# Individual cell sag



## evmetro (Apr 9, 2012)

I have 34 gbs gen 3 cells, and when I accelerate, I see two cells sag to 2.8 volts, while the other 32 cells sag to 3.1 volts. These cells are top balanced with an orion bms. I have tried adding a little bit of energy to them to make them read as the highest two cells, but they still sag lower during acceleration,. The orion is now drawing them down first, and they are almost back to perfect balance. The two cells in question are the first cells after a run of 0/2, in two of my five battery boxes. The first cells in the other three boxes are looking great so I am having doubts about location being an issue. Any ideas? Bad cells?


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## frodus (Apr 12, 2008)

Did you ever equalize them before you built the pack? I.e. did they each get individually charged with a single cell charger... or get all hooked in parallel?

If not... it'll take a few (maybe more) cycles up and down to balance them. 

Either start top balanced... or wait for a few more cycles for them to balance out. You could charge those cells up after a full charge with a single cell charger to ensure theyre in-fact top balanced.


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## major (Apr 4, 2008)

evmetro said:


> .... and when I accelerate, I see two cells sag to 2.8 volts, while the other 32 cells sag to 3.1 volts.


It sounds like those 2 cells have a higher than normal internal resistance. If so, it is not due to balance or SOC. I'd first check the connections to be sure the high resistance isn't on the terminal. If it still shows low cell voltage under load, maybe switch position with known good cells and see if the problem follows the cells. If it does, then you've pretty much confirmed those two cells have high internal resistance and I suggest to replace them. An extra 300mV sag is going to cause extra heat in those cells so they will get weaker and weaker and eventually experience a premature death. And they will cause your BMS to signal _empty_ well before the other cells have discharged far enough, or in other words, reduce your range.

Good catch. At least you know something is wrong


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## evmetro (Apr 9, 2012)

I top balanced them way back at 3.4 v cv until amps were down to 1.something...took about two weeks. I drove the car around a little trying different profiles, but never got one dialed in before tearing the car apart for overhaul. A few months later it is back on the road and I am getting very familiar with the orion. I have the delta set for .005, and have been leaving it plugged in all night after my commute so that it can draw down the high cells after the orion shuts down the charge enable. I understand that .005 may be a little far out, but I figured I would leave it this way for awhile to insure a nice balance, and then up it a little later. The cells are reading from 3.333 to 3.335 after charger shuts down and cells are not being drawn down. I snuck a few minutes of 3.4 volts into the two cells in question, and they are now the ones that disable the charge enable, and read 3.335 while the rest are at 3.334 and 3.333. They look great sitting at a red light, but they sag exponentially faster than the rest of the cells during acceleration. I can't push the pedal to the floor without fear of sagging beyond what is unthinkable...meanwhile, the other 32 are not sagging below 3.0xx. I have removed the bus bars to check out my connections and they still look good and tight.


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## major (Apr 4, 2008)

Balancing and/or state of charge will not compensate for high internal resistance cells. If they have high internal resistance there is nothing you can do to correct the problem short of replacing the cells or living with them the way they are.


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## ndplume (May 31, 2010)

major said:


> Balancing and/or state of charge will not compensate for high internal resistance cells. If they have high internal resistance there is nothing you can do to correct the problem short of replacing the cells or living with them the way they are.


But as Major mentioned earlier, make sure the resistance is not external. 

Here is where I'd look :
I have GBS cells (Gen 2) and they came pre-jumpered in 4 cell packs. The jumpers have multiple laminations, make sure the lamination count is the same among sagging and non-sagging batteries. Mine have 3 laminations (100AH cells).
I'd also try taking the jumpers off, cleaning the terminals and putting them back on and test. 
Swap the Orion lead to another cell operating correctly and test. 
If there are crimps on on the cables going to the 1st cells in the banks, check to make sure they are crimped correctly. Test. 
Last would be to swap the battery (or pack of 4 batteries) to another location and test.

I'd be interested to hear what you find out.


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## evmetro (Apr 9, 2012)

Major, I am not well educated on internal resistence, but I have added high and a low internal resistance gauges to the dash, and they show .4mv for the lowest cell, and 2.0 for the highest cell. I can hook the laptop up when I get to the shop tommorow and run some graphs for those cells and graph the resistence and voltage under load if it can help pinpoint the trouble.


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## steven4601 (Nov 11, 2010)

Hi,

I have two Orion BMS modules working together in the Z3 (string of 131 taps)
Orion BMS allows you (if you have enough spare cell-tap inputs on the BMS) to eliminate the chance of measuring external impedances. 
Either by the galvanic isolated tap-groups or by entering external impedance (bus-bar impedance noted in the Orion bms pc program). The first solution works best without experimenting. The bus-bar impedance correction is more meant for correcting hv power cable impedance. (at least from my point of view)

To determine the busbar (cable impedance?) you'd need to measure it (the voltage drop @ specific current) on the cells in question. 

However, a bad cell is a very realistic scenario. You can verify this with a carbon pile or any other programmable load and test the individual cell.

//Steven


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## evmetro (Apr 9, 2012)

Here is a test drive going through the gears. I picked 4 cells for comparison, and 14 and 31 are the ones in question. The other two are just for comparison. Red is 14, green is 31.










Same drive, but cell resistance instead.


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## mora (Nov 11, 2009)

0.8 ohms and 1.8 ohms are pretty far apart from each other. You can calculate your voltage drop even though your graphs and experience already tell the same tale.

R = U / I where R is resistance, U is voltage and I is current.
R * I = U

Let's assume you pull 100A from those cells.

0.0018 ohm * 100A = 0.18V. This is your voltage drop in theory. 

let's check the better cell at same amps:

0.0007 ohm * 100A = 0.07V. See, there is 0.1V difference already.

The more you stress the bad cells the more they will sag. I'd say bad cells but would remove them and test separately before drawing final conclusion.


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## evmetro (Apr 9, 2012)

I do not have cell testing equipment, but I have her up on the lift right now and am pulling the pack containing cell 14. I have come to call that pack the gas tank, since it is a 12 cell pack that fits where the gas tank went. That box contains 14 through 25. I figured I can rotate the first four pack of cells to the middle position in that tray, which will mean that 14 will become 18. I have had the tray down once before for this problem, but I only cleaned and inspected all of my connections, but this time I will have a better look at the bus bars. The bus bars are not solid stock, but rather braided cable. I had to fabricate many of these with 3/4" copper pipe squashed onto some stripped 0/2 cable and then soldered.


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## evmetro (Apr 9, 2012)

Ok, I have the fuel tank box out, and I don't feel confident about the 0/2 cable connection to #14. I have an anderson connector for this box, and the cable from the anderson connector to 14 neg has a crimp on lug on it that came with the cells. My understanding of these crimp style lugs is that you should imagine your kid on a tire swing with the tire attached to this lug, and this one looked like it was made for the next smaller gauge wire. I got out my copper pipe, chop saw, torch and solder and did the right thing. I went ahead and pulled all the bus bars (120 screws for 12 cells on gbs cells, 5 per terminal, 10 per cell) and hooked them up in parallel for the night. I will drive one of the the gross polluting gas sucking ICE metros home tonight, and rebuild the fuel tank box tomorrow, and rearrange the order of the 3 four packs of cells. If there is still a problem after this, I will order a couple cells.


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## steven4601 (Nov 11, 2010)

So everyone understands:
cell 14 , is the first cell, after a longer than usual (including anderson & long cable) connection? In fact, is it the longest?

Do you have the BMS wired in such a way you'll not measure the voltage over the cables and connectors? Each fuse / connector should be on a galvanic isolated interval/possition on the BMS. If you wire up the BMS with the regular cell-string over the cable/connector you'll see this behaviour.


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## evmetro (Apr 9, 2012)

The length of cable may be the longest, but I am not sure. There are several around the same length. This one is the one that has the bms hall effect sensor for the bms, and has the most complex routing. I have the voltage taps for this centralized bms wired with crimp on ring terminals mounted on top of the bus bars and cables. Each box has a molex connector as well for the cell taps. The molex and anderson facilitates rapid removal of each group of cells, but I am open to other options since the function of the cells takes priority over the design of the mounting system.


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## major (Apr 4, 2008)

evmetro said:


> The length of cable may be the longest, but I am not sure. There are several around the same length. This one is the one that has the bms hall effect sensor for the bms, and has the most complex routing. I have the voltage taps for this centralized bms wired with crimp on ring terminals mounted on top of the bus bars and cables. Each box has a molex connector as well for the cell taps. The molex and anderson facilitates rapid removal of each group of cells, but I am open to other options since the function of the cells takes priority over the design of the mounting system.


The resistance can be internal or external to the cell. The magnitude of resistance and voltage drop you are seeing is much greater than I would expect for a meter or two of 2/0 even with an Anderson. So I would go ahead and relocate the questionable cell and see if the problem follows the cell or stays with the cable.


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## evmetro (Apr 9, 2012)

I have all three four packs of cells rearranged now, and am installing all the bus bars. This is one of the downsides of the gbs cells. I do not know if the connection integrity is any better with these five screws instead of the traditional big bolt in the center, but it is a pain in the [email protected]# to put this many fasteners on. The bus bars that I got with the batteries were the braided cable type, but they were just long enough to go from one cell to the next, but only within the four pack. The four packs have end plates banded into the bundle, so when you place two four packs side by side the bus bar between four packs needs to be a little longer to compensate for the end plates. Of course when you build these bus bars, you need to drill ten screw holes into each bus bar that line up perfectly with the holes on the terminals. My home made bus bars needed to be flexible as well, since the square five screw lugs on the cells are not precision lined up. it looks like they are pressed onto a spline that goes into the cell, so if all the terminals are at various angles somewhere in the ball park of where they are supposed to be. My home made bus bars are lengths of 0/2 with the casing stripped off and then smashed flat inside of 1" lengths of 3/4" copper pipe, then soldered and drilled. I quickly learned to leave the strands of 0/2 clamped in the vice directly under the smashed pipe ends while I soldered them. The first attempt rendered a very stiff bus bar with solder going way past the copper pipe ends. When they cable is clamped in the vice, it keeps it cooler than the end of the bus bar that is being soldered, so it stays flexible for mounting between the random angles of the cell terminals.

You can see the bars that I made between the four packs and how they are a little longer than the other bars. A closer look shows how the terminal lugs are not all lined up the same. This pic is from months ago, so unfortunately I still have close to 100 screw left to install... I am still building the backside of this pic. 60 screws on this side, 60 screws on the other ends


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## evmetro (Apr 9, 2012)

Just took her for a test drive with the cables reworked and cells in new locations, and the verdict is in. The fault stayed behind, and #14 still sags! This is very good news to me, because this means that I am not in the market for new cells. I am not sure where the problem is now, but at least it is not in the cells. I have not studied the other end of the cable yet, where it departs the five cells under the passenger seat, but that is on my agenda. this also leaves other areas such as the crimps in the anderson connector, and perhaps the cell tap lead. The cell taps for each of the five battery locations all go through a molex connector. I am ruling out the anderson connector as a bad connection method until last, since there are other anderson connectors that are behaving normally.


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## steven4601 (Nov 11, 2010)

I couldn't find the right naming convention Ewert used for the BMS, its CELL GROUP/BANK.

You likely do not have the anderson in the right place in your layout.
Did you strictly follow this conceptual diagram?
*No cell-taps accros a fuse/disconnect or long cable run?*


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## evmetro (Apr 9, 2012)

The cell tap leads routing is isolated from the 0/2 routing, no cell taps cross the 0/2. I am still processing the image above...


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## frodus (Apr 12, 2008)

What he means is: On the 0/2 HV cable between cell 13/14/15, are there long cable runs, disconnects or any fuses?


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## evmetro (Apr 9, 2012)

From what I understand of that diagram, it says to not have any disconnects between cells 1 through 36. I have an anderson on each of my 5 battery boxes, so 5 andersons would be 10 connections in the circuit. My 1st box has cells 1 through 8, the next box has 9 through 13, then 14 through 25, then 26 through 30, and the 31 through 34. There is also a hv fuse, contactor, and + and - termination posts. Because there are andersons on each box, this means that the current flows through one anderson pin leaving one box, and then through a second pin to get into the next. Two pins per run. The andersons are placed with no reguard to the above ewert diagram.


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## frodus (Apr 12, 2008)

Thats likely the problem. Theres voltage drop across every connection... every long run.... so your bms may be seeing a lower than real voltage under load. Because from one cell to the next.. theyre not right next to eachother.


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## evmetro (Apr 9, 2012)

Is this only a problem in the voltage values displayed, or are these cells really sagging? I see in the cell population settings that there is a bus bar impedance area where I can adjust the values, but I have not felt comfortable making any changes without having a solid understanding of what I am adjusting. If at all possible, I would like to have these 5 battery packs set up for rapid removal and am willing to do a lot in the design to keep it super easy to work on later. The way that it is built right now, any of the 5 packs can be removed in under 5 minutes with an electric ratchet, and unplug an anderson and a molex. Obviously, function of the vehicle is the priority, but I would like to rise to the challange of maintaining the easy to work on theme of the vehicle.


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## frodus (Apr 12, 2008)

That's probably the issue. the BMS isn't accounting for the voltage drop (bussbar) from cell 13 to 14.

You said each of those cells is at the beginning of a box. One is cell 14. So the BMS needs the POSITIVE tap from cell 13 and the POSITIVE tap from cell 14. So the BMS is measuring the voltage of that cell MINUS the volage drop across that cable/connector. Since there's a cable in between, you get voltage drop across it while pulling amps. It could me 0.1V or even higher if you didn't choose the right gauge cable. This is worsened when you use anderson connectors, which have a higher-than-cable resistance.

Can you add a bussbar impedance between cell 13 and 14?


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## evmetro (Apr 9, 2012)

I am not sure how to come up with a value for the busbar impedance between 13 and 14. If I understand correcly, the impedance would change with the load? I see under cell population setting that it is possible to make adjustments to this... might I be able to adjust this out, or is the long and complex busbar actually causing those cells to sag and work harder than the others?


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## frodus (Apr 12, 2008)

Well, one way to tell is to attach a meter on that one cell, and drive the car. Compare measured value with the Orion value and see if they're the same. If not, then it's likely due to voltage drop across that long/high-impedance connection.

My hunch is that the cell is fine, and the Orion is fine, but you haven't configured it correctly to account for long/high-impedance connections between cells.

Consider calling Orion to ask them what settings to populate inside the Orion BMS. they're responsive.


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## evmetro (Apr 9, 2012)

I went ahead and played with the bus bar settings in the orion. They were all set to 0 for default settings, so if know what to set them back to if need be. I studied the resistance values on a test drive the orion showed higher resistance on all of the cable runs compared to the standard bus bars, so I corrected for them .01 Mohm at a time, and the individual cell sag began disappearing .01 Mohm at a time. I am able to correct all the cells in the pack to read the same resistance and then all the cells Iin the pack sag together. What I do not know however, is if these corrections provide the true picture of what is really going on. I am able to rotate cells to other locations with the adjusted bus bar values and have the cells all sag together, so it seems to be working.


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## Karter2 (Nov 17, 2011)

As previously mentioned, why dont you just hook a MMeter or a celllog direct onto the cells in question, bypassing ALL the existing wiring ?


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## Russco (Dec 23, 2008)

Karter2 said:


> As previously mentioned, why dont you just hook a MMeter or a celllog direct onto the cells in question, bypassing ALL the existing wiring ?


Same problem, cell log or ANY monitoring system looks at the positive of one cell and the negative of the next cell , since it uses one sensing wire. If ALL cells are in one location and ALL cells use the same factory bus bars all is fairly OK. But, if one bus bar is different, such as a long wire, it will cause an incorrect voltage reading during current draw due to the increased resistance of that one different wire, since E = IR. 

Reason is each (one) wire on BMS is shared with two connections. It looks like your BMS can be configured to calculate the voltage drop across this one (or another) battery to battery connection if the resistance (impedance) of the one connection is entered into the software, the BMS calculates the voltage drop using ohm's law under all and any current draw and makes a correction on the cell voltage readout.

That's the problem with cell to cell connections. If they aren't all the same, the voltage drops under load will vary. A Kelvin method of sensing would take care of that.


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## Karter2 (Nov 17, 2011)

Maybe i didnt make myself clear...


Karter2 said:


> As previously mentioned, why dont you just hook a MMeter or a celllog direct onto the cells in question,* bypassing ALL the existing wiring *?


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## Tesseract (Sep 27, 2008)

Karter2 said:


> Maybe i didnt make myself clear...


Actually, you didn't understand what Russco just explained. 

Rather than reword his explanation, I simulated the two different means of monitoring a stack of cells in SPICE and posted a screenshot of the simulation below. The 1m resistors represent the resistance of the cell to cell interconnects. When significant current flows through the battery pack there will be noticeable voltage drops along those interconnects that will affect the measurement of the cell voltages in the typical BMS which sends a single wire to each cell to cell junction.


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## Russco (Dec 23, 2008)

Tesseract said:


> Actually, you didn't understand what Russco just explained.
> 
> Rather than reword his explanation, I simulated the two different means of monitoring a stack of cells in SPICE and posted a screenshot of the simulation below. The 1m resistors represent the resistance of the cell to cell interconnects. When significant current flows through the battery pack there will be noticeable voltage drops along those interconnects that will affect the measurement of the cell voltages in the typical BMS which sends a single wire to each cell to cell junction.


The fix-it is to have a sensing wire to each battery screw instead of sharing two battery screws with the one sensing wire. That will double the number of wires going to the BMS.


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## evmetro (Apr 9, 2012)

Now I understand why the orion has a screen to calibrate the busbars.


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## Siwastaja (Aug 1, 2012)

This is one of the reasons why a distributed BMS both works better (is more accurate) and is easier with less wire spaghetti. It takes two wires to accurately measure a cell voltage, but digital communication can be done using just one wire.


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## Karter2 (Nov 17, 2011)

Tesseract said:


> Actually, you didn't understand what Russco just explained. .


 Actually i did ! ..what makes you think otherwise ?
Russco said..


> The fix-it is to have a sensing wire to each battery screw


 Which is what i also implied.


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## Tesseract (Sep 27, 2008)

Russco said:


> The fix-it is to have a sensing wire to each battery screw instead of sharing two battery screws with the one sensing wire. That will double the number of wires going to the BMS.


Nope - all that will do is cut the measurement error in half. For why, see below response to Karter2:



Karter2 said:


> Actually i did ! ..what makes you think otherwise ?
> ....


The attached picture shows why your idea won't work, and why I thought you did not understand what Russco wrote. A true Kelvin connection is only possible if *each* voltage monitoring device is floating. If the positive-most terminal of one voltage measuring circuit is connected to the negative-most terminal of the next voltage measuring circuit then the best you can do by using individual wires to the respective terminals on the series-connected cells anyway is to cut the measurement error in half.

The reason why is because the doubled-up wires simply provide an alternate pathway for pack current to flow. If there is a voltage drop on the high current cell-to-cell interconnects then that will cause current to flow along the doubled-up BMS wires which are essentially connected in parallel with the cell-to-cell interconnects. The amount of current that flows through the BMS wiring will depend on its total resistance, but its total voltage drop will be the same as the drop across the cell-to-cell interconnect. EDIT: to clarify, note that the voltmeters monitor the "center-tap" of the individual BMS wire resistances, so the measurement error will be cut in half *if* both BMS wires have the same resistance.


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## Russco (Dec 23, 2008)

Russco said:


> Same problem, cell log or ANY monitoring system looks at the positive of one cell and the negative of the next cell , since it uses one sensing wire. If ALL cells are in one location and ALL cells use the same factory bus bars all is fairly OK. But, if one bus bar is different, such as a long wire, it will cause an incorrect voltage reading during current draw due to the increased resistance of that one different wire, since E = IR.
> 
> Reason is each (one) wire on BMS is shared with two connections. It looks like your BMS can be configured to calculate the voltage drop across this one (or another) battery to battery connection if the resistance (impedance) of the one connection is entered into the software, the BMS calculates the voltage drop using ohm's law under all and any current draw and makes a correction on the cell voltage readout.
> 
> ...


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## evmetro (Apr 9, 2012)

I talked to support at orion today, and they confirmed that my calibrating the bus bars was correct, and that while my five box configuration may not have complied with the installation manual, it will work fine. If anybody is considering an orion bms, I would like to say that they really stand behind their product, and that they have been very patient dealing with all of my questions. For those of you who do not know, I plunged into ev conversions as a layman, and have had to learn a lot of stuff to install this equipment. This means that ewert (orion) had to have provided strong support. Between frodus and orion, this would not have happened for the layman.


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## frodus (Apr 12, 2008)

Glad you are getting it all figured out... I didn't mind helping at all. Its really you doing all the hard stuff! I'm just going through what would I do.

Gotta say though.. You've caught on well and are way better at explaining things than when you started... That helps a ton.

Good luck.. Hope it all gets working like ya want.


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