# Is this LiFePO4 BMS idea impossible?



## MN Driver (Sep 29, 2009)

Whenever I look at the Lithium chemistry (whether LiIon or LiFePO4) standard charging BMS operation style, I scratch my head a little.

The reason why I scratch my head is that I can't figure out why there isn't a better way. We charge the battery as a pack and when cells reach the maximum voltage we start pumping the current going to that cell through a load, typically a resistive shunt, that it wastes that current as heat.

What I can't figure out is why there isn't something that I can find, which doesn't seem to be out there, that will trip a relay and disconnect/bypass that cell that reaches its max voltage and then drop the total max charging voltage of the charger to match the remaining number of cells being charged, stepping down until each cell reaches their max voltage, or at least for the top half or quarter of the cells or so, which by that time the rest shouldn't be far behind if they were balanced well initially. ...then come back charging all the cells with a lower current charge but now limiting to the level of total current needed to maintain the max charge voltage set based on the highest voltage cell in the pack. Once the finishing charge amperage is reached, then have the relays bypass each cell or at least half of them and then terminate charging as at this time they should all be charged within a few percentage points of SOC. ...or if you were really picky it could somehow step down until all cells reach the termination point or shunt them at this low amperage point to make them all completely equal, which I think they would be close enough as it is anyway so it shouldn't need to be that complicated. With a method like this I don't see the need for shunts, there isn't any wasted power through the use of a shunt and less of a chance of having the shunts get too hot, especially with a higher amperage charge and this method seems like it would be the best out there if it exists.

If not a relay, then something else that could pass the excess current from the highest voltage cells to the lowest voltage cells which would also balance the pack, basically shunting the cells to match voltage instead of out into a resistor, some sort of controlled MOSFET setup or something perhaps. It just seems that relays might be easier and cheaper way of doing this but maybe not.

Is there a charger or BMS out there that operates like this? When I read literature from Tesla and GM and see the testing videos from GM, it seems like they have a system to bypass cells during charge and discharge if they go out of range, maybe I'm wrong but it reads and looks as if they do.

...am I missing a critical detail that prevents something like this from being created. It seems that most people who want to do something similar to this seems to think that individual chargers for each cell would be best but I don't think it would be neccessary. When I look at lower voltage packs with less than 40 cells with cells that hold 200Ah it seems like this would be better than shunting out until they match due to the larger size of the cell it seems there could be more waste or danger especially if we charge something like this with a charger that runs off of 220 volts at 30 or so that could possibly be pumping in around 50+ amps or so into each cell for a 0.25C charge. As far as an efficiency standpoint it seems to make sense to have something like it. Is this possible?


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## dimitri (May 16, 2008)

What you propose would make BMS more expensive than the battery itself, its simply not economical to do it this way.

Shunting/balancing only happens for few minutes at the very end of the charging cycle, when current is already low, like 1-2 amps. Shunting this little current for few minutes does not make much waste at all.

LFP cells tend to stay well balanced, so this process is just a long term assurance that it will stay balanced over long time.

Trust me , I have been doing it daily and its not an issue.....


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## MN Driver (Sep 29, 2009)

When thinking about it, I realize that this idea might make the charger more expensive if it needs to be designed to step down voltage a bunch of times to match the voltage. If we went with the other approach I mentioned where we could keep the same chargers that already being used but instead have a BMS set up so that way when cells reach the voltage set point for the cells that it starts to shunt(if this word is even correct in this instance) the excess current to the lower voltage cells. This way the charger can scale down normally without losing the extra energy to heat and the cells will be balanced. If we did this, I don't see how it would make the BMS more expensive than the batteries, we are just pushing the current to another cell rather than to a resistor.


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## dimitri (May 16, 2008)

All cells are connected in series and make a single path for high current when charging or discharging. What you propose would require commutation of high current, i.e. commutation of the high current path, i.e. replacing hard links between cells with very large IGBTs or contactors, i.e. at least one high power commutation device per cell plus all the logic around it, plus all the losses associated with high power commutation, etc etc. This will quickly add up the cost of BMS and I bet the thermal losses in commutation devices will be higher than what you shunt with simple BMS, so bottom line is worse than what you started with.


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## MN Driver (Sep 29, 2009)

A device to bleed the current that would normally cause a cell to over your voltage setpoint to another cell would require a complex commutation device? I'm not talking about relays, bypasses, or any form of a disconnect anymore. Since with many BMS devices, the cells have additional connections or wiring to a common point, couldn't it be controlled in a similar way to the shunting of the cell to a resistor but instead have that power shunt to a different cell through the pair of wires going to the lowest cell instead. All of the wires end up at the same general place in the BMS as it is. ...at least with a BMS that looks like something like this as opposed to the ones mounted directly to a cell. http://www.evcomponents.com/ProductDetails.asp?ProductCode=BMS

I could be missing a detail or two though, like everyone here we have something to learn, experiences to share and even experts and the experienced will learn something new from time to time, which is our common goal.

I'm not saying that the current use of a BMS is bad as they are or even all that wasteful because I believe that they aren't too bad, but if nobody questions the way things already are, then nothing would ever be improved upon, therefore I take a look at the box from the inside, the outside, and see how the box can be reconstructed for better efficiency, performance, etc. Pioneers of progress and innovation moved us away from lead acid batteries and the same progress and innovation has brought LiFePO4 cells to us as a cheaper and slightly more abuse tolerant version of secondary Lithium cell technology.


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## TheAtomicAss (Feb 19, 2009)

I'd have to agree with everyone else here that your idea, at least as I'm perceiving it, is grossly more difficult and expensive than what is currently popular, that is the shunting of individual cells to load resistors.

My concept is a little different, and reduces some of the losses of both of the previous ideas, at the potential expense of longer times to full charge on a pack that has significant capacity AND dis-balance.

My concept is bulk charging the entire pack until one cell reaches the cutoff voltage, the bulk charger shuts off and then the pack controller individually boosts the cells that haven't reached cutoff yet using the same wire that is used to read the cell voltage. A properly designed pack controller/charger combo could handle all of this.


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## MN Driver (Sep 29, 2009)

TheAtomicAss said:


> ...until one cell reaches the cutoff voltage, the bulk charger shuts off and then the pack controller individually boosts the cells that haven't reached cutoff yet using the same wire that is used to read the cell voltage. A properly designed pack controller/charger combo could handle all of this.


The two problems I see with this is and it seems to share a similar issue as my idea but maybe a little bigger of an issue with yours. When you use a wire to read a cells voltage it can be very thin wire such as 18 AWG wire, or even thinner if you aren't too worried and are very careful about the thin insulation abrading off or the wire snapping if it accidentally gets pulled. That same 18 AWG wire only can carry a small handful of amps before it gets warm. If it were a matter of 2Ah of difference, then it would be fine, but if it stops when the highest cell is reached and you have a 200Ah pack where the worst cell managed to be 5% out of balance after you went on a vacation for a few weeks and then ran it low on SOC right after, it could take 5 hours to balance out the 10Ah discrepancy with that method. With the normal shunting method, the current level drops and less gets shunted but that cell can get more than a few amps at a time because it gets the bulk load too. Since you want to use the same wire that you measured voltage with to charge it, you wouldn't be able to accurately measure the voltage provided because voltage drop will cause the voltage measuring device to read high while the cell is receiving a lower voltage which would be a problem unless you want to either use larger than usual gauge wiring for the voltage measurement or you have the patience or smaller cells to work with it.

The second issue that I see is that you will need to have some sort of power supply to provide a smaller voltage for the individual cells to charge. If it was a power supply that provided the voltage that would be standard for a per-cell level, you would need to either have it be capable of providing a high amperage for multiple cells to charge at the same time, but if it comes from a single power supply if you connect the positive and negative to cell 1 for example and the same positive and negative leads to cell 4 then the negative lead is going to essentially get a short circuit connection from the negative of cell 1 the positive of cell 3(neg of 4) and the positive of cell 1 to the positive of cell 4(neg of cell 5), or however you choose to look at that. The only way to do this properly would be to have seperate supplies for each cell or be able to isolate every charged connection somehow which is why I scrapped the relay/bypass/disconnect idea after dimitri mentioned the loss behind IGBTs and basically explained that the pack would require every cell to be disconnect from the series connection for something like this to work which isn't practical. 

So basically your idea would have to work with each individual cell, one at a time which would take quite a long time.

My idea of one cell shunting its extra amperage out to another doesn't really work either due to the same series connection issue so I'm not really getting around the problem. It seemed good in theory but I didn't have the details to understand how it wouldn't work. For some reason I thought there would be a way to seperate them through the use of diodes in some special way or something but it doesn't seem to work out. $13.99 isn't a bad price per each shunt and sure needs less thinking and schematic design as it's already laid out as a kit with parts provided especially with only a 34-36 cell pack. Thanks for your thoughts guys!


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## Gavin1977 (Sep 2, 2008)

The device you are looking for is an isolated DC-DC converter. Whether the amount of inbalance warrants it though is another matter. I dont have any personal experience with high Ahr lithiums though so cannot make a comment.


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## TheAtomicAss (Feb 19, 2009)

MN Driver said:


> If it were a matter of 2Ah of difference, then it would be fine, but if it stops when the highest cell is reached and you have a 200Ah pack where the worst cell managed to be 5% out of balance after you went on a vacation for a few weeks and then ran it low on SOC right after, it could take 5 hours to balance out the 10Ah discrepancy with that method. With the normal shunting method, the current level drops and less gets shunted but that cell can get more than a few amps at a time because it gets the bulk load too. Since you want to use the same wire that you measured voltage with to charge it, you wouldn't be able to accurately measure the voltage provided because voltage drop will cause the voltage measuring device to read high while the cell is receiving a lower voltage which would be a problem unless you want to either use larger than usual gauge wiring for the voltage measurement or you have the patience or smaller cells to work with it.


I'm fine with it taking many hours to finish. Unlike others here, I'm in no rush. I can justify charging a 50KWh battery pack from a 120V, 20A outlet. I can therefore justify a 10+ hour balancing procedure, assuming a SERIOUSLY out of balance cell, or cells. The setup I envision, will be able to balance ALL of the cells simultaneously, though.

As for reading the voltage of the cell, that shouldn't be necessary during balancing. Feed the cell say, 3.9V, then wait for the current to drop to level X. I'd say 18g wire is perfectly fine for this purpose.



MN Driver said:


> The second issue that I see is that you will need to have some sort of power supply to provide a smaller voltage for the individual cells to charge. If it was a power supply that provided the voltage that would be standard for a per-cell level, you would need to either have it be capable of providing a high amperage for multiple cells to charge at the same time, but if it comes from a single power supply if you connect the positive and negative to cell 1 for example and the same positive and negative leads to cell 4 then the negative lead is going to essentially get a short circuit connection from the negative of cell 1 the positive of cell 3(neg of 4) and the positive of cell 1 to the positive of cell 4(neg of cell 5), or however you choose to look at that. The only way to do this properly would be to have seperate supplies for each cell or be able to isolate every charged connection somehow which is why I scrapped the relay/bypass/disconnect idea after dimitri mentioned the loss behind IGBTs and basically explained that the pack would require every cell to be disconnect from the series connection for something like this to work which isn't practical.


I don't claim to know much about electronics, but I do know it should be possible to charge 2 batteries in a series pack from a single point, without disconnecting them. In the guitar world, there is a 9v battery supplement designed for older pedals without adapter inputs, and the company claims that 2 of them can be used, from the same 9v power source, to power an 18v pedal, due to their isolation. Like I said, I don't know how it's wired, but I know people that use them, and they work. Damned if I can remember the name, though.



MN Driver said:


> My idea of one cell shunting its extra amperage out to another doesn't really work either due to the same series connection issue so I'm not really getting around the problem. It seemed good in theory but I didn't have the details to understand how it wouldn't work. For some reason I thought there would be a way to seperate them through the use of diodes in some special way or something but it doesn't seem to work out. $13.99 isn't a bad price per each shunt and sure needs less thinking and schematic design as it's already laid out as a kit with parts provided especially with only a 34-36 cell pack. Thanks for your thoughts guys!


I just don't like the extra heat, especially in close proximity to my batteries. Even at just 4A, that is still a lot of heat in one small semiconductor.

My pack is going to be 90-100 batteries in series, so the heat that could be generated if half the cells were out of balance is immense, 540W, assuming 4A at 3V per cell. I have space heaters that use little more than that.


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## Overlander23 (Jun 15, 2009)

I, too, am worried about the potential heat generated by a shunt system... especially if the heat source is contained in the same box as the cells. I'm considering running leads to an external connector which would then connect to a remote shunt system (remote VoltBlocher's, if you will). 18AWG wire should be enough to handle the 1.5 amp shunt current. And the idea would be to plug the shunt system in occasionally to balance the pack... not necessarily on every charge (though it could certainly be done that way)

I also believe that your idea has merit, though I don't have much electronics experience either. My idea was to have a "digital valve" at every cell along with other electronics. Voltage would be monitored and converted to a data stream at each cell. One cable would essentially series link the battery modules to a central controller which would be able to monitor encoded voltage, or whatever data the battery module can handle. Once a first cell hits a limit, the main charger is shut down. A "common rail" of low current power runs to each cell module, and the central processor opens the valve, coordinated with other modules, in pulses to equalize the charge levels at an individual cell level. Or something like that... The goal wouldn't be to charge the pack quickly in that last phase, nor would it be to "max" the cell out, it would be an equalization process, so the cell level "switch" wouldn't have to handle large current.

If the idea could work, I wonder if it could be active all the time, slowly balancing cells even as large amounts of energy are drawn out while driving?


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