# What are the failure modes of parallel connected LiFePo4 cells?



## ga2500ev (Apr 20, 2008)

I've been trying to get up to speed in terms of lithium battery protection, monitoring, and management. I've learned quite a bit about BMS, top and bottom balancing, and the like.

What I've finally realized is while all these tools and techniques make sense for series connected battery packs, that it isn't so clear what happens in terms of cells connected in parallel.

The objective is to make sure that a cell's state of charge (SOC) stays within a valid range. Cell damage, and a corresponding reduction of useful cycles, occurs both when a cell is continued to be charged when it is full and discharged when it is empty.

Balancing is a process that ensures that all of the cells in a battery have either the same SOC or same amount of charge (note these are different values), so that when charging and discharging the battery no single cell falls into an invalid SOC range.

The two common types of balancing are called top and bottom balancing, one which manages each of the two states above. Top balancing ensures that each cell in the battery is charged to a 100% SOC before use. Bottom balancing on the other hand brings every cell in the battery to a 0% SOC, then charges until the first cell in a pack reaches a 100% SOC.

Each have advantages and disadvantages. Both deal with the issue of the differences between cells. In an ideal battery each cell would have exactly the same capacity. In this case either balancing method works if all the cells are at the same SOC because they will reach the same state (full or empty) at exactly the same time. This maximizes the available energy of the pack.

But in the real world, there are variations in the capacities of the cells. Some hold less energy, some more. This causes problems in both the charging and discharging states of the battery because as soon as dynamic energy movement occurs in either direction, the individual SOC of the cells will start to deviate from one another.

With top balancing, this means that one of the cells (usually the one with the least overall capacity) will reach a 100% SOC before the others when charging. Since with top balancing the system is trying to charge all cells to 100% SOC, continued charging of this cell, and all the others cells that reach 100% SOC, will cause them to overcharge above 100% SOC. In this state the terminal voltage (TV) skyrockets, heat buildup occurs in the cell, and damage starts to occur.

Top balancing has issues on discharge too. Again because one cell, which I will refer to as the Runt, has less capacity than the others, it will reach a 0% SOC before the others, even though all the cells started at 100% SOC. This is equally bad because continued use of the cell destroys it. Again in this situation as the Runt runs out of energy, its terminal voltage drops off.

So clearly with top balancing, it's critical to watch the terminal voltage of each cell and do something when a cells TV goes out of range.

Bottom balancing on the other hand acknowledges that the Runt is the limiter of the pack and therefore manages the pack based on its limitation. By emptying everyone, then charging only until the Runt gets to 100% SOC, all cells are protected from overcharging.
Note that all cells other than the Runt are at a SOC of less than 100%. On the discharge end, since each cell has exactly the same amount of charge (equal to the Runt's), each cell will run out of energy at exactly the same time, when the Runt runs out of energy.

Since a fixed charge is put into the battery, in theory the charger can be set to deliver a set charge to the entire battery without having to monitor the cell voltage. Since the pack's cell all runs out of energy at the same time, again the theory there's no danger at the bottom end of the discharge.

But in practice it's difficult to manage a bottom balanced pack without knowing the individual cell voltages. The charger still needs to be cut off when the first cell hits the 100% limit. Also to protect the entire pack, discharge needs to be disabled when the pack voltage starts to drop off.

So at the end of the day, it really seems that running a LiFePo4 pack without cell level monitoring is like driving blind: an accident waiting to happen.

With the surge of the powerful yet moderately priced A123 20 Ah pouch cells, it seems that a new strategy of paralleling cells emerges. What I'm trying to figure out is what happens on the top and bottom of the discharge curve with paralleled cells.

Generally LiFePo4 cells hold a steady charging and discharging TV line when used. Charging is about 3.5V for a cell for virtually the entire charging cycle, while discharge is a strong 3.3-3.4V/cell until the very bottom of the discharge curve.

Now cells in parallel will equalize to a common TV. But each group has a Runt, just like the overall battery. So in both directions the runt will reach the SOC endpoints first. But it seems that as the Runt tries to drag the TV of the group either above or below those bright lines above, that the group will drag it back in line. So for charging, the Runt (and any other cell that's at 100% SOC) will charge the other cells in the group that have not reached the 100% SOC state, and that on discharge as the Runt (and others) drag below the steady state, that the ones that have not reached the 0% SOC will pull them back up, presumably by charging them. So it would seem that as a group they can be treated as a single cell that will only raise TV after all cells are at 100 SOC and drop off only when all cells in the group are exhausted.

So from a monitoring standpoint, a parallel group of cells should be treated as a single cell, right? So it would seem there's no good reason to do individual cell monitoring. That's good. 

So I guess my last question is how exactly can a pack be run without a BMS? If top balancing, something has to be done at both ends on a cell/parallel cell group basis. Same for bottom balancing in terms of a fixed charge to the pack and a LVC when the pack voltage starts to drop like a brick.

Any thoughts would be welcome.

ga2500ev


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## GizmoEV (Nov 28, 2009)

ga2500ev said:


> But in practice it's difficult to manage a bottom balanced pack without knowing the individual cell voltages. The charger still needs to be cut off when the first cell hits the 100% limit. Also to protect the entire pack, discharge needs to be disabled when the pack voltage starts to drop off.
> 
> So at the end of the day, it really seems that running a LiFePo4 pack without cell level monitoring is like driving blind: an accident waiting to happen.


Not quite blind, just blurry vision. With a bottom balanced pack they all will reach "empty" at the same time and the car will just stop. With a top balanced pack it is much easier to get a voltage based charger to stop charging at the correct point. Right now I'm using an Ah counter (CycleAnalyst) and a 1/2 pack voltage monitor you can read about in my blog. Even with my pack at about 10%SOC the voltage difference between halves of my pack stayed under about 0.15V difference under a 1.5C load.



> With the surge of the powerful yet moderately priced A123 20 Ah pouch cells, it seems that a new strategy of paralleling cells emerges. What I'm trying to figure out is what happens on the top and bottom of the discharge curve with paralleled cells.


They act like a single cell due to the wonderful SOC vs Voltage curve of the LiFePO4 cells. I've directly connected a discharged 40Ah TS cell with a charged 40Ah TS cell and the initial current was about 28A through a 50A 50mV shunt so maybe 100A if no shunt were used. The current dropped off quite rapidly. I've been running a parallel pack of TS-LFP100AHA cells for over 2 years and 11kmiles and have not had any issues with them. I don't over charge them (charging stops at 3.465vpc with low current) and I haven't over discharge them.



> Generally LiFePo4 cells hold a steady charging and discharging TV line when used. Charging is about 3.5V for a cell for virtually the entire charging cycle, while discharge is a strong 3.3-3.4V/cell until the very bottom of the discharge curve.


Your numbers are a little on the high side compared to what I see with my pack. Even at 0.25C charging rate the terminal voltage is below 3.4V for much of the charging time. At a 0.5C discharge rate with 20°C cells I'm seeing 3.2V average on discharge with my TS cells. A resting LiFePO4 cell which has not been over charged will sit at just under 3.4V.



> Now cells in parallel will equalize to a common TV. But each group has a Runt, just like the overall battery. So in both directions the runt will reach the SOC endpoints first. But it seems that as the Runt tries to drag the TV of the group either above or below those bright lines above, that the group will drag it back in line. So for charging, the Runt (and any other cell that's at 100% SOC) will charge the other cells in the group that have not reached the 100% SOC state, and that on discharge as the Runt (and others) drag below the steady state, that the ones that have not reached the 0% SOC will pull them back up, presumably by charging them. So it would seem that as a group they can be treated as a single cell that will only raise TV after all cells are at 100 SOC and drop off only when all cells in the group are exhausted.


That is basically it.



> So from a monitoring standpoint, a parallel group of cells should be treated as a single cell, right? So it would seem there's no good reason to do individual cell monitoring. That's good.


That has been my experience and the experience of others.



> So I guess my last question is how exactly can a pack be run without a BMS? If top balancing, something has to be done at both ends on a cell/parallel cell group basis. Same for bottom balancing in terms of a fixed charge to the pack and a LVC when the pack voltage starts to drop like a brick.


If by BMS you mean a cell level BMS then it all depends on what you have for charge cutoff and controller cutoff. If you have a charger like the Zivan I have which doesn't have the option of cutting off when the current drops to a particular point then charging a top balanced pack is probably the best way to go. I have a 1/2 pack voltage monitor for those times I'm pushing the range of my pack. The most I've discharged my pack is to 10%SOC and the voltage of each half was still nearly the same so I assume that no cell pair went too low.

I went a year with my BMS boards installed but didn't use the balancing function and found that it took that long for the difference between the high and low cell to go over 0.1V. I was still getting dust and water into my pack so that could have been due to imbalanced discharging through the crud on top of the cells. I am about half way through a similar test without the BMS boards installed. So far I'm not really seeing any cell drift to speak of. The cells usually "move around" with respect to each other between one monthly test and the next.

For bottom balancing you just have to make sure your charger doesn't over charge the pack. You should really make sure it follows the charging procedure of charging to 3.6Vpc and ending when the current drops to 0.05C. It is better to undercharge than to over charge. A the bottom end there really isn't any thing to worry about because all the cells will drop out together. Just program the controller to not allow the cells to go below 2Vpc. If you set it higher then the controller may cut back on a hard acceleration because of voltage sag which will include any voltage drop in the lines between the battery pack and the controller.

What ever you do, I think it is foolish to not have at least a volt meter and an Ah counter of some sort.

As for failure modes, some have cautioned me that if one of my cells were to have an internal short then the other cell will dump into the shorted one. I don't see this as any different than having a 200Ah cell which develops an internal short. If I had the resources I would parallel two 100Ah cells, fully charge them then drive a spike through just one of them and record the results. Based on what I've seen posted on line I don't things would catch fire but there would/could be some venting.


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