# CC/CV and BMS shunt



## green caveman (Oct 2, 2009)

If I have a BMS that shunts cells in the CV charging range shouldn't the voltage be decreased to allow for the cell that has been shunted?

So, if I'm charging, for the sake of simplicity say 4 cells. My charger is putting out 14.6V (3.65V/cell) for the final charge (the 3.65V is irrelevant, so pick any other terminal value you like if 3.65V is not your choice):

0-|-|-|-|-14.6V

Now I shunt one:

0-|-|=|=|- XV

shouldn't the X be 10.95V rather than 14.6V?


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## rwaudio (May 22, 2008)

green caveman said:


> If I have a BMS that shunts cells in the CV charging range shouldn't the voltage be decreased to allow for the cell that has been shunted?
> 
> So, if I'm charging, for the sake of simplicity say 4 cells. My charger is putting out 14.6V (3.65V/cell) for the final charge (the 3.65V is irrelevant, so pick any other terminal value you like if 3.65V is not your choice):
> 
> ...


Shunting doesn't mean taking it out of the circuit, shunting means wasting some of the current through a resistor as heat in an attempt to keep the voltage from going to far over the set point.
If you had:
3.65v
3.65v
3.50v
3.80v
That still totals your 14.6v however you don't want that cell to hit 3.8v so a BMS with a shunt may kick in at 3.65v and try and get rid of the extra energy so keep it from drastically overcharging while cell 3 increase from 3.5v


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## green caveman (Oct 2, 2009)

rwaudio said:


> Shunting doesn't mean taking it out of the circuit, shunting means wasting some of the current through a resistor as heat in an attempt to keep the voltage from going to far over the set point.
> If you had:
> 3.65v
> 3.65v
> ...


Where's the "extra" energy coming from? The charger or the cell? That is, is the goal of the shunt/resistor to waste energy or to cut the cell out of the charging circuit?

If I put a perfect shunt - a 0 ohm resistance - across the 3.8V cell. With the charger still putting out 14.6V, assuming no shunts on the other cells and perfectly balanced cells. If you just left it, would the other cells (theoretically) finally max out at 4.87V/cell (14.6/3) or at some other voltage? If not, why not, and does the resistance of the shunt actually matter?

I'm not entirely sure of the effect of a short across the cell (while the charger is attached - I'm fairly clear in other circumstances), so let's imagine an ideal MOSFET or similar uni-directional short.


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## Moltenmetal (Mar 20, 2014)

The purpose of the shunt is just that- to shunt some current around a cell which is close to its high voltage cutoff, to allow the other cells to catch up in SOC before the particular cell becomes overcharged. 

Since all the cells (or parallel cell groups) are in series in a pack, the total charge current from the charger flows through every cell. The shunt resistor in parallel with a particular cell simply diverts some of the current AROUND the cell in question. 

Depending on the value of the resistor being used, the cell being shunted will either continue to rise in voltage somewhat (i.e. it will receive some share of the current flowing from the charger) or if the resistor is sufficiently low in resistance, it will discharge the cell somewhat through the resistor.

Numerical example: charger's constant current balancing charge current is 1.25 A. If a particular cell is measured at 3.6 V while being shunted with a 3.6 ohm resistor in parallel across its terminals, 1A of current will be flowing through the resistor and 0.25 A will flow into the cell. That 0.25 A will gradually raise the cell's SOC and hence its voltage, and gradually the fraction of current flowing into the cell will reduce- until it reaches the HVC and the BMS trips the charger. Drop the shunt resistor to half that value and now 2A will be flowing through the resistor- 1.25A from the charger and 0.75A out of the cell being shunted. Its voltage will gradually fall as its SOC decreases.

So no, during constant current charging (which is used during top balancing), you don't need to reduce the charger voltage as you shunt cells. You merely allow the voltage to rise as the cells reach 100% SOC- and you STOP the charger when the voltage in total across all cells reaches a certain average value per cell. Regrettably, if the cells are not perfectly in balance, one or two cells will go above their high voltage limits before the charger voltage rises that high- hence the need for the BMS.

The shunt can be automatic in a BMS celltop board, or it can be any resistor you add manually in order to accomplish the same thing (during manual top-balancing). I've been using a 12V 50W lightbulb which will work well as a shunt for one to three cells, and which gives you a visual indication when it is connected properly.


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## green caveman (Oct 2, 2009)

Moltenmetal said:


> So no, during constant current charging (which is used during top balancing), you don't need to reduce the charger voltage as you shunt cells. You merely allow the voltage to rise as the cells reach 100% SOC- and you STOP the charger when the voltage in total across all cells reaches a certain average value per cell.


Umm, now you have me questioning charging...

I think that the start of charging is CC and that you're in big trouble if you have to shunt in that phase of charging.

The final stage is CV, that is, the charger sits at the desired voltage (so 14.6V in the 4 cell example) until the current drops to a desired value, say 0.2A.

In CC mode I would agree with your logic, in CV mode not so much...


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## Moltenmetal (Mar 20, 2014)

Sorry to have confused you. My ElCon charger has three stages: constant current (max current), then constant voltage, then a limited current balancing step which in my case is 1.25 A. Voltage during that last step is not constant- in my case (32 LiFePO4 cells) it starts around 112 V and the charger shuts off at 116.8V.

BMS shunting only occurs during that balancing step, and the celltop boards can shunt only a very limited current.


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## green caveman (Oct 2, 2009)

Moltenmetal said:


> Sorry to have confused you. My ElCon charger has three stages: constant current (max current), then constant voltage, then a limited current balancing step which in my case is 1.25 A. Voltage during that last step is not constant- in my case (32 LiFePO4 cells) it starts around 112 V and the charger shuts off at 116.8V.
> 
> BMS shunting only occurs during that balancing step, and the celltop boards can shunt only a very limited current.


[OFFTOPIC]It's an interesting curve, I wonder if it is better than CC/CV. I'm intrigued by what happens at 112V (3.5V/cell). Does it just sit there until the current is near zero (or 1.25A?). I wonder if that does help balance, although I doubt it because it's just a CV step at a slightly lower voltage. It might make sense if there was a delay after that CV step to allow the cells to settle (or it might not).[/OFFTOPIC]

OK, back the the flow of the thread...

I think that, in essence the final _current balancing step_ is also just CV.

Assuming that your cells are perfectly balanced and charged if the charger is putting out 116.8V the current is (very close to) zero (that's the only choice, it can't be 1.25A). At 112V, it's probably also not 1.25A (or not for too long), so the charger will ramp the voltage to get the current to 1.25A and I would guess that it fairly quickly reaches 116.8V at which point the current will steadily decrease. (The alternative is that the charger puts out more than 116.8V to maintain the current, but that seems unlikely).

So, once the _current balancing step_ reaches 116.8V it's the standard CV and the BMS shunts will have the same effect (whatever that is). There really shouldn't be much balancing at 1.25A (that's quite an out-of-balance pack) so your situation with the Elcon is not much different.

If this wasn't a charging circuit with the cells as voltage sources as well as sinks, then the answer would be simple - shorting out a cell would mean you'd need to lower the voltage. But (to me at least) it's not as obvious when you short out something that has the ability to provide a voltage.

A few things are leading me to believe that the CV voltage should be cut. This diagram (from Elithion who I feel might know a little about charging and BMSs).

http://liionbms.com/php/wp_cccv_charging.php

Which leads you to the concept of a "regulated charger". That seems to be pictured here. If I understand this concept correctly, each cell is charged separately:

http://www.apriliaforum.com/forums/showthread.php?242120-Lithium-Iron-Battery-Primer-(LiFePO4)&

Both suggest that just shunting is not a good strategy. And the only "why" I can think of is this voltage problem - BUT I'm not even close to sure of this.


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## PStechPaul (May 1, 2012)

The Elithion article shows how a shunting BMS can protect cells from overvoltage and eventually top balance a pack. It even states that a good BMS will work even with an unregulated charger, although it must be reasonably limited and the BMS must be able to shut it off when all cells are fully charged. Even that may not be totally necessary, as long as the BMS shunt can handle the full current of the charger at a voltage less than the cell maximum.

The cell voltage is not a fixed value, and depends on temperature, current (in and out), and time after a charge or discharge. This is partially due to internal resistance and partially due to chemistry. But when I did some testing of the charge/discharge characteristics of LiFePO4, Li-Ion, and NiMH cells, they all showed somewhat similar behavior. There is a "rest voltage", which is stable after sitting for a few hours at a constant temperature, and is fairly constant from perhaps 10% SOC to 90%. The self-discharge is minimal (except for NiMH), so that voltage will remain constant for months or years.

When charging, especially for a weak or barely charged cell, the voltage will rise significantly at first, as a certain amount of current is applied. As the CC charge continues, the voltage will continue to rise, but by only a small amount, and when the charge is removed, the voltage will immediately drop by a certain amount, and then continue to drop over an hour or so to the "rest" voltage. If you monitor the cell voltage while you charge with CC, at about 90% SOC the voltage will start to climb at an increased rate, at which point charging can be discontinued, or you can enter the CV stage where the maximum specified voltage is held and the current slowly drops, and charge should be terminated at C/10 to C/20. After this CV stage, the cell voltage will stabilize at the high end of its rest voltage range.

A similar effect occurs during discharge, where the cell voltage drops when the load is applied (indicating the internal resistance), and then slowly drops until about 10% SOC where it drops more quickly. When the load is removed, the cell voltage will recover, and perhaps even continue to rise a bit until it reaches its rest voltage.

This non-linear effect allows the use of BMS shunting to control the charging process just about as well as charging each cell individually. But the BMS must assume that the charger is connected and supplying a certain current. Since it only monitors the cell voltage, it will not do anything (other than report that voltage) until it rises above a certain level, at which point it starts to apply the shunt load. The cell may be represented by a resistance in series with a voltage, but each of those parameters may change. 

However, for simplicity and as applies to the cell at the point of maximum charge, assume the fixed voltage is 3.6 and the charge current is 10 amps, at which the cell voltage reaches the 3.7 volt setpoint. The internal resistance is 0.1/10 = 0.01 ohms. If you apply a shunt resistor of 3.6/10 = 0.36 ohms, the charge current will produce 3.6 volts on the resistor and the terminals of the cell, and since its internal voltage is also 3.6 volts, there is zero volts across its internal resistance, and no current will flow in or out. In reality, the applied resistance may be in the form of an equivalent PWM load, and the voltages and currents will be waveforms that on average accomplish the same thing, but there will be some small amount of charge/discharge happening at some frequency. This may be a good reason to disconnect the charger when all cells are at full SOC.

The BMS may then remain connected and can be used to detect and prevent excessive discharge and reverse polarity when the pack is providing power.

It is possible to charge cells (or groups of cells) in a pack individually, but this is usually impractical and unnecessary. Also, the second reference shows how it may be possible to monitor individual cells in a series/parallel matrix, but if all cells are the same chemistry and of similar capacity, it is better and more practical to connect several in parallel, and then connect these groups in series, so the cells in parallel will share the load and charge appropriately due to their identical voltages. The disadvantage is that if one cell in a parallel set shorts, or changes characteristics drastically, it can damage the other cells. It is possible to add protection in the form of fuses or other devices to remove a bad cell in such a case, but it will make the pack weaker because of the lower capacity of that "link in the chain".


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## green caveman (Oct 2, 2009)

PStechPaul said:


> The Elithion article shows how a shunting BMS can protect cells from overvoltage and eventually top balance a pack. It even states that a good BMS will work even with an unregulated charger, although it must be reasonably limited and the BMS must be able to shut it off when all cells are fully charged. Even that may not be totally necessary, as long as the BMS shunt can handle the full current of the charger at a voltage less than the cell maximum.


That's not my reading of that article. To me it suggests that the charger just shuts off when any cell reaches the "shunt" voltage and then restarts at a later time hoping that cell has stabilized.



PStechPaul said:


> However, for simplicity and as applies to the cell at the point of maximum charge, assume the fixed voltage is 3.6 and the charge current is 10 amps, at which the cell voltage reaches the 3.7 volt setpoint. The internal resistance is 0.1/10 = 0.01 ohms. If you apply a shunt resistor of 3.6/10 = 0.36 ohms, the charge current will produce 3.6 volts on the resistor and the terminals of the cell, and since its internal voltage is also 3.6 volts, there is zero volts across its internal resistance, and no current will flow in or out. In reality, the applied resistance may be in the form of an equivalent PWM load, and the voltages and currents will be waveforms that on average accomplish the same thing, but there will be some small amount of charge/discharge happening at some frequency. This may be a good reason to disconnect the charger when all cells are at full SOC.


If you're shunting at 10A you have a real problem. Apart from anything else you need a 40W resistor in your example. Most BMS's will shunt less than an amp, often 500ma or less. If the pack is reasonably balanced then the shunting should not start until almost the end of the CV phase. During CV the current is dropping as the cells reach full charge and the phase ends when the current is ~0.

At 100ma your 0.36Ω resistor is only dropping 0.036V and so, if your rational is correct, the rest of the cells are seeing (basically) the whole voltage of the charger, which is too much. Maybe the answer is that a perfect BMS/Charger combo should modify the charge voltage depending upon the number of shunts that are activated as well as the current in the circuit. I just don't think so.

However, I think that your logic has some merit, the error maybe in the calculation of the voltage drop across the cells. Or assuming that there is a voltage drop.

We know that at the asymptotic end of an ideal CV phase the current is exactly zero. This means that there cannot be any voltage drop across any shunts, resistor, whatever connected to the cells.

Let's put together a perfect 10-cell pack. The CC phase ends when the (ideal) charger reaches 36.5V and the charger enters the CV phase. There is no voltage "drop" across any of the cells. The sum of the cell voltages == the charger voltage (that's the definition of CV).

If you shunt with a resistor (any resistor) there's going to be a voltage drop across the resistor that changes with the current of the CV phase. But regardless of the resistance, that voltage drop will approach zero as you reach the end of the CV phase and zero current.

Now I'm wondering why MOST BMS designs use a resistor. Are the BMS resistors discharging the cell they are shunting or just preventing further charging? A perfect, 0Ω shunt should be the best - say an ideal MOSFET. I *think* what should happen in this case is the shunted cell stops charging, that is, it stops sinking current and the current in the circuit drops to 90ma. BUT the voltage seen by all the cells stays the same (there is still no voltage drop across the shunted cell).

Problem is that for current to flow, there has to be a voltage difference somewhere. Now I just can't see where that is, which, presumably, means that I misunderstand something and so am wrong (as usual).


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## Moltenmetal (Mar 20, 2014)

I haven't observed that carefully but what seems to happen is that around 112 v the charge current starts to drop off but voltage doesn't rise, ie this is a constant voltage step. Once at 1.25 A, the charger allows voltage to rise again. I haven't hit 116.8 v yet yo see the charger stop - my BMS trips the charger before I get beyond 116 v.


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## PStechPaul (May 1, 2012)

I may have used an unrealistic example with shunting 10 amps, but the principles still apply. In the presence of a charging current through the entire pack in series, a resistor can be added across any cell which will "shunt" a portion of the current, and the remaining lower amount of current will be applied through the cell. At some point you can add a low enough resistance that the entire charging current will go through it and the cell will see zero. If you further decrease the resistance, current will be drawn from the cell to discharge it.

A BMS can be designed so that it will shut off the charging when the first cell reaches the maximum and starts shunting, which may be safer and more efficient than waiting until all cells are being shunted for top balancing. If the pack is bottom balanced, it may be best to stop charging when the weakest cell is full. This should maintain the bottom balance condition, where all cells will have an equal charge even though they are all at various percentages of being full.


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## green caveman (Oct 2, 2009)

PStechPaul said:


> I may have used an unrealistic example with shunting 10 amps, but the principles still apply. In the presence of a charging current through the entire pack in series, a resistor can be added across any cell which will "shunt" a portion of the current, and the remaining lower amount of current will be applied through the cell. At some point you can add a low enough resistance that the entire charging current will go through it and the cell will see zero. If you further decrease the resistance, current will be drawn from the cell to discharge it.


I'm not convinced that last sentence is true (or false). 

Notwithstanding. What would you answer to the original question which is, should the CV voltage be decreased to account for the shunted cells (and by how much)?



PStechPaul said:


> A BMS can be designed so that it will shut off the charging when the first cell reaches the maximum and starts shunting, which may be safer and more efficient than waiting until all cells are being shunted for top balancing. If the pack is bottom balanced, it may be best to stop charging when the weakest cell is full. This should maintain the bottom balance condition, where all cells will have an equal charge even though they are all at various percentages of being full.


Top balancing. I'm thinking of adding BlueTooth to Dave Mellick most excellent Zivan NG3 mod (http://www.diyelectriccar.com/forums/showthread.php?t=64827). I have built up Ian Hooper's BMS http://zeva.com.au/Research/BMS/ with BlueTooth (It uses Darlingtons as shunts). And have an Asus Memo Pad Android Tablet as a display/controller - Android builder is here - http://www.ergotech.com (Disclaimer - I have an affiliation with that organization). The Zeva BMS is a little calibration challenged and I'm moving the calibration to the tablet along with the shunting logic (with the board as default if communication is lost/missing).

With BlueTooth on the Zivan it would be simple to change the CV voltage (or stop charging, etc. etc.) based on the number of cells that are shunting, but I'm trying to figure out whether that approach is correct.


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## PStechPaul (May 1, 2012)

Once the pack has been top balanced, each time you charge, the shunting should start happening on almost all cells simultaneously. So it won't matter too much if the charger is turned off with the first cell or all of them. And to answer your question, I think the overall pack CV voltage should remain the same, because there is no way to make the effective voltage of the fully charged cell any lower except by drawing more current through the shunt, or eventually by discharging the cell.

OTOH, when the pack is discharging, the weakest cell will be the first to signal a low voltage condition, which could result in reverse polarity. There is not much that the BMS can do except shut down the controller. But it may be possible to devise a system where the weak cell could be removed from the pack and replaced by a short or a high current diode. That may not be a practical idea, but it might work in an emergency or where one cell has become very weak and needs to be replaced anyway. 

Even having the body diode of the MOSFET in the circuit would route the discharge current through it and the shunt resistor, although it wouldn't provide enough current to operate the vehicle. A high current SPDT relay would work, but not for a high power vehicle drawing hundreds of amps. It may be more practical to identify such a failed cell and manually remove it from the pack and connect with one less cell.

Just tossing some ideas out there...


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## dcb (Dec 5, 2009)

Numerous knowledgeable folks here will recommend bottom balancing, since over discharge seems to be more detrimental to a cell. The cv/shunt facilitates a top charge, which you don't want. Top balance is only useful if you have active cell balancing bms that can transfer charge between cells. And that is only useful if for some reason your cells are very mismatched.



green caveman said:


> Notwithstanding. What would you answer to the original question which is, should the CV voltage be decreased to account for the shunted cells (and by how much)?


It is an invalid question. You should turn off the charger when one or more cells are fully charged in CV mode. Otherwise leave the pack voltage constant.


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## green caveman (Oct 2, 2009)

dcb said:


> Numerous knowledgeable folks here will recommend bottom balancing, since over discharge seems to be more detrimental to a cell. The cv/shunt facilitates a top charge, which you don't want. Top balance is only useful if you have active cell balancing bms that can transfer charge between cells. And that is only useful if for some reason your cells are very mismatched.
> 
> It is an invalid question. You should turn off the charger when one or more cells are fully charged in CV mode. Otherwise leave the pack voltage constant.


Thanks for your input. There are many threads discussing top vs. bottom balancing. I'd rather that this didn't become another one, since I'm really interested in the answer to the question I asked. Which, to rephrase, is this:

_When top balancing with a shunting BMS should the charger voltage in the CV phase be decreased to compensate for the cells that are shunted?_


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## Sunking (Aug 10, 2009)

green caveman said:


> _When top balancing with a shunting BMS should the charger voltage in the CV phase be decreased to compensate for the cells that are shunted?_


No because the cells have not been removed from the circuit, just a resistor to bypass the majority of the current around the fully charged cell. Think of this way you are charging two cells in series or 7.2 volts. The two cells form a voltage divider of roughly 3.6 volts each across each cell. 

When an LFP cell reaches full charge, charge current basically stops flowing through the cell assuming a correct Float Voltage or CV is used. Without a shunt, charging current would stop when the first cell reaches full charge leaving the rest of the cells in series at a lower state of charge. To get around that road block you use a resistive shunt to bypass the fully charged cell(s)


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## green caveman (Oct 2, 2009)

Sunking said:


> No because the cells have not been removed from the circuit, just a resistor to bypass the majority of the current around the fully charged cell. Think of this way you are charging two cells in series or 7.2 volts. The two cells form a voltage divider of roughly 3.6 volts each across each cell.
> 
> When an LFP cell reaches full charge, charge current basically stops flowing through the cell assuming a correct Float Voltage or CV is used. Without a shunt, charging current would stop when the first cell reaches full charge leaving the rest of the cells in series at a lower state of charge. To get around that road block you use a resistive shunt to bypass the fully charged cell(s)


Increasingly I think that you are correct and I think I follow your reasoning. The biggest thing to me is that at the absolute end of the CV phase, at zero current, the charger is still seeing the cell voltages. With zero current I think that the shunt is irrelevant and it's hard for me to believe that there is a step function where at 1ma the shunt starts to have an impact on the sum of the cell voltages.

However, if this is the case, why are resistors used and why do people think that the value of the resistor is significant? For example this:



dmwahl said:


> For a 2W balance resistor, P = V^2/R is the equation you want. Target about 50% of the max resistor power, so for a 4.2V final cell voltage you'll want around 18-20ohms, for 3.65V 13-15. The full rated resistor power is probably at around 150C and assumes more space for airflow than most people give it. If you want 2W of actual balance power, then you'll want a ~5W resistor. Hope that helps.


From: http://www.diyelectriccar.com/forums/showpost.php?p=522034&postcount=3


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## tomofreno (Mar 3, 2009)

green caveman said:


> Increasingly I think that you are correct and I think I follow your reasoning. The biggest thing to me is that at the absolute end of the CV phase, at zero current, the charger is still seeing the cell voltages. With zero current I think that the shunt is irrelevant and it's hard for me to believe that there is a step function where at 1ma the shunt starts to have an impact on the sum of the cell voltages.
> 
> However, if this is the case, why are resistors used and why do people think that the value of the resistor is significant? For example this:
> 
> ...


The BMS boards are designed so that the shunt circuit starts conducting through the resistor at a predetermined voltage, typically about 3.5V for LiFePO4. When the shunt turns on the cell voltage remains the same, some current just starts bleeding through the shunt circuit because the cell resistance has risen a bit higher than those that are not yet shunting. Cell voltage starts increasing exponentially with added charge above around 3.45V. The shunt just decreases the rate of rise of the cell's voltage a bit. If you follow the cell manufacturers' specs, the pack is charged to some limit voltage, say 3.6V per cell (I use 3.55V), then pack V is held constant and charge current decreases due to rising cell resistance until charge is terminated at typically 0.05C (5A for 100Ah cells). The shunting cells are still being charged up to the point of termination. Typical shunt current is 0.8A or less, so the shunting cells are still being charged at least at 4.2A for a 100Ah cell. If this results in a cell exceeding the HVC setting for the BMS, charging will be terminated to prevent that cell from overcharging. This is the way most BMS/chargers work, and at least the way Ian's older design did, I'm not familiar with his later design. The value of resistance is chosen to limit the value of the shunt current. Higher shunt currents result in hotter resistors which can be a fire hazard - as Jack R. discovered years ago when he used a Darlington pair shunting at 3A, then declared all BMS a fire hazard. If your cells are well matched, well balanced initially, and fully charged most charges, then 0.2A shunt current is sufficient, which is what I think Orion and Elithion BMS use. Lots of info on this in white papers on the Elithion site. The minibms shunts about 3/4A and the resistors feel just a bit warm when you hold a finger on them.

The problem with the NG3 is that it just keeps tapering charge current down. Max spec'ed cell voltage is not exceeded, but it just keeps stuffing in more charge at lower and lower currents. You will find that the "rest" voltage several hours after charge is significantly higher this way than if the pack is charged following the 0.05C spec. If you let charging continue this way for a long time you may overcharge a cell. Lots of stuff posted on this site by David Nelson on this. IIRC his solution was to set up the NG3 to charge to a lower voltage, say 3.45V per cell. Of course the shunts will be shunting all the charge current if the the charger current gets down low enough, so that will help.


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## Sunking (Aug 10, 2009)

green caveman said:


> However, if this is the case, why are resistors used and why do people think that the value of the resistor is significant?


Ohm's Law for one thing. But a Shunt or Bleeder Balance Board is not a set or fixed amount of resistance, it is variable and uses a FET in series with a power resistor. So when the FET is Turned Full ON the Bleeder Resistor bares most of the power.

Consider this: Lets say when you module switches to Balance Top Charge, charge current is limited to 1 amp. A FET forward full On Voltage is 1 volt. We want to limit the voltage across the cell to say 3.7 volts as an example. The Resistance required to do that is 3.7 - 1 volt = 2.7 volts. So now we know the resistor has to be 2.7 volts / 1 amp = 2.7 Ohms. At full bypass the Resistor wil dissipate 1 amp x 2.7 volts = 2.7 watts. SO we are looking at a 2.7 Ohm 5 watt resistor.

It is not an easy subject, nor are the methods easy as it takes some processing to pull off. TI has a really good article and this should show you what is going on. It walks you through both Passive and Active Balance.


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## green caveman (Oct 2, 2009)

tomofreno said:


> If you follow the cell manufacturers' specs, the pack is charged to some limit voltage, say 3.6V per cell (I use 3.55V), then pack V is held constant and charge current decreases due to rising cell resistance until charge is terminated at typically 0.05C (5A for 100Ah cells).


5A seems to be a very high cutoff for the CV phase of charging.



tomofreno said:


> The value of resistance is chosen to limit the value of the shunt current. Higher shunt currents result in hotter resistors which can be a fire hazard - as Jack R. discovered years ago when he used a Darlington pair shunting at 3A, then declared all BMS a fire hazard.


Although not really relevant to this discussion, these are two different technologies. 

If I understand correctly, with the resistor you essentially make a tradeoff between how much you are willing to continue to charge the cell and how big (wattage) a resistor you are willing to use.

With a Darlington (assuming you can find one that will withstand your charging current without overheating) you are (in an ideal case) shorting the cell out and should essentially stop the cell charging.



tomofreno said:


> The problem with the NG3 is that it just keeps tapering charge current down. Max spec'ed cell voltage is not exceeded, but it just keeps stuffing in more charge at lower and lower currents.


I think that pretty much defines the CV stage of any charger. The only question is the cutoff current.



tomofreno said:


> You will find that the "rest" voltage several hours after charge is significantly higher this way than if the pack is charged following the 0.05C spec. If you let charging continue this way for a long time you may overcharge a cell. Lots of stuff posted on this site by David Nelson on this. IIRC his solution was to set up the NG3 to charge to a lower voltage, say 3.45V per cell. Of course the shunts will be shunting all the charge current if the the charger current gets down low enough, so that will help.


I haven't really started on the charge curve for the NG3. With Dave Mellick Zivan NG3 mod (http://www.diyelectriccar.com/forums...ad.php?t=64827) I can put in any charge curve I like. (Well I could, if I stopped posting to the forum and did that instead ). Currently it's CC to a fairly conservative voltage (3.5V/cell IIRC) and then the CV. But I think it doesn't quit until about 200ma. Increasing that would probably help the balancing problem and probably not impact the charging much. Not sure about 5A on a 100Ah pack though. Do you have a reference for the 0.05C end point? I don't see it in the battery spec, eg this:

http://en.winston-battery.com/index.php/products/power-battery/item/wb-lyp100ahaa-2?category_id=176


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## PStechPaul (May 1, 2012)

green caveman said:


> 5A seems to be a very high cutoff for the CV phase of charging.


It may seem high but it's still C/20. However, this may accentuate the need for a shunting BMS to be capable of fairly high power levels, in the order of 15 watts per cell, or more.



> With a Darlington (assuming you can find one that will withstand your charging current without overheating) you are (in an ideal case) shorting the cell out and should essentially stop the cell charging.


Not really. A Darlington can be controlled in linear mode rather easily, as it has a well-defined current gain, typically about 1 amp / mA. Adding a base resistor makes it voltage-controlled. Monitoring the cell voltage provides the feedback for simple analog control. It can be done without a resistor, if the control loop is adequately compensated and the device has enough heat sink to maintain safe temperature. But a resistor is cheap and handles overloads better, and provides a maximum level of current shunting even if the semiconductor device fails shorted. 

A MOSFET is more difficult to control in the linear region because it is a voltage-controlled device with a rather sharp turn-on curve. But it can be used in PWM mode with a resistor so that the device sees very little heating and can be very small (and cheap). PWM may have its own problems because it alternately allows a higher cell voltage and then a much lower voltage that may actually drain the cell. However this can be minimized by using an inductor to provide the bypass current path. It may still require a resistor to dissipate the power, but it is also possible to use a coupled inductor (transformer) and transfer the power to charging a capacitor or even back to the main charging circuit. This is most efficient, but also complex and costly. However, it may be worth it for large battery packs, and it may also allow for the use of the total energy of the pack from top balance to bottom balance.


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## green caveman (Oct 2, 2009)

PStechPaul said:


> It may seem high but it's still C/20. However, this may accentuate the need for a shunting BMS to be capable of fairly high power levels, in the order of 15 watts per cell, or more.


Clear on that, but regardless of the units it still seems high. I've been looking for a reference, but haven't yet found one. I haven't seen ANY value quoted and referenced, so I'm not disputing the value either. I've seen lots of values suggested, but not one with data (testing or manufacturers data sheet) to back it up.

The Elcon approach (mentioned earlier in this thread) may be a good balancing approach. CC/CV to some low voltage ((3.5V/cell, but I might choose lower if I was using this approach). Then what was described as a CC stage but which would rapidly become a CV stage. With a low enough amperage you could shunt in this stage.


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## Moltenmetal (Mar 20, 2014)

In my pack, it's much ado about not very much charge- squeezing the last Ah or three out of 180 just to say it's full. Charger terminates at 3.65V per cell and my BMS trips at 3.68, at least on the cell that usually fills first. That's too close a margin to be of much use. The Sinoploy cell data sheet says you can go as high as 3.8 v before HVC but there's not much point and some risk in doing that. So This spring when the car is legally on the road, I'll see if a spfew more cycles get it in balance sufficiently to allow the charger to shut off, or I'll have to let the BMS end the charge each time.


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## dcb (Dec 5, 2009)

green caveman said:


> Thanks for your input. There are many threads discussing top vs. bottom balancing. I'd rather that this didn't become another one, since I'm really interested in the answer to the question I asked. Which, to rephrase, is this:
> 
> _When top balancing with a shunting BMS should the charger voltage in the CV phase be decreased to compensate for the cells that are shunted?_


Maybe you don't understand the difference between top and bottom balance and why you should bottom balance (in lieu of active balance, which you are not describing here)? Your pack shouldnt need to be balanced constantly, more like an oil change type interval. As long as your bms tells you when something is amiss on one cell you are good to go.

Here is jack ricard on it, he gets a lot of flack, but he knows batteries. Top balancing is pointless (and a bit dangerous) with a shunt bms.
http://evtv.me/2009/11/get-rid-of-those-shunt-balancing-circuits/


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## kennybobby (Aug 10, 2012)

green caveman said:


> Clear on that, but regardless of the units it still seems high. I've been looking for a reference, but haven't yet found one. I haven't seen ANY value quoted and referenced, so I'm not disputing the value either. I've seen lots of values suggested, but not one with data (testing or manufacturers data sheet) to back it up.


Here's one for Bestgo cells, note the standard charging method in the Specifications table--that is very typical of almost all recent LiFePO datasheets.

Think of shunting as partial-shunting or slow-overcharge. Unless the shunt resistance is significantly lower than the cell internal resistance most of the current is still going thru the cell since the shunt is in parallel to the cell.

I think the answer to your posted question is No, because if you lower the voltage then you have no potential difference for current to flow--the charger voltage must be higher than the pack for charging to occur.


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## dcb (Dec 5, 2009)

the follow up question is how do you intend to detect when a cell is shunting and modify the voltage applied to the pack?


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## green caveman (Oct 2, 2009)

dcb said:


> the follow up question is how do you intend to detect when a cell is shunting and modify the voltage applied to the pack?


That was in post 12:

http://www.diyelectriccar.com/forums/showpost.php?p=524442&postcount=12


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## green caveman (Oct 2, 2009)

kennybobby said:


> Here's one for Bestgo cells, note the standard charging method in the Specifications table--that is very typical of almost all recent LiFePO datasheets.
> 
> Think of shunting as partial-shunting or slow-overcharge. Unless the shunt resistance is significantly lower than the cell internal resistance most of the current is still going thru the cell since the shunt is in parallel to the cell.
> 
> I think the answer to your posted question is No, because if you lower the voltage then you have no potential difference for current to flow--the charger voltage must be higher than the pack for charging to occur.


Excellent - thanks for the datasheet. That's pretty clear. I agree with you that if one manufacturer states C/20 the others won't be too different.

Despite this, I'm not sure how many chargers cut off at C/20. Elcon seems to have an AH/30 and AH/180 option (For 3.65V curves)

Earlier in the thread was a discussion of Elcon and a 1.25A balancing phase, so presumably the cutoff was < 1.25A. Moltenmetal doesn't specify the pack size - maybe many 25Ah cells.

There are references to and assumptions of low current cutoff elsewhere on various threads and some BMS manuals.

I too have 100Ah cells - so C/20 is 5A. Shunting any significant fraction of 5A+ is no mean feat. If you're going to shunt an insignificant fraction then maybe you should start sooner. Interestingly, I think that this means that shunting decisions should be made based on the whole pack, not an individual cell voltage. So you could, crudely, imagine that all cells above the pack average should be shunted regardless of the voltage of the cell. (That's too simplistic, but you get the idea).

At the same time, I have a feeling that if I cut off charging at 5A the cells will be better balanced. That is, much of the imbalance is introduced in the later (and it seems incorrect) parts of the CV phase. Or rather, some cells become overcharged in that phase.

OK, time to change the program in the NG3 (well, tomorrow, if I get a chance). I should really modify Dave's NG3 board design so that the PIC serial port pins are the spares and pull them out and put a BlueTooth module on them. That would allow control of the charger as well as monitoring. Knowing the current from the charger and each cell voltage from the BMS it should be possible to count Coulombs/cell. 

Oh, and yes, I agree that the answer to my original question is "no".


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## dcb (Dec 5, 2009)

green caveman said:


> That was in post 12:
> 
> http://www.diyelectriccar.com/forums/showpost.php?p=524442&postcount=12


OK, so you have a bms board with three outputs, and source code to the bms. And the bms has 3 outputs (can be chained), and you have some charger that you can control voltage to, and an android.

You *could* use the android to emulate a constant current phase, by continually modifying the voltage. And modify the bms firmware to just blip the bleed signal when it is time to switch to constant voltage. Though that is a lot of flakey layers and possibly not fail-safe.


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## green caveman (Oct 2, 2009)

dcb said:


> OK, so you have a bms board with three outputs, and source code to the bms. And the bms has 3 outputs (can be chained), and you have some charger that you can control voltage to, and an android.
> 
> You *could* use the android to emulate a constant current phase, by continually modifying the voltage. And modify the bms firmware to just blip the bleed signal when it is time to switch to constant voltage. Though that is a lot of flakey layers and possibly not fail-safe.


I have bluetooth on the BMS so that digital outputs are irrelevant except for failsafe (and until I have bluetooth on the charger).

I have the source code for the charger also. Between the two I could do almost anything, including bottom balance - although that would take quite a while because of the vast number of coulombs that have to be turned into heat.

With bluetooth on all the components (currently missing on the charger, but that's fixable, with some not-too-great effort). The you can build the system with the tablet controlling charger voltage/current and the shunts (and everything else) on the BMS. 

The failsafe - based on a watchdog timer on the communication link - is the traditional design, BMS shunt at 3.XV HVC at 4.0V and the charger does it's thing.

A potentially better design would be to daisy-chain the RS232 ports of the BMS and put a BT module on each end. Request goes in one end, each board resends the whole message to the next board then adds its own data. Whole packet is read from the BT module at the end of the chain. Reason for this is that I'm hitting the maximum allowable number of connected BT device (which, if memory serves is 7). With only two BT boards the design is more scalable (if I care).

I might yet make this change because, with the charger, controller and 6 BMS boards I'll be at 8 devices. Although I can't think of a time the charger and controller need to be connected at the same time. There's also no real reason to always stay connected to the BMS boards, so you could constantly connect/disconnect, but that's potentially slow.


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## dcb (Dec 5, 2009)

bluetooth goes under the heading of "flakey layer", as does android in general. I could see using bluetooth/android for display purposes, but not for control logic. Your best bet is to chain the ssr outputs so that when one opens the main controller (in the charger that you have source to?) gets a signal to change mode or stop charging, IMHO. Androids hang and bluetooth glitches, you don't want mode change/charge termination/bleeds dependant on any of that. This is surely not a product meant for production. But if you wanted to send charge status/current/pack voltage over bluetooth to an android from the controller then that makes some sense, though an onboard lcd would be more reliable.


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## green caveman (Oct 2, 2009)

dcb said:


> bluetooth goes under the heading of "flakey layer", as does android in general. I could see using bluetooth/android for display purposes, but not for control logic. Your best bet is to chain the ssr outputs so that when one opens the main controller (in the charger that you have source to?) gets a signal to change mode or stop charging, IMHO. Androids hang and bluetooth glitches, you don't want mode change/charge termination/bleeds dependant on any of that. This is surely not a product meant for production. But if you wanted to send charge status/current/pack voltage over bluetooth to an android from the controller then that makes some sense, though an onboard lcd would be more reliable.


I'm willing to bet that I have software on more Android devices installed in monitoring/supervisory control applications including some fairly nasty industrial environment (we develop and sell an app-builder for Android - that was mentioned in post 12 also). Android devices do not, routinely, have stability problems (that includes the $60 no-name tablets as well as the $500 name-brand).

That said, tablets, PCs, phones, Google Glass, etc. are usually limited to "supervisory control" (there are non-consumer grade versions of some systems that are a little more trustworthy). That is, they send commands and receive data from a device somewhere that will also ensure system/human safety if the supervisory controller goes to heck.

There's no reason to pick on BlueTooth. ALL communications are "flaky" - it's just a matter of degree. Whatever you install (Serial, Ethernet, WIFI, BT, Device Net, CAN, Profibus, etc. etc. ) needs error recovery (and preferably a fail-safe).

This system is incredibly slow. The timeframes are in the many seconds - plenty of time to recover a lost BT connection and re-establish communications. The BT modules are also moderately likely to hard-fail they are $5 easily replaceable parts, but again, this just dumps failsafe back to the BMS.

If I was looking for troubling failure points (and potentially catastrophic rather than recoverable) I would expect hard failures of any high voltage/high current parts of the system. I haven't seen MTBF numbers on BMS systems, but my guess would be the most failures are in shunts and the immediate surrounding circuitry due to overheating, mis-installation, etc. Certainly mine, and I suspect most, use consumer grade parts not industrial temperature or mil-spec. At some level Jack R is undoubtedly correct, these are one more point of failure - possibly, but it seems rarely, catastrophic.


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## dcb (Dec 5, 2009)

I don't buy it. There are countless stories of folks using high reliability networking in industrial settings, then watching things fail when an update occurs on a windows box "controller". Too many variables for my tastes. And after adding 7 bluetooth modules so it can communicate with an android, it will still be a shunt top balancer. That doesn't seem even little ridiculous to you?


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## green caveman (Oct 2, 2009)

dcb said:


> I don't buy it. There are countless stories of folks using high reliability networking in industrial settings, then watching things fail when an update occurs on a windows box "controller". Too many variables for my tastes. And after adding 7 bluetooth modules so it can communicate with an android, it will still be a shunt top balancer. That doesn't seem even little ridiculous to you?


I'm not exactly sure what you're objecting to. 

Yes you can design a system badly so that failure of a consumer grade product brings the whole system down. Similarly you could assume that any network is 100% reliable and introduce a failure. Having installed these systems for a good many years, I would advise against making any of those errors.

Design is always a trade-off. Android is much more than an LCD display. This is my car and there are things that I really want that a $70 Android tablet provides wonderfully - and BT is one way to get the data there. So, here's what I can get:

1) Real-time display of battery voltages. In the warm kitchen while the car is charging. On a dash (or more likely a window-mount) while the car is running.

2) Coulomb Count - charge and discharge. Possibly per cell.

3) History. SQLite on Android is wonderful for this. I can get all the voltages every few seconds, along with the accelerometer data, the current speed, temperatures, etc. (I could get the weather too).

4) Reports on history. 

5) Real-time display while driving, battery voltages (as above), lowest voltage cell, current speed, incline angle (that's just me). There are more, direction (compass), etc. etc. 

6) A dash-cam, in case something bad happens. (To me, that's enough justification on its own to have the window-mounted tablet).

There are many more things, but I don't care about them. For example, integration with Google Maps/Navigation (given the range of my car I consider this useless).

(An FM radio might have been a good addition, but they are now rare on tablets).

I can leave control to the charger/BMS, but if the tablet's running then it has cycles to spare and I can get clever. (What "clever" means in this context is sort of where this thread has morphed to). If the tablet's not running, not communicating, etc. I've lost nothing critical.

Also, I could get all this (and would still consider it worthwhile) if I was bottom balancing (I'm just not).


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## dcb (Dec 5, 2009)

I don't care if it is just personal use (well I do a little bit, but you don't seem to be receptive), your disclaimer of affiliation made it seem otherwise though. Have a ball and good luck


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## green caveman (Oct 2, 2009)

Don't know when I'll get to install the BMS boards (it's cold and wet outside and I have to travel next week) but here are a couple of Android screen shots with simulated inputs. This is running on the Memo Pad 7". The logic is (almost) all there. You can calibrate each cell by long-clicking and entering the measured voltage. Data is stored to SQLite (historical and configuration). 

If it keeps raining, I may break it into a couple of screens - moving things is easy enough. The whole app has taken a couple of days with most of that being calibration (they'll surely be some problems show up again the real devices). I think that the voltage values for each cell are not useful on the driving screen and could be moved to another screen (used mainly during charging). That would leave the min, max, average and range on the main screen. The colors change at 3.65 and 4.0 for high and 3.1 and 2.9 for lows. The simulated data is all over the place so the colors are extreme, hopefully it will look better with real values.

Trend across the bottom is total voltage and speed. Thoughts or ideas welcome...


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