME" impact on a pack "commissioned" with either top or bottom balancing in that the limit at the other extreme will cause the remaining cells to "share" the dead cells normal voltage.


I would have to disagree with that. A bottom balanced pack will see all the cells drop off together at the low end of a charge cycle, but not all rise the same at the charged end. A loss of a cell would not be "shared," it will primarily be felt by the cells closest to full. In the same way, a top balanced pack has to be watched if you want to push the bottom of a charge, because they won't be dropping off the same. If all the cells are close enough to the same it won't be an issue -- either way.

My pack generally only drops to 2.8 vpc at 5C. I set my controller minimum pack voltage to 2.5 vpc. It doesn't interfere with normal driving until the last 10% of available charge. In my range test I found the car not worth driving (serious acceleration limitations) before any of the cells where below 3.0 volt after the drive.


----------



## Jerry Liebler (Feb 1, 2011)

EVfun said:


> I would have to disagree with that. A bottom balanced pack will see all the cells drop off together at the low end of a charge cycle, but not all rise the same at the charged end. A loss of a cell would not be "shared," it will primarily be felt by the cells closest to full. In the same way, a top balanced pack has to be watched if you want to push the bottom of a charge, because they won't be dropping off the same. If all the cells are close enough to the same it won't be an issue -- either way.
> 
> My pack generally only drops to 2.8 vpc at 5C. I set my controller minimum pack voltage to 2.5 vpc. It doesn't interfere with normal driving until the last 10% of available charge. In my range test I found the car not worth driving (serious acceleration limitations) before any of the cells where below 3.0 volt after the drive.


You can, INCORRECTLY diss-agree if you choose. Think of a simple case of a ten cell pack. With pack voltage limits of 35 volts and 25 volts with all cells present and exactly matched, each cell sees a voltage range of 3.5 (35/10) volts to 2.5 (25/10) volts. Short one cell and keep those limits and the exactly matched cells will be seeing 3.5/9=3.88 volts at the top! BINGO OVERCHARGE DESTRUCTIVE!


----------



## Duncan (Dec 8, 2008)

Jerry Liebler said:


> You can, INCORRECTLY diss-agree if you choose. Think of a simple case of a ten cell pack. With pack voltage limits of 35 volts and 25 volts with all cells present and exactly matched, each cell sees a voltage range of 3.5 (35/10) volts to 2.5 (25/10) volts. Short one cell and keep those limits and the exactly matched cells will be seeing 3.5/9=3.88 volts at the top! BINGO OVERCHARGE DESTRUCTIVE!


I see where you are missing the point

_Short one cell _

That is not a balance problem but a single cell failure problem - as such it needs a different solution
I like the Battbridge solution
http://www.evdl.org/pages/battbridge.html

If running with a top/bottom balanced pack you can eliminate the need for a BMS (with its attendant cost and risks) but you still need method to warn if a cell has failed


----------



## Jerry Liebler (Feb 1, 2011)

Duncan said:


> I see where you are missing the point
> 
> _Short one cell _
> 
> ...


What point am I missing??

I have simply made the assertion that one can create a "manual BMS" by a commissioning process that initially balances the individual cell voltages and establishes two limits on the pack's total voltage, an upper limit and a lower limit. The purpose of the balancing step is to maximize usable capacity between the limits so it is done at either end of the SOC vs cell voltage "curve" where the rate of change of voltage per unit of SOC is greatest. After the "commissioning", operation between the set limits guarantees that the individual cells are in their "safe" operational range if ALL CELLS SEE IDENTICAL CURRENT (series connection with ZERO parasitic load imbalance ), and each cells SOC vs voltage graph remains the same.
IT MAKES NO DIFFERENCE WHATSOEVER WHICH END WAS BALANCED! However, a SINGLE cell failure cancels the guarantee and may lead to destruction of the remaining cells.
Thank you for the link, Battbridge is an excellent tool, which quite elegantly, will detect a single cell's degradation.


----------



## kennybobby (Aug 10, 2012)

I think Jerry has made some excellent observations and i agree with his pont. We recently experienced exactly this shorted-cell scenario in a bottom balanced pack in Paul's Celica. 

Last year we spent many hours to accurately measure the charge capacity of each and every cell and exactly bottom balance with mV precision, etc. The pack was 'commissioned' and put into service, and promptly run to empty within the first month of operation on two occassions when exuberant enjoyment and expectations exceeded range. There was no onboard voltage measurement at the time since the assumption was that a bottom-balanced pack would protect itself--they would all go empty at the same time and the car would just stop, no harm no foul. Only ran it empty twice. 

Over time gauges were added to monitor pack voltage and it seemed that the after-charging voltage of the pack was down about 3 volts-- measured all the cells and found one dead cell, 0V dead short. 

Dissassembled the cell and found that all the copper had plated onto the aluminum foils. It seems that if a cell is pulled below 2.0 V then copper ions go into electrolyte solution. Then when the voltage is brought back up via charging the copper plates out onto whatever surface it is near. Over time the copper plated until the cell shorted internally. What was amazing is that the vehicle could still run and that the short was strong enough to carry full load currents without fusing the thin foils of the current collector tabs. 

Anyway a good lesson was learned the hard way with minor expense and no fire or destruction. 

From what i have read it appears that the opposite plating reaction can occur due to overcharging--that aluminum will plate out onto the copper foils. And the end result of this failure mode is a thermal event. Anybody have experience with this?


----------



## EVfun (Mar 14, 2010)

Jerry Liebler said:


> You can, INCORRECTLY diss-agree if you choose. Think of a simple case of a ten cell pack. With pack voltage limits of 35 volts and 25 volts with all cells present and exactly matched, each cell sees a voltage range of 3.5 (35/10) volts to 2.5 (25/10) volts. Short one cell and keep those limits and the exactly matched cells will be seeing 3.5/9=3.88 volts at the top! BINGO OVERCHARGE DESTRUCTIVE!


Try that thought experiment again with a 39 cell pack that is regularly charged to 3.5 vpc. The rest don't even reach the 3.6 vpc level used to initially balance the pack. 

I would also point out that 3.88 vpc isn't instant destruction. It isn't good for their life, particularly if repeated, but it doesn't ruin them unless you hold them to that voltage for an extended time. As a test I've charged a cell to 4.00 volts and then cycled it. One such overcharge didn't produce any measurable change in the cell capacity.


----------



## Moltenmetal (Mar 20, 2014)

One note of caution about the batt bridge, which is a simple and elegant solution: note that most of the superbright LEDs I've played with are 3 v rather than 1.5 volt devices. Test first! A 3v led will miss the problem you're trying to detect.


----------



## Jerry Liebler (Feb 1, 2011)

EVfun said:


> Try that thought experiment again with a 39 cell pack that is regularly charged to 3.5 vpc. The rest don't even reach the 3.6 vpc level used to initially balance the pack.
> 
> I would also point out that 3.88 vpc isn't instant destruction. It isn't good for their life, particularly if repeated, but it doesn't ruin them unless you hold them to that voltage for an extended time. As a test I've charged a cell to 4.00 volts and then cycled it. One such overcharge didn't produce any measurable change in the cell capacity.


 Sure the AVERAGE cell voltage is fully charged to 3.592 voltswith 38 of 39 cells good and one shorted. Now discharge the pack to LVC of 2.5X39=97.5 and we'll have 97.5/38 = 2.56 volts per cell. There still is NO difference whether the pack was top or bottom balanced! The added cells avoid catastrophe for an unmonitored pack.


----------



## EVfun (Mar 14, 2010)

The difference is where you find the ragged edge of the pack. If the cells are top balanced then over-discharging pack quickly results in widely varying cell voltages. If the cells are bottom balanced then overcharging quickly results in widely varying cell voltages. 

You should build an EV with a LiFePO4 pack, drive it, and find out how the cells behave.


----------



## ga2500ev (Apr 20, 2008)

ga2500ev said:


> With top balancing each charge puts a different amount of energy into each of the cells. Each will start off with a 100% SOC. However, because they each have different capacities, as soon as you start discharging, the SOC's of each cell goes ragged and is different from the SOC of each of the other cells. So now you're in a situation where both the SOC and the amount of energy for each of the cells is different. And this happens in a discharging profile where the voltage is virtually constant all the way through the discharge.





mora said:


> I understand that every cell is different, even though it might be very minor difference, but how come charger puts different amount of energy into each cell if they are in series? I see this is possible if shunt regulators burn some energy as heat.


Because each cell has a different capacity. Top balancing charges each cell to its maximum capacity.

Say that we're working with three 100 Ahr cells. Cell 1 has a capacity of 99.7 Ahr, cell 2 101.2 Ahr, and cell thre 101.7 Ahrs. They are wired into a series string with a shunting BMS.

When you charge the string, cell 1 will fill to capacity first. It's voltage will rise off the mainline to the cutoff for the shunt. The shunt will bypass the cell and only send energy to the two remaining charging cells. Cell 1 is at a 100% SOC, and is no longer charging because of the shunt.

Eventually Cell 2 will charge too and the shunt will bypass it so that only Cell 3 continues charging. Cell 3 finishes and the charger shuts off.

Each cell is at a 100% SOC, but each carries a different amount of energy.

On discharge cell 1 will empty first and its voltage starts to drop off. Unless caught by a low voltage cutoff, the other cells will continue to drive the vehicle until they empty. But by that time cell 1 has been destroyed by overdischarge.

Of course there are ways to manage this, but it requires cell level monitoring complexity to pull it off. If the BMS fails, cell destruction occurs.

With bottom balancing, the battery is charged until cell 1 fills to 99.7 Ahrs. The charger is then cut off. Presuming that the cells are all balanced in terms of energy, cells 2 and 3 also have 99.7 Ahrs of energy. Now in an over discharge situation, they all run out of energy together. Also the low voltage cut off can really work because the voltage dropoff is 3x that of a single cell.



> Even in top balance scheme every single cell is not at 100% SOC. Not if it is done by setting every cell to same voltage.


The voltage only rises above the mainline charging voltage when it's full. So it definitely is at 100% SOC for top balancing.



> Amount of energy taken off and put back into pack remains the same unless there are shunt regulators or something that drains cells unevenly. I assume that all monitoring circuits draw exactly same amount of power. Only SOC differs between cells then.


The SOC and the capacity are linked. As outlined above the shunt bypasses a full lower capacity cell to charge a not full higher capacity cell. Only when all cells are charged are they all at 100% SOC.

ga2500ev


----------



## mora (Nov 11, 2009)

I have an example here. Or two actually. Two cells reach their rated capacity, 90Ah, when charged to 4.00V per cell. Other cells I have are very close or more than their rated capacity at 3.65V. If I put them in series and top balance to 3.65V per cell then those two exceptions are not at 100% SOC when charger stops.

If every cell would need be charged to 100% SOC then their individual management boards (BMS) should adapt to that exact cell. This might mean different charge voltage for every cell, even in top balance scheme.

But then, if 100Ah cell at 100% SOC actually has 101Ah, does it make it 101% SOC? Or if that cell receives only 100Ah is it really 99% SOC? Hehehe. I'm saying that 100% SOC is reached when cell is charged to its rated capacity. If this is not true then some cells never reach their rated capacity and remain at less than 100% SOC.

I'm splitting some hair here. Shunt resistor itself doesn't charge other cells. Only active BMS could transfer charge from one cell to another.


----------



## ga2500ev (Apr 20, 2008)

Jerry Liebler said:


> I see in reading through this thread repeated assertions that a pack that is "bottom balanced" behaves somehow differently than a pack that is "top balanced. this is simple BS! The only thing balanced by either balancing method is the cell voltage!


I don't think so sir. I've been very consistent in my discussions. Bottom balancing puts the same amount of (now fixing my mistake) charge into each cell, regardless of voltage or state of charge. A charged bottom balanced pack will have each cell at a different state of charge and possible varying voltages.


> Each and every cell has a unique SOC vs cell voltage and it is constant over time (or individual cell monitoring would ALWAYS be required).


I can agree to this if you change the phrase to "SOC changes slowly over time for a single cell."


> Charge is amp hours or coulombs, it is NOT energy! Cells charged or discharged in series connection receive or loose IDENTICAL amounts of charge NOT ENERGY!


I do apologize for playing fast and loose with the terminology. This page on electrical potential describes the difference. Be sure to take a look at the example of the AA battery as opposed to the D battery.


> The energy stored in each cell is the area under it's state of charge versus cell voltage graph!


Which defines the amount of work that can be done. 

Even though I was mistaken in the naming of my units, the concept still works because of the cell chemistry on disharge has a nearly flat terminal voltage curve regardless of the state of charge.


> With either balancing technique TWO voltage endpoints are necessary to keep each an every cell in the pack in it's "safe" range. For bottom balanced packs it is well understood that an upper limit on pack voltage (determined by identifying the first cell to hit the upper limit of it's "safe operating range) needs to be set. What is not understood is that a similar limit, on total pack voltage must be established and observed for the top balanced pack (by a discharge test to identify the first cell to hit the lower limit of it's "safe" operating range and establishing the corresponding pack voltage as the ABSOLUTE minimum. A shunting BMS repeats the top balancing with every charge cycle. Running without individual cell monitoring depends on the fact that cell characteristics are stable and proper limits on pack voltage excursions are observed by TWO tests during "commissioning". Balancing the cells at either end REQUIRES a test at the other end. Either balancing technique will identify the SAME lowest capacity cell! A shorted cell failure will have the SAME" impact on a pack "commissioned" with either top or bottom balancing in that the limit at the other extreme will cause the remaining cells to "share" the dead cells normal voltage.


Can you clarify exactly what happens on the discharge end for both techniques? It's clear to me that with bottom balancing that cells have equal charge. So on discharge, regardless of the individual cell terminal voltages that all the cells wired in series must exhaust all their charge at the same time. If this is incorrect, please explain what really happens.

On the other hand shunting facilitates different cells to have different absolute charge amounts (not state of charge, as that is the ratio of charge to capacity)
So on discharge cells will drain of charge at different times. So it would seem that a battery terminal voltage measurement would be unreliable.

Any assistance in understanding would be appreciated.

ga2500ev


----------



## Jerry Liebler (Feb 1, 2011)

ga2500ev said:


> Because each cell has a different capacity. Top balancing charges each cell to its maximum capacity.
> 
> Say that we're working with three 100 Ahr cells. Cell 1 has a capacity of 99.7 Ahr, cell 2 101.2 Ahr, and cell thre 101.7 Ahrs. They are wired into a series string with a shunting BMS.
> 
> ...



ENERGY IS IRRELEVANT! Series connected cells ALL see Identical CURRENT FLOW! Current flow has a time integral of CHARGE. SERIES CONNECTED CELLS EXPERIENCE IDENTICAL CHARGE!!!! Each cell of a series pack (without shunting) delivers or absorbs a, potentially at least, different amount of ENERGY in the course of being charged or discharged, but they all have an IDENTICAL amount of CHARGE change. 

The problem here is ignoring the need to set a lower limit on pack voltage during discharge, even if one has "BMS" hardware!


----------



## Jerry Liebler (Feb 1, 2011)

ga2500ev said:


> I don't think so sir. I've been very consistent in my discussions. Bottom balancing puts the same amount of (now fixing my mistake) charge into each cell, regardless of voltage or state of charge. A charged bottom balanced pack will have each cell at a different state of charge and possible varying voltages.
> 
> I can agree to this if you change the phrase to "SOC changes slowly over time for a single cell."
> 
> ...


Each and every cell has it's own charge vs terminal voltage 'characteristic' which changes slowly or not at all over time and use. At an individual cell level they behave the same over thousands of charge discharge cycles. The pack is the total of the cells, however while the cells are in series with no shunting they all will receive the same current and thereby the same changes in the charge they hold. At any time the pack voltage is the sum of the cell's voltages. All "balancing" means is adjusting the SOC to reflect equal cell voltage. As use takes the pack from the "balanced" condition, the lowest capacity cell will show itself by it's voltage changing faster than the others. It maters not whether the balance was " top" and the following cycle was discharge or the balance was "bottom" and the following cycle was charge, the weakest cell leads the pack.

Let me correct a miss understanding. The cells in a bottom balanced pack, at the balance point have whatever charge their characteristic says they will have AT THAT VOLTAGE, the amount of absolute charge each cell contains is not necessarily equal at all!.


----------



## kennybobby (Aug 10, 2012)

*Can a shunt really bypass a cell?*



ga2500ev said:


> ... When you charge the string, cell 1 will fill to capacity first. It's voltage will rise off the mainline to the cutoff for the shunt. [The shunt will bypass the cell and only send energy to the two remaining charging cells.] Cell 1 is at a 100% SOC, and is no longer charging because of the shunt.
> 
> ... The SOC and the capacity are linked. As outlined above the shunt bypasses a full lower capacity cell to charge a not full higher capacity cell..
> 
> ga2500ev


Are the shunt circuit and the cell in parallel? Even if the shunt resistance were lower than the internal resistance of the battery, there would still be current flowing thru the cell. There may be some off-loading of current thru the shunt, but it can't really bypass the cell can it? How does this affect your scenario?


----------



## Jerry Liebler (Feb 1, 2011)

*Re: Can a shunt really bypass a cell?*



kennybobby said:


> Are the shunt circuit and the cell in parallel? Even if the shunt resistance were lower than the internal resistance of the battery, there would still be current flowing thru the cell. There may be some off-loading of current thru the shunt, but it can't really bypass the cell can it? How does this affect your scenario?


During charging, the internal resistance of a cell rises rapidly as a fully charged state approaches. Eventually a situation develops where attempting to add more charge results in just heat and cell destruction. Assuming a constant voltage charging source and series connected cells the rise in internal resistance, of the first cell to reach fully charged, limits charge current to the remaining cells. The whole purpose of the shunts is to allow the higher capacity (those below the shunting voltage) to be charged more fully.


----------



## ga2500ev (Apr 20, 2008)

Jerry Liebler said:


> Each and every cell has it's own charge vs terminal voltage 'characteristic' which changes slowly or not at all over time and use. At an individual cell level they behave the same over thousands of charge discharge cycles. The pack is the total of the cells, however while the cells are in series with no shunting they all will receive the same current and thereby the same changes in the charge they hold. At any time the pack voltage is the sum of the cell's voltages. All "balancing" means is adjusting the SOC to reflect equal cell voltage. As use takes the pack from the "balanced" condition, the lowest capacity cell will show itself by it's voltage changing faster than the others. It maters not whether the balance was " top" and the following cycle was discharge or the balance was "bottom" and the following cycle was charge, the weakest cell leads the pack.
> 
> Let me correct a miss understanding. The cells in a bottom balanced pack, at the balance point have whatever charge their characteristic says they will have AT THAT VOLTAGE, the amount of absolute charge each cell contains is not necessarily equal at all!.


This is helpful. I hope that you don't mind me summarizing:

1) Charging/Discharging a series string of cells without shunting will add/remove the same amount of charge from each cell.

2) A bottom balanced set of cells will have different absolute charges at a certain voltage (say 2.6V). However, it is presumed that if you take a series string of those cells and cycle them with the same amount of charge in a charge/discharge cycle, that all those cells will return to nearly the same voltage.

3) The characteristics of the cells (voltage at a certain charge point, capacity) may change but slowly over time.

So I guess my last question is that if the string is of similar cells (same nominal capacity, same chemistry, same manufacturer) that it's possible to extrapolate the charge to voltage relationship? In other words if you take a bottom balanced pack (all cells starting at 2.6V for example) and charge/discharge the pack to 3.0V/cell, what will be the spread on the terminal voltage of the individual cells? How likely will one be a 2.7V and another still mainlined at 3.3V when both still reach 2.6V together?
I wonder about this because 2.9-3.1V/cell is the usual window for the low voltage cutoff. It would be helpful to have assurances that a bottom balanced pack will behave and all the cells drop through that window at nearly the same time.

ga2500ev


----------



## ga2500ev (Apr 20, 2008)

Jerry Liebler said:


> ENERGY IS IRRELEVANT! Series connected cells ALL see Identical CURRENT FLOW! Current flow has a time integral of CHARGE. SERIES CONNECTED CELLS EXPERIENCE IDENTICAL CHARGE!!!! Each cell of a series pack (without shunting) delivers or absorbs a, potentially at least, different amount of ENERGY in the course of being charged or discharged, but they all have an IDENTICAL amount of CHARGE change.
> 
> The problem here is ignoring the need to set a lower limit on pack voltage during discharge, even if one has "BMS" hardware!


I thought that I had apologized for this energy/charge confusion in an earlier post.

The low voltage cutoff only works if all the cells' terminal voltages in the pack remain tightly grouped together. Trying to get clarity on absolute charge, change in charge, state of charge, and terminal voltage/charge relationships is the key to understanding how each type of balancing works on both the top and bottom ends of the charge/discharge scale.

ga2500ev


----------



## IamIan (Mar 29, 2009)

*Re: Can a shunt really bypass a cell?*



Jerry Liebler said:


> During charging, the internal resistance of a cell rises rapidly as a fully charged state approaches.


Terminal Voltage rises ... Internal resistance actually decreases at higher SoC.

See attached

- - - - - - 
Also a note to the casual reader:
Energy is not irrelevant to SoC change.
A higher resistance cell will heat more ... even in series ... and even with identical resistance the cells in the center are more insulated ... any temperature difference in a series pack can result in different SoC% changes from the same __Ah that flowed.

See Attached


----------



## Jerry Liebler (Feb 1, 2011)

*Re: Can a shunt really bypass a cell?*



IamIan said:


> Terminal Voltage rises ... Internal resistance actually decreases at higher SoC.
> 
> See attached
> 
> ...


NO, NO, NO! Energy heat temperature are absolutely irrelevant If the cells are in series with no parasitic external shunting they all have EXACTLY the same change in stored charge (Amp hours). The series connection forces the current to be IDENTICAL!!!!!


----------



## Jerry Liebler (Feb 1, 2011)

ga2500ev said:


> This is helpful. I hope that you don't mind me summarizing:
> 
> 1) Charging/Discharging a series string of cells without shunting will add/remove the same amount of charge from each cell.
> 
> ...


You gave a fine summary!
Short answer to your question is yes but it takes a detailed testing. 
In your example you picked 3.0 volts, for reasons I do not understand. The Lifepo4 cells have a cell voltage vs state of charge that changes very little over 90%, or more, of their capacity with open circuit room temperature cell voltages in this region only changing from 3.20 volts to 3.00volts. This is the region you want to operate within. You want to balance the cells outside this region because the voltage change per unit of charge is much greater. I think your question is: if I I balance a pack of a reputable manufacturer's cells at 2.6 volts, then charge them till I see the first cell hit 3.5 volts at which time I measure the pack voltage and reset the charger with this voltage as the final charge voltage. Then I diss- charge the pack, what is it safe to set the lowest discharge voltage to? If that is the question the answer is ANY voltage greater than 2.6 times the number of cells.


----------



## bwjunkie (Jul 31, 2013)

*Re: Can a shunt really bypass a cell?*



IamIan said:


> Terminal Voltage rises ... Internal resistance actually decreases at higher SoC. See Attached


Which graph shows Resistance to SoC? 
I only see temp graphs.
-josh


----------



## major (Apr 4, 2008)

*Re: Can a shunt really bypass a cell?*



bwjunkie said:


> Which graph shows Resistance to SoC?
> I only see temp graphs.
> -josh












Resistance is on the y-axis (vertical). SOC is the family of curves. You can see that the lower red line representing 100% SOC is the lowest resistance for all the temperature range.

I thought Iam did a nice short post with some relevant data. It would have been nice for him to state his source. I too thought Jerry was off base with his statement that resistance rises with SOC. For practical purposes I thought it remained about the same. You can see from the graphs that it does not change much at 25ºC until you get nearly drained. 

This also shows nicely how drastic the change is for cold temperature. 

Iam? What cells were these? Was there any indication this is representative for other Lithium types?


----------



## tomofreno (Mar 3, 2009)

*Re: Can a shunt really bypass a cell?*



major said:


> Iam? What cells were these? Was there any indication this is representative for other Lithium types?


 Caption at the bottom of the figure says A123: _ResistanceVsTemperatureA123.JPG (1 of 2)_
Maybe you meant pouch or cylindrical type? Would guess most LiFePO4 behave similarly in terms of temperature. It's been pointed out before that there isn't much change with temperature above about 10C (50F).

I think Jerry was likely referring to the upper end of the charge curve (exponential part of V versus Ah) when he said cell resistance increases with SOC.


----------



## ga2500ev (Apr 20, 2008)

ga2500ev said:


> This is helpful. I hope that you don't mind me summarizing:
> 
> 1) Charging/Discharging a series string of cells without shunting will add/remove the same amount of charge from each cell.
> 
> ...





Jerry Liebler said:


> You gave a fine summary!
> Short answer to your question is yes but it takes a detailed testing.
> In your example you picked 3.0 volts, for reasons I do not understand. The Lifepo4 cells have a cell voltage vs state of charge that changes very little over 90%, or more, of their capacity with open circuit room temperature cell voltages in this region only changing from 3.20 volts to 3.00volts. This is the region you want to operate within. You want to balance the cells outside this region because the voltage change per unit of charge is much greater. I think your question is: if I I balance a pack of a reputable manufacturer's cells at 2.6 volts, then charge them till I see the first cell hit 3.5 volts at which time I measure the pack voltage and reset the charger with this voltage as the final charge voltage. Then I diss- charge the pack, what is it safe to set the lowest discharge voltage to? If that is the question the answer is ANY voltage greater than 2.6 times the number of cells.


The 3.0 was arbitrary to a point. I wanted to discuss the region near the knee of the discharge curve. At 3.3V the cell is at a mainline discharge voltage. There's only minimal changes until you get to the knee. At 2.5-2.7V the cell is significantly drained. However, at 3.0V the cell has a state of charge where it can continue to operate (as you pointed out), but is on the knee of the discharge curve. I was wondering would a bottom balanced matched set of cells all be in the ballpark of that voltage all together or would there be significant voltage spread across the knee.

I'd have the same question for lower down the curve too. If the pack is at 2.75V*numberofcells, then is it reasonable to expect all the cells to be near 2.75V?

ga2500ev


----------



## dougingraham (Jul 26, 2011)

There is no low voltage cutoff that is usable when the temp is low. Just set your controller to 1.6 volts per cell. That is the maximum power point. In my case this would be 82 volts but my DC-DC doesn't like going below 90 volts much so I just set it to 90 volts. When it is warm you will never get anywhere near that. When it is cold out you won't be able to reasonably drive the car with a low voltage cutoff of anything higher than 2 volts per cell. The sag is simply too severe. By cold I mean around freezing (0C or less).

Best Wishes!


----------



## IamIan (Mar 29, 2009)

*Re:*



major said:


> I thought Iam did a nice short post with some relevant data.


Thanks 
Short posts are not one of my strengths (as some may already know)... I'm trying to improve.


Sadly too much to cover (reply to) , for the bellow to be as short.




major said:


> It would have been nice for him to state his source.


A power point summary of key points from a paper by:
Dr. Languang Lu

August 2011

Title of paper was:
LiFePO4 battery performance testing and analysis for BMS 

I don't think it was A123 specifically ... the power point summary I have doesn't specially list the 97 cells they used for testing ... but sense in other power point slides they show ~70Ah cell discharge capacity , I doubt A123 specifically ... like most things different brands , etc ... may give slightly different results ... but the core / general chemistry concepts was what I was pointing to... I was not claiming an exact match to those graphs.

The concept does generally agree with my own testing of 55 of the A123 (20Ah) Pouch cells in particular ... although it is far more pronounced effect in the previously posted graph ... the low temperature testing makes the SoC point separation far more pronounced than the room temperature SoC testing I've done myself ... but at room temperature with my own testing I only saw ~1mOhm variation in the Resistance ... the PL8 did show a variation in Ohms of Impedance also.

It's one of the same references I brought up previously in other LiFePO4 discussions ... although I think last time the piece I referenced from it was the variation in cycle efficiency SoC / SoE at different rates ... maybe you'll recognize that one ... (see attached "CycleEfficiency")

Slightly different concept ... but along a similar over all concept of variation with varying % of battery fill ... My own testing has also shown a variation in cycle efficiency SoC and SoE depending on how high a % the battery is taken to during that cycle.... see attached "20AhA123CycleEfficiencyVsSoE" , and "20AhA123CycleEfficiencyVsSoC".

- - - - - - 



tomofreno said:


> I think Jerry was likely referring to the upper end of the charge curve (exponential part of V versus Ah) when he said cell resistance increases with SOC.


He might have been ... but that would still be incorrect.

The exponential dV part at the upper end of the charge curve you refer to ... is not the result of changing resistance ... as he incorrectly claimed.

- - - - - - - 



Jerry Liebler said:


> NO, NO, NO! Energy heat temperature are absolutely irrelevant If the cells are in series with no parasitic external shunting they all have EXACTLY the same change in stored charge (Amp hours). The series connection forces the current to be IDENTICAL!!!!!


I request:
Please clearly indicate (bold , color change, etc) your alterations to my (or others if you do it to anyone else) ... comments , when you claim you are quoting me (or someone else) ... Do not as you just did , in post #49 ... Altering what I wrote , and then still presenting it falsely as a quote from me.

- - - - - - 

FYI:
Repetition , and capital letters do not make you any less incorrect ... and are not persuasive.

Sense you don't seem to have put it together with the short previous post ... I'll walk you through it step by step.

#1> Apply charge current to series string battery pack.

#2> As stated previously:


IamIan said:


> any temperature difference in a series pack can result in different SoC% changes from the same __Ah that flowed


#3> A 1Ah change is a ~6.25% change in SoC for a 16Ah cell ... but that same 1Ah is a ~5.88% change in SoC for a 17Ah cell.

#4> 6.25 is not = to 5.88

#5> The effect of temperatures on a cells usable capacity does not vanish when the cells are in series.

#6> Even without the impact of temperature variation among the cells in the pack ... just having different cells of any difference in Ah of capacity will also lead to that same 1Ah not being the same % of change of the SoC ... for the same reasons and steps as shown above.


----------



## Jerry Liebler (Feb 1, 2011)

*Re:*



IamIan said:


> Thanks
> Short posts are not one of my strengths (as some may already know)... I'm trying to improve.
> 
> 
> ...



You are conflating state of charge with charge! I am not wrong on this!
In a series string of cells without shunting all cells see the IDENTICAL CURRENT and the integral of that IDENTICAL CURRENT over time results in IDENTICAL CHARGE! 

"The exponential dV part at the upper end of the charge curve you refer to ... is not the result of changing resistance ... as he incorrectly claimed."
If it walks like a duck,quakes like a duck, it just might be a duck! If the effect is not correctly called resistance please give us your preferred label. We are talking of the electronic circuit model of a cell. Resistance is the only model parameter that reflects power dissipation. Once a cell is "full" any additional charging current flow through it results only in heat or dissipation.


----------



## Jerry Liebler (Feb 1, 2011)

ga2500ev said:


> The 3.0 was arbitrary to a point. I wanted to discuss the region near the knee of the discharge curve. At 3.3V the cell is at a mainline discharge voltage. There's only minimal changes until you get to the knee. At 2.5-2.7V the cell is significantly drained. However, at 3.0V the cell has a state of charge where it can continue to operate (as you pointed out), but is on the knee of the discharge curve. I was wondering would a bottom balanced matched set of cells all be in the ballpark of that voltage all together or would there be significant voltage spread across the knee.
> 
> I'd have the same question for lower down the curve too. If the pack is at 2.75V*numberofcells, then is it reasonable to expect all the cells to be near 2.75V?
> 
> ga2500ev


The simple quick answer is YES. With the balance at one end of the charge range and set the other pack voltage limit at the other end,based on a measurement of all individual cell being within the"safe" voltage range, we are assured that all individual cell voltages will be in the "safe" range while the pack voltage remains between the voltage set at balance and the other limit. Through out the pack operating voltage range the cell voltages will have less variation than they do at the unbalanced extreme.


----------



## major (Apr 4, 2008)

*Re:*



Jerry Liebler said:


> Once a cell is "full" any additional charging current flow through it results only in heat or dissipation.


That may be, but your previous statement clearly relates to SOC less than "full". 



Jerry Liebler said:


> During charging, the internal resistance of a cell rises rapidly as a fully charged state approaches.


Which I feel is incorrect. Do you have supporting data or reference?


----------



## IamIan (Mar 29, 2009)

*Re:*



Jerry Liebler said:


> You are conflating state of charge with charge!


Re-Read previous post more carefully... I am not conflating (SoC) with Charge... the difference is clearly indicated in Step #3 of my Post #56.


Jerry Liebler said:


> I am not wrong on this!


Depends on which 'this' you refer to.


Jerry Liebler said:


> During charging, the internal resistance of a cell rises rapidly as a fully charged state approaches.


This is incorrect.

A reasonable source has been given clearly showing your error ... post a better source showing the rising internal resistance (at rising SoC) you claim ... or deal with it , that your wrong about it.


Jerry Liebler said:


> If the effect is not correctly called resistance please give us your preferred label.


It isn't about a 'preferred label' ... it is not resistance ... resistance is a specific thing , this dV is not resistance.

A battery is a chemical reaction ... this is not electronics 101 with ideal systems that get to ignore real world chemistry... in a battery the terminal voltage can change without a change to the resistance ... the terminal voltage can go up as the resistance does down ... and the terminal voltage can go down as the resistance goes up... it is not a simple V=IR system.


----------



## tomofreno (Mar 3, 2009)

*Re:*



IamIan said:


> It isn't about a 'preferred label' ... it is not resistance ... resistance is a specific thing , this dV is not resistance...


 I guess you don't consider polarization resistance "resistance", only Ohmic resistance? From page 18 of the reference you gave: 
"_The polarization resistance is strongly concern with SOC, and influenced greatly by temperature. The lower temperature and the low SOC ,the higher the polarization resistance._" 

If I recall correctly, this is the Warburg effect, diffusion limited supply of Li ions in the cathode at high discharge currents, and is reduced by using thinner films - the main difference in "power" cells and "energy" cells. It is modeled as a variable resistance.

Also, iirc the exponential rise of voltage with charge added to a cell near the end of charge is because all the available lithium is gone from the cathode, so the lithium ion current to the anode decreases - which might be modeled as an increasing resistance in the cell at end of charge.


----------



## Jerry Liebler (Feb 1, 2011)

*Re:*



IamIan said:


> Re-Read previous post more carefully... I am not conflating (SoC) with Charge... the difference is clearly indicated in Step #3 of my Post #56.
> 
> Depends on which 'this' you refer to.
> 
> ...



Simple experiment charge a SINGLE LIFEPO4 cell with a constant voltage source set at 3.6 volts watch the current, does it increase or decrease with time?
Disconnect the charger wait 24 hours and measure the cell voltage. subtract the measured voltage from 3.6 then divide by the current when charging was stopped, the result is a resistance (ohm's law). 
Discharge half the cell's capacity then recharge, again with a 3.6 volt constant voltage source but this time stop at about 5 times the final current of the first charge. Again wait 24 hours and perform the same calculation.
Perhaps it is not cell resistance but whatever it is It does allow shunt BMS circuitry to work and work well.


----------



## IamIan (Mar 29, 2009)

*Re:*



tomofreno said:


> I guess you don't consider polarization resistance "resistance", only Ohmic resistance?


Neither would make Jerry Liebler's previous claim I was replying to (in that post) correct.

Both effects decrease with increasing SoC (see previous graph) ... that is the opposite of Jerry Liebler's previous claim (I was replying to in that quote of mine you reference there).

Also the mechanism that creates the exponential dV at top end SoC being referenced in this part of the discussion ... is not caused by either of those 'resistance'... or any type of 'resistance' for that matter.

More specifically / directly toward your question ... although in the case of this discussion , the distinction would be largely academic (insignificant) ... yes , strictly speaking polarization resistance is not actually 100% accurately using the term 'resistance' (as it is not the same thing being described by Ohms Law) ... Polarization Resistance does not for example always give rise to exothermic heat generation as the same amount of actual resistance (as in Ohms law) would ... it (polarization resistance) is however (right or wrong) a common usage describing the electrochemical kinetics voltage effect in chemical reactions ... See Tafel Equation



tomofreno said:


> If I recall correctly, this is the Warburg effect, diffusion limited supply of Li ions in the cathode at high discharge currents, and is reduced by using thinner films - the main difference in "power" cells and "energy" cells. It is modeled as a variable resistance.


I don't see the connection you are making from the Warburg Effect in bio-chemistry to the LiFePO4 chemistry cells described here ... Please elaborate on you PoV?

Diffusion rates significantly increase at higher charge transfer rates (high amp current rates ) ... in the LiFePO4 cells being described here ... See Attached.

- - - - - - - - - 



Jerry Liebler said:


> Perhaps it is not cell resistance but whatever it is It does allow shunt BMS circuitry to work and work well.


Correct ... it is not resistance.

- - - - - - 

The question raised by some about shunt BMS circuitry is not about that effect ... nor about a properly functioning BMS ... it's more about the MTBF ... and weighing the net life time benefit vs alternatives.

With a 50 cell pack that has a per cell level BMS always connected ... all of those circuits would need to each have over 438,000 Hours MTBF rates just to only be expecting to have (on average) 1 failure per 1 year... ~nearly 5million hours MTBF for the circuit to have (on average) only 1 failure per 10 years ... etc... getting into the millions of hours for MTBF rates starts to become a potentially serious design and component logistical issue... especially at affordable prices compared to the cost of alternatives.

The BMS itself is not free ... there is a cost for it ( $ , Joules , Space, Weight, etc ) ... thus it get's compared to other alternatives... and for some people a cell level BMS is their choice ... while others side with the alternatives ... among the alternatives there is a functional / operational difference between a top balanced pack and a bottom balanced pack... as was discussed some previously above in this thread.

- - - - - - 

FYI the chemical process of the voltage settling you refereed to ... that is not resistance based ... that is chemical reaction based... and is the wrong kind of experiment for determining resistance.

Depending on the temperature of the cell the attached graph shows what one can expect from an experiment as you described.

- - - - - -

The kind of experiment you need to do to see a resistance based dV is under much smaller time scales ... seconds or fractions of a second ... you need to see what the dV is between the loaded cell's terminal voltage and the unloaded cells terminal voltage ... not the rested (after many hours) unloaded cells terminal voltage , just unloaded ... and you need to know accurately what that load was ... than you can use that to 'estimate' , 'effective' resistance ... which is not 100% the same as actual 'resistance' but 99% of the time the difference is not significant in the context of BEV BMS usage.

If you do that kind of proper resistance testing you will see what was shown in the previous posted graph data ... 'resistance' goes down as SoC goes up.


----------



## Jerry Liebler (Feb 1, 2011)

*Re:*



IamIan said:


> Neither would make Jerry Liebler's previous claim I was replying to (in that post) correct.
> 
> Both effects decrease with increasing SoC (see previous graph) ... that is the opposite of Jerry Liebler's previous claim (I was replying to in that quote of mine you reference there).
> 
> ...


It appears that the only resistance values you accept are during discharge (words like "loaded cell"). I have been and am talking ONLY about while charging. The rested cell reflects the state of charge while the immediate effects include what should be modeled as capacitive effects. Without a doubt these cells are complex enough that cell resistance has a different value charging and discharging.


----------



## tomofreno (Mar 3, 2009)

*Re:*



IamIan said:


> ...I don't see the connection you are making from the Warburg Effect in bio-chemistry to the LiFePO4 chemistry cells described here ... Please elaborate on you PoV?


 Description of equivalent circuit and incorporation of Warburg effect with variable resistance (edit: variable resistance is just my simple model for the effect) here:
http://www.ri.cmu.edu/video_view.html?video_id=60&menu_id=387


----------



## IamIan (Mar 29, 2009)

*Re:*



Jerry Liebler said:


> It appears that the only resistance values you accept are during discharge (words like "loaded cell").


No.
A load can be either +Amps or -Amps ... or +Watts or -Watts ... either is still a load... load as term is itself a sign-less descriptor.

Just like acceleration can be either + or - ... either would still be acceleration... or force can be + or - ... or charge can be + or - ... or voltage can be + or - ... etc.



Jerry Liebler said:


> I have been and am talking ONLY about while charging.


Doesn't change previous corrections about :
(**) = resistance vs top end SoC ... nor dCharge vs dSoC%... nor relevant effects of energy transfer (even on a series pack of cells) as noted previously.



Jerry Liebler said:


> The rested cell reflects the state of charge while the immediate effects include what should be modeled as capacitive effects.


Capacitive effects are very short lived ... and yes they are part of the short term dV ... so is the cell's inductive effects ... etc ... but only the short term testing will give you an indication of 'Resistance'.

The only way a rested cell value is relevant to resistance testing discussion is if you apply the previously described short period of time (seconds or less) type of testing to a rested cell.

example:


Rested cell V noted @ zero amps
Within a very short period of time (seconds or fractions of) , to go from zero amps to your loaded value , a CC rate ... doesn't mater if it is charging(+) or Discharging(-).
Record the dV from #1 shortly (seconds or less) after reaching CC #2.
 


Jerry Liebler said:


> Without a doubt these cells are complex enough that cell resistance has a different value charging and discharging.


Correct.
But not enough to change any the results of the previous corrections (**)... See Attached Butler-Volmer Kinetics bellow.

Short version ... I've seen charge 'resistance' as high as about ~60% above Discharge resistance ... which is partially indirectly related to the difference in effective usable pulse power rates charge vs discharge ... See graph for A123 20Ah pouch cell attached... be careful though ... that pulse power rates is not 100% about different resistance of charge vs discharge ... other factors are also involved.

- - - - - - - - - 



tomofreno said:


> IamIan said:
> 
> 
> > ...I don't see the connection you are making from the Warburg Effect in bio-chemistry to the LiFePO4 chemistry cells described here ... Please elaborate on you PoV?
> ...


Thanks for the link ... but that link took me to a basic rudimentary video on LiFePO4 chemistry... I did not see "Warburg Effect" shown or mentioned.

If you follow the link I provided previously you see that the term "Warburg Effect" referrers to one of two effects:

#A> 


Wikipedia said:


> In plant physiology, the Warburg effect is the decrease of photosynthesis by high oxygen concentration.


#B>


Wikipedia said:


> In oncology, the Warburg effect is the observation that most cancer cells predominantly produce energy by a high rate of glycolysis followed by lactic acid fermentation in the cytosol, rather than by a comparatively low rate of glycolysis followed by oxidation of pyruvate in mitochondria as in most normal cells.


The only reference I've seen at all in your link to "Warburg" at all is not about the "Warburg Effect" ... but about the "Warburg Coefficient".

Did you actually intend to refer to the "Warburg Coefficient" or "Warburg Constant" ... and not either type of the "Warburg Effect" ??? 

Warburg Coefficient:


Wikipedia said:


> The *Warburg coefficient* (or Warburg constant),
> 
> 
> 
> ...


- - - - - - - 

Are you trying to take a round about path to referring to a Peukert Effect in LiFePO4 cells?

If so ... yes , the Usable capacity (Ah or Wh) reduces at higher rates (Amps or Watts)... no argument from me about that.

However the actual diffusion rates increase at higher amp rates (as posted previously) ... the coating of electrodes is a different mechanism ... not either of the types of "Warburg Effect"... The Warbug Coefficient is used (AFAIK) in modeling the behavior of the organic solvent (Electrolyte).

- - - - - - 

As for a Peukert Effect in LiFePO4 cells ... I've seen very very little ... ie a very low 'k' value.

For my A123 20Ah pouch testing , I've consistently seen around ~0.8% loss of usable Ah (~4% Wh loss) going from 10A rates to 40A rates ... my higher Amp rate testing is not yet completed ... eventually I will have my own up to 120A testing for my batch of cells ... but from what I've seen from others who have already done that kind of high rates ... they also still see comparatively low Peukert loss rates... especially for the Ah side ... it's a bigger impact on the Wh side... but still comparatively small ... 10 to 40 only a comparatively tiny % loss.

If the resistance even just stayed the same , I would expect to see a nearly matched increase in loss with a increase in rate (amps or Watts) ... but that isn't what happens ... the amount of loss is much less than the amount of increased rate (amps or watts).

A similar effect is also seen indirectly in the previously posted Ah and Wh cycle efficiency effects at different rates ... higher rates yielding lower cycle efficiencies Wh ... but again we see , the amount of loss is less than the amount of increased rate... and partially due to the increased diffusion rates we see a net higher rate of Ah cycle efficiency at higher Amp rates.


----------



## tomofreno (Mar 3, 2009)

*Re:*

Warburg solved the diffusion equation for the boundary condition of a source concentration varying periodically in time. This results in a solution that has the form of a wave equation, gradient squared = derivative wrt time. It has many applications in different fields of study. One is in describing ion flow in lithium batteries. Whitacre draws a circuit with it and gives some brief comments on it in the video I gave a link to. There are also lots of papers on it easily found with google. I don't care to go any further into it here. Since the overall effect is to modulate diffusion rate of Li ions, I just think of it as a variable resistance. There is of course energy loss associated with this since the ions undergo collisions with the solvent molecules and lattice atoms in the electrode coating.


----------



## IamIan (Mar 29, 2009)

*Re:*



IamIan said:


> The only reference I've seen at all in your link to "Warburg" at all is not about the "Warburg Effect" ... but about the "Warburg Coefficient".
> 
> Did you actually intend to refer to the "Warburg Coefficient" or "Warburg Constant" ... and not either type of the "Warburg Effect" ???
> 
> ...





tomofreno said:


> Warburg solved the diffusion equation for the boundary condition of a source concentration varying periodically in time. This results in a solution that has the form of a wave equation, gradient squared = derivative wrt time. It has many applications in different fields of study. One is in describing ion flow in lithium batteries. Whitacre draws a circuit with it and gives some brief comments on it in the video I gave a link to.


Yes.
That is the same "Warbug Coefficient"... or "Warbug Constant" ... I listed.

The "Warbug Effect" are two entirely different things.



tomofreno said:


> There are also lots of papers on it easily found with google. I don't care to go any further into it here.


That's fine.
I think we are on the same page now. 
I was just previously confused ... not making the connection .. by the reference to the "Warbug Effect"... I was mistakenly thinking of the two types of "Warburg Effect".

Thanks for clarification.


----------

