# BMS About Face



## dcb (Dec 5, 2009)

I'm still on the learning curve, but I believed Jack Rickard when he did a video about the dangers of shunt regulators, and the effects of once cell going low on discharge, the benefits of bottom balancing, as well as the loss of cell life if constantly cycled from %100 to %0 SOC.


It is good to know when one cell is high or low (I.e. 80%/20%), and temperature probes, with some authority over the charger and the throttle perhaps. But actual balancing should probably be something done rarely IMHO, and you don't really need to display every cell voltage in the pack, just min/max and cell # for both. 

When charging, the weakest cell should hit the upper limit first, when discharging the weakest cell should hit the lower limit first, if the same cell isn't the one triggering the upper and lower limits, then the pack is out of balance, and the "idiot light" should go on.

Just thinking out loud.


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## dougingraham (Jul 26, 2011)

Derek, You may find that your low voltage set points are way too high for those days when the batteries are near or below 0 degrees C. I know that in the part of the country where you live this seldom happens. I set my low voltage cutoff to 90 volts because that is where the DC-DC is supposed to start having problems. This would be 1.73 volts per cell. Before I had the DC-DC installed and had just a 33 cell pack I set the cutoff to 1.65 volts. This allowed me to drive the car in the winter. When the temps are good the cells never sag this low anyway and your bottom balance will protect you from yourself. I have driven the car to the point of battery exhaustion (once) and with a bottom balanced pack the feeling is just like someone turned off a switch. One moment you have good power and when the batteries fall off the cliff you have none. When they are bottom balanced this happens to all of them at the same time so no one cell goes into reversal. And as we know it is reversal that kills lithium type cells almost instantly.


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## dcb (Dec 5, 2009)

I should note that with such an arrangement as in post #2, "balancing" would consist of noting which cell triggered the low limit (L), charging the pack till a cell hits the upper limit (H), then clipping on an isolated single cell charger to L and bringing it up to H (as opposed to going through the entire pack with a resistor and a voltmeter).


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## Sunking (Aug 10, 2009)

dougingraham said:


> Derek, You may find that your low voltage set points are way too high for those days when the batteries are near or below 0 degrees C. I know that in the part of the country where you live this seldom happens. I set my low voltage cutoff to 90 volts because that is where the DC-DC is supposed to start having problems. This would be *1.73 vpc*


Doug thank you for your comments and they are appreciated. Confession time this is for a NEV, a converted golf cart running 48 volts 16S 100 AH using a AC9 motor and Curtis 650 amp controller. I don't think that changes anything other than total pack voltages. I am using GBS 100 AH cells as a point of reference.

I can program the LVD to anything I want but where are you coming up with *1.65 and 1.73 volts*. I understand 2 volts is the line to never cross. I do not want to even tip my toe into the 0 or 100% SOC area. More like 10 to 90% SOC. Range is not an issue for me as 100 AH is more than enough. I want maximum cycle life, not range. 

You are correct in NTX winter temps are moderate. Rarely goes below freezing here.


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## Hollie Maea (Dec 9, 2009)

Just a quick comment, not taking a stand one way or another, but if you are doing top balancing with shunting, that doesn't force you to go all the way to 100SOC--you can have your CV/shunting threshold a little lower for the sake of cycle life. The one spot you are balanced won't quite be the top, but you are going to have imbalance at the bottom regardless.

The bottom line is, you will have imbalance on one side or the other. With top balance and shunting, you are balanced and safe on the charging side, and have to stay away from a fully discharged pack. With bottom balance, you are balanced and safe on the driving side and have to stay away from a fully charged pack. Without active balancing, you can't avoid having an imbalance on one end or another. I guess a lot of it come down to what you have more trust in: that your BMS circuits will do what they are told or that none of your cells have no self discharge.


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## dougingraham (Jul 26, 2011)

Sunking said:


> Doug thank you for your comments and they are appreciated. Confession time this is for a NEV, a converted golf cart running 48 volts 16S 100 AH using a AC9 motor and Curtis 650 amp controller. I don't think that changes anything other than total pack voltages. I am using GBS 100 AH cells as a point of reference.


I was pretty sure this was for your NEV and the only thing it changes is that if it is that cold it would be really unlikely that you would be taking the vehicle out for a spin.



Sunking said:


> I can program the LVD to anything I want but where are you coming up with *1.65 and 1.73 volts*. I understand 2 volts is the line to never cross. I do not want to even tip my toe into the 0 or 100% SOC area. More like 10 to 90% SOC. Range is not an issue for me as 100 AH is more than enough. I want maximum cycle life, not range.


The Cell voltage under load can pretty much be ignored as a cutoff factor when it is cold. I chose the 1.65 volts per cell number because it is half of the nominal voltage of 3.3. It should probably have been 1.6 volts as that is half of a nominal 3.2 volts. At half the nominal voltage you get maximum power. Below that power drops off because the voltage is too low. This is the voltage the the Motor controller will not allow the battery to go below. The 1.73 volts number is what I am using now. This is 90 volts (the DC-DC converter minimum voltage) divided by 52 cells. The car is kind of wimpy because it hits that 1.73 volts at a lower battery current than it did when I set it to the earlier 1.65 volts per cell (before DC-DC). Setting the low voltage limit to 1.65 is not an attempt to get more range. It is just so I can drive the car when it is cold. The lowest I have discharged the pack since that one time when I ran it till it wouldn't go anymore was 78% (22% SOC).

Hope that clarifies my earlier comments. Best Wishes.


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## Moltenmetal (Mar 20, 2014)

My opinion, which should not be considered fully informed-

A recent post of a talk by a guy from Dalhousie University made it clear to me that setting your top balance trip voltage too high is bad news, as is holding your cells at that high voltage trip point for any longer than necessary. Electro chemically driven oxidative damage to the electrolyte occurs at high voltage, resulting in fouling of the electrode pores, which leads to capacity loss and ultimate cell failure. The damage is duration of exposure, voltage and temperature dependent and of course is more severe for some Li ion chemistries than for others. They put magic soups in the electrolyte to prolong its life, but these components aren't perpetually self-regenerating- eventually they can be overwhelmed, so it's best not to challenge them.

That suggests to me that shunt charging for top balancing is a bad idea long term. It does little to store charge that is safe to use, and does this at a risk of decreased cycle life for the cells from oxidative damage. A better strategy would be to bottom balance, then stop charge when the lowest capacity cell reaches a conservatively set high voltage cutoff. Forget about using total pack voltage as a trip point for the charger- you'll never get there unless you set it based on what you observe when the first cell goes out AND it never changes- to be sure of that without a BMS you'd need to be very conservative indeed to avoid damaging the lowest capacity cell. Going this route would seem to abandon some unused available capacity in the pack, but in practice it is very little capacity in practical terms- ie capacity you can safely use without putting the pack at risk of damage at either end.

Bottom balancing gives you protection from cell reversal, but in practice if you care about cycle life you should stay above 30% SOC as a target. If you follow that by means of a good SOC monitor and good choices, bottom balancing too is pretty much irrelevant.

Can you get by without a BMS? I'd suggest it's necessary insurance against overcharging and against cell reversal in a pack which hasn't been bottom balanced ever, or for a while. But what you can do without is shunt charging and a charger with a complex curve. At the end of the day, it's HVC detected by the BMS that should end the charge if cycle life is your main concern. And overheating the pack also needs to be avoided if at ll possible. 

Regrettably, I have shunting that I can't shut off. But I can set my charger for a curve with one less cell than I have, and hopefully I can also get my BMS wiring working properly so the first cell hitting HVC will trip my charger and latch that way until I cycle the keyswitch (the way the BMS designer intended it to be used). That, and parking it on the hottest days, will make things optimal for cycle life for my pack. Or at least, that's my hope! I certainly can't afford another pack...!


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## glyndwr1998 (Apr 27, 2013)

I'm not sure what bms you have but if I had one that shunted, and I really didn't want to shunt, des older the shunt resistor out of the pcb, now no shunting, pretty crude but I'm sure it would work, if you chaged you mind just solder the resistor back in.

Just a thought!


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## Moltenmetal (Mar 20, 2014)

Nah, not going to risk damage to the celltop boards which are all working great at the moment. The BMS boards trip HVC at 3.65 V which Is conservative relative to the battery vendor's recommended max of 3.8 V- and with the top balance done well, we're talking about a fairly brief period of shunt charging before the high cell tops out. But the way I originally had it wired, which allowed the charger to restart again and again until the charger reached its top voltage and shut off, is not a good idea based on what I know now- if I'm right that is!


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## PStechPaul (May 1, 2012)

As I mentioned elsewhere, my latest idea for a BMS would be for groups of 8 cells maximum (24-30V), and would use the same resistor for charge shunting and bottom balancing. I agree that it may be best to limit charging to the point where the weakest cell reaches about 90-95%, and then have adequate SOC indication when driving to flash a warning at about 20% and shut down at 10%. All cells should deplete at the same time so the voltage drop of the entire pack at the low end should be quite apparent, and the BMS might not even be needed. But unless you have a different setup for charging and driving, the BMS will probably be connected at all times, and it will be important to reduce idle current drain and especially make sure it is not much greater for some cells than others.

I used my little circuit to charge a 1250 mA-h LiFePO4 cell to about 3.36V, and when I disconnected the charger, it soon dropped to 3.34V. I let it sit for a week, blinking an LED with 5% duty cycle, and it dropped to about 3.29V. The total load was a cheap DMM with 1meg input, a 110k resistor divider, and the PIC itself. I figure the drain was about 500 uA so after 1 week it (168 hours) would have used 84 mA-h or about 5% of capacity. I have it programmed to go to a 1% duty cycle flash at a LV cutoff of about 2.8V, and I could have it go to sleep at, say, 2.5V where it would draw only about 50 uA which would deplete this small cell to half capacity in about 12000 hours or 1.5 years.


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## Sunking (Aug 10, 2009)

PStechPaul said:


> important to reduce idle current drain and especially make sure it is not much greater for some cells than others.


Paul thanks for the comments but that does not make sense to me. The cells are in series and any current drain will always be equal in series circuits.

Are you thinking something like a tap for 12 volts because you are to cheap to buy a converter?


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## JRP3 (Mar 7, 2008)

Sunking said:


> However I can set and have a accurate ZERO reference point which is what I really want to know, not the top. So when I charge the pack, I use Constant Current until the first cell reaches 3.65 volts. That cell is the weakest in the pack, and when it hits 3.65 volts has the same capacity of the stronger cells. The magic is under discharge conditions because all cells will reach 0% at the same time.
> 
> Comments welcome.


3.65V might be a bit high, that cell will be consistently getting charged to a higher SOC than your other cells, and will likely age faster. With my 5+ year old pack my smallest capacity cell never goes above 3.5V unless I'm doing a maximum range charge, which is rare.


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## PStechPaul (May 1, 2012)

Sunking said:


> Paul thanks for the comments but that does not make sense to me. The cells are in series and any current drain will always be equal in series circuits.
> 
> Are you thinking something like a tap for 12 volts because you are to cheap to buy a converter?


Any simple set of voltage dividers on a series string of cells will draw more from the bottom than the top. See this, where I added resistors to the design that uses differential amplifiers, to balance the individual cell current drain:










Of course, having an individual isolated module on each cell will draw equal current, but it adds to the complexity and cost, and involves some way of extracting the information over the isolation devices.

I don't know what you mean about a 12V tap, or being "too cheap to buy a converter". It just seems more efficient to me, to use an 8 channel analog MUX with nearly infinite input impedance to sample each of up to 8 cells in a block of up to 24-30 VDC, and have that module communicate with maybe a dozen or so other modules, and/or a central module, that can display information and perform other functions such as adjusting the charge or limiting discharge by the motor controller.


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## IamIan (Mar 29, 2009)

Sunking said:


> The magic is under discharge conditions because all cells will reach 0% at the same time.


Unless ... cell drift.

Unless ... there is a significant variation in temperature from cell to cell.


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## JRP3 (Mar 7, 2008)

IamIan said:


> Unless ... cell drift.


Haven't seen it to any significant degree in 5 years, nor have most others going without a BMS.



> Unless ... there is a significant variation in temperature from cell to cell.


Again, hasn't been an issue for me, and though all cells are in the engine bay one bank is is in the bottom while the other two are at the top and get more motor and inverter heat.


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## IamIan (Mar 29, 2009)

JRP3 said:


> Haven't seen it to any significant degree in 5 years, nor have most others going without a BMS.


'significant' ... and 'most' ... being the key words.

That's why I am personally an advocate for at least the minimum of a middle pack differential .. as 'worst case insurance' to protect the sizable battery pack investment... cheap and easy to do... and as a freebie the same simple center differential can also detect a 'bad cell' that fails.



JRP3 said:


> Again, hasn't been an issue for me, and though all cells are in the engine bay one bank is is in the bottom while the other two are at the top and get more motor and inverter heat.


A good design prevents significant temperature difference between cells ... AFAIK temperature variation effects above 20Celcius are pretty small. 

But winter over night (10+ Hr) low temperatures .. it is a design concern to prevent the 'coldest' cells from getting into that range ... and prevent those most 'external' cells from being a 'significantly' colder temperature than the center of pack cells ... if not designed / accounted for .. that temperature variation .. can remove that nice 'all together' at bottom effect... especially from a terminal voltage PoV.


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## JRP3 (Mar 7, 2008)

I only used "most" as a qualifier because I am obviously not aware of the status of all packs, and "significant" as in causing any actual negative effects.


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## IamIan (Mar 29, 2009)

JRP3 said:


> I only used "most" as a qualifier because I am obviously not aware of the status of all packs, and "significant" as in causing any actual negative effects.


I'll still agree with 'most' ... But I am aware of a few instances that would fall outside of that 'most'.

I'll still agree with 'significant' .. especially combined with 'most'.

But neither changes the accuracy of what I already posted about conditions that effect the "reach 0% at the same time".

For example/clarification effort:
Winning the lottery is also unlikely .. especially to win anything 'significant' ... but if someone says 'it never happens to anyone' I'd disagree with them.

- - - - 

In addition to the previous 2 images related to temperature variation effects the Self Discharge rate would play a role in the 'cell drift'.


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## Sunking (Aug 10, 2009)

PStechPaul said:


> I don't know what you mean about a 12V tap, or being "too cheap to buy a converter".


A common mistake a lot of golf cart owners do to run lights and radio from the traction battery. Carts us 6 and 12 volt batteries to make 48 volt systems. Instead of buying and using a 48 DC to 12 DC converter will tap the batteries to get 12 volts.


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