New EVE 12-cell Marine House Bank

[Dave McCampbell]

We've just commissioned our new 3P4S 12v EVE LF280K prismatic cell house bank with useable capacity around 900ahrs. Max charge will be about 100amps, max load about 150amps. We are using our Electrodacus SBMS0 for BMS. I have wired it as per the attached photo. A couple of questions:

1. Should I double up on the 3 connecting bars between the 4S sets of cells?

2. Are the main battery cables placed properly at the final cells on opposite ends of the U shape configuration?

3. What is the proper terminal nut torque for these welded studs cells?

4. Any other suggestions for improvement?

[sailingharry]

My suggestions:

*  Where possible, you should connect to the middle cell of each 3P.  For the battery cables, move in one cell.  For the U-Turn, move to the middle cell.  For the first and third, it would be awesome if you could use a cable from middle-to-middle, but that may be a bridge too far.  At the very least, the jumpers (the ones between each 3P) should be doubled up.

*  Measure the actual copper in those jumpers, and compare that to this site:  https://www.copper.org/applications/electrical/busbar/bus_table1.html  You will find that the busbar vendors massively overrate their bars.  They are woefully inadequate.  Look at your fuse size (or at the very least your realistic ongoing current), and see the cross section required.  If you are fused at under 200A, you are probably OK.

*  You have two problems and one solution.  Your batteries are not held down, and you have a risk of shorting terminals across the cells.  If you lay a 2x2 board, held down at each end, over each row of cells, you will minimize risk of dropping a wrench across the terminals, and also hold them down.  I make a groove over each vent spot so I don't block them.

*  The solution above doesn't protect between bank 1/2 and 3/4.  Consider that.

*  You also have a risk between 1 and 4.  You have a barrier plate running down the length, but I can't tell how high it is.  Mine protrudes up, so it again minimizes shorting risk.

*  In those hold-down boards I also ran a groove length wise, and inserted a string of thermal fuses (about 150F, I forget the actual number) set over each cell, so that if any cell overheats, it trips the contactor.

[Dacian Todea (electrodacus)]

1. Not sure what the characteristics of those bars are but at 150A max load is likely fine.

2. It is not optimum but for your application it will work fine.

3. 6Nm

4. Optimum will have been to have the groups of 3p cells rotated by 90 degrees.  That way you can have 3 connections. But I do not think you have the space in that box to rotate those cells as 3x72mm = 216mm so more than width of a cell 207mm and box seems to be exact right size as it is.

Just apply max possible load for 15 minutes then check if all connections are around ambient temperature or if you have a thermal camera even better check battery but also all your other connections up to your load. 

[Dave McCampbell]

Thanks to Harry and Dacian for the comments.
1 Regarding cell bussbar carrying capacity, I found on BatteryDesign.net that the capacity for good quality Cu bars was 5.0-5.9A/sq mm. All bussbars that I've seen provided with cells are 2mmx20mm.  I was able to find quality 3mmx20mm connecting bars, so 60mm squared times 5 equals a minimum of 300A carrying capacity. Plenty I think for my needs, and the extra 3mm threaded hole in these bussbars is perfect for connecting voltage sense leads, etc. without having to use the cell stud connection.  The flexible braided Cu bussbars, like Dacians, have 5mm thick terminal ends, so would not work with 10mm long EVE studs.
2 I cleaned all the cell bussbar connections with vinegar/water mix and isopropyl alcohol and torqued them to 5 Nm so there was no danger of exceeding EVE torque spec of 6 Nm max.  The small electronic torque guage available on the internet for about $30 is perfect for doing this.
3 Since the EVE cell terminal stud length is just 10mm there is not enough room to do more than the required in some places double the 3mm thick bussbars and add the nice 4mm thick provided nut with the bussbars.  That should be plenty for my use. 
 
Dacian, I should have checked first here before I built my box so I could have gotten the optimum arrangement.  Next time.  I will check to see if any connections develop heat.
Harry, thanks for the several suggestions.  As mentioned above, given the constraints with the short cell studs, I can't do much more than I have already done with additional connections, ie two 30mm bussbars stacked on top of each other.  Connecting anything more to the middle cells with already two bussbars connected would be impossible.  I do have a hold down system, not shown, to keep the cells in place if the boat inverts.  Re protection for the cell sides all have plastic between cells and between cells and box sides.  Also, there is a significant amount of cushioning under the box and between the box and bottom of the cells.

[Dacian Todea (electrodacus)]

the 60mm^2 bars about 8cm long will have a resistance of around 0.025mOhm so at 300A you get about 2.25W of heat and with the cells terminals helping with cooling it should not be a problem.  The contact resistance needs to be low but if you cleaned the surface and applied 5Nm then it should be OK.

[sailingharry]

The important part of my comment regarding the bus bars is to ensure you design for it.  You've done that, so you're set!  Ali is the most diverse source for bus bars, but some vendors are a little....loose?... with their stated specs.

[Dave McCampbell]

Thanks Dacian and Harry.  I have one more issue:

For some time now both our temporary 4 cell 4S battery and our new 12 cell 4S3P battery have had a problem with the first and last cell voltages going high early during charging.  See the screen shot on our Node Red display below.  This divergence starts at about 3.42vpc with cells 1 and 4 shooting high and cells 3 and 4 remaining around 3.42vpc until the end of charging.  At HVD 3.55vpc the spread between the two high and 2 low cells is well over 100mv.   The drop in voltage between the first and second spikes was because we had to turn off charging for a while, but the end result near the top of the charge is the same. 

For the past year, while hauled out under a shed with no solar, we have been using our shore charger charging at about 45 amps.  This rate was less than .2C for both batteries.  Although the temporary battery may have had balance problems to begin with, we carefully top balanced the entire new battery cells, but it still exhibits what appears to be the same cell divergence problem.

Our supply/charge positive and negative cables are connected at the first and last cells as shown in the image above.  I am wondering if the positioning of those cables would have anything to do with the uneven divergence of the charge voltages.  As an option I could make the cable connections to the battery at cell terminals 4 negative and 9 positive.

[Dacian Todea (electrodacus)]

Dave,

There is zero problems with any of the cells. This is typical behavior for LiFePO4.

The cells start to diverge for the last 10 minutes and average charge current over those last 10 minutes seems to be 30A (starting from around 40A and dropping to 20A).

30A * 10/60 = 5Ah 

5Ah/280Ah = 1.8% delta between cells.  While this seems a bit more than typical 0.5% to 1% that I observed on my batteries is not significantly high.

Not sure if you have cell balancing enabled or not as that may explain the higher closer to 2% deviation vs less than 1% expected. Or maybe this is not a 280Ah battery as I assumed.

If this is the 4S3P battery in the first post then that will be a 840Ah battery and then 5Ah / 840Ah = 0.6% deviation and will be as expected and as I also measured on my cells.

Where the cells sense are located makes little if any difference at this low charge currents so that has nothing to do with your readings at the end of charge. And the place you chose to connect the sense wires are perfectly reasonable for that setup.

[Dave McCampbell]

Thanks for that Dacian.  The balancing was not on at that time of the screen shot and it is for the larger 4S3P 840ahr battery.  My concern is that cells 2 and 3 seem to stop charging completely while the others, 1 and 4, continue to receive all the charge, although the connections were all cleaned and torqued evenly.  So you don't think my connecting of the main battery cable at cells 1 and 4 have anything to do with those cells continuing to charge while the other two do not?

[Dacian Todea (electrodacus)]

Dave,

Not sure I understand what you mean by "cell 1 and 4 continue to receive charge" cells 2 and 3 where also charging. It is impossible for just some of the cells to charge. Is either all or none.

I asumme that a few seconds after you took that screenshot the charging stopped as cell 1 exceeded 3.55V.

[Dave McCampbell]

Dacian. 

By my statement above that  'cells 2 and 3 seem to stop charging completely while cells 1 and 4 continue to receive charge', I mean that cells 1 and 4 voltages continue to rise while cells 2 and 3 voltages stop rising and just remain the same for the final few minutes of charging before HVD.  This happens  starting around 3.420vpc which seems rather early. 

I would have expected that all voltages would continue to rise, but those for cells 1 and 4 would just rise faster than those of cells 2 and 3.  This exact same thing happened with cells in the same positions while using the temporary 4S battery earlier, but using entirely different cells.  What would could cause voltages for cells 2 and 3 to completely stop at that point while voltages for cells 1 and 4 continue to rise?  Maybe something to do with my wiring or the connection point for my main battery cables on cells 1 and 4?

[Dacian Todea (electrodacus)]

Same current flows trough all cells especially since you disabled the cell balancing.

Voltage very slightly decreases on cells 2 and 3 because the charge current decreases from a bit over 40A down to around 20A over that 10 minute period.

Cells 1 and 4 where already full so they had no more space to store electrons thus the voltage rapidly increased.

From around 12:30 to 12:45 you can better see all cells voltage continue to increase just cells 1 and 4 at a faster rate. Not quite sure what stopped the charging then since no cell got to 3.55V and some similar thing happened around 12:01 when charging stopped for no apparent reason for a few minutes.

Also a bit before 11:00 when charging first started you see that initial change in voltage over maybe around 10 minutes after charging started but then all cell voltage remained constant for the next hour or so.

If you where able to continue charging it will have taken maybe no more than 10 to 15 minutes before cell 2 and cell 3 will have also reached 3.55V.

Cells are not equal they have different capacity and internal impedance. So every time you charge and discharge them some of the energy will be lost as heat and that amount of energy lost as heat is not equal for all cells.

The cell balancing will keep the pack from exceeding 1% imbalance but can not keep them at 0% imbalance (not for LiFePO4). For other type of lithium cells where charge discharge curve is less flat cell balancing can do a better job and keep the pack at maybe as low as 0.1% imbalance.

If you disable cell balancing you could see the imbalance of the pack increase by about 2% per month (based on my experiment a few years ago). But if you keep the cell balancing enabled then pack imbalance will be maintained below 1% typical around 0.5%

The point is that there is nothing abnormal about your pack and you can not expect to have better pack balance unless you are willing to waste a lot of time and energy at the end of each end of charge to just decrease the imbalance from typical 0.5% to maybe 0.1%.  There is nothing to be gained by doing so.

You can see the same thing on my pack.

At around 9:07 I had a full charge for the day and I was charging at that time with about 1000W / 26V = 38A so about the same current as in your case is just that I have a 8s2p

It just happens that cell 1 was also the first to get to 3.55V and charging stopped.

It is a bit hard to see due to cell balancing but cell 6 and 7 are at around 3.47V and if you look at the graph cell 1 was at that voltage with 2 minutes earlier meaning that cell 6 and 7 are just 38A / (60/2) = 1.26Ah behind cell 1 so 1.26Ah/560Ah = 0.225% 

Cell 3 looks like the lowest after charge ended but during charging cell 5 was lowest at around 3.43V and cell 1 was around there about 4 minutes earlier thus around 0.45% delta from cell 5

At this point my battery is a few years old and this cell delta deviation remains around this typical 0.5% delta. 

[Dave McCampbell]

Many thanks for that detailed explanation Dacian.  It is interesting that your first and last cells, 1 and 8, are exhibiting the same performance as my first and last cells, 1 and 4.  We spent quite a bit of time capacity testing and top balancing these 12 well matched EVE cells, so I think their balancing is  pretty close.  Are your main battery cables also connected to your first and last cells? 

I turned off the SBMS0 balancing temporarily to better see what was happening to the cell voltages during charging.  Now I will turn it back on.  Also, I manually turned off the charging for a bit in order to reset the shore charger as it only charges for an hour at a time.  It is a pretty lame Sterling charger which is advertised to have a LFP charging setting but doesn't really.  So we have to use a custom setting that only goes for an hour.  I'll be glad when we can get back to using solar for charging through our two Victron MPPTs.  Dave

[Dacian Todea (electrodacus)]

This sort of cells have fairly low internal impedance so the busbars and connections can add some significant increase to the internal impedance.

Here is a photo of my battery cell 1 is top right and cell 8 is bottom right. So yes cell 1 voltage measurement does not include a bussbar  but cell 8 does.

If cells will have been perfectly equal in characteristics (capacity and internal impedance) and the cell connections and bussbars where also perfectly equal then cells will stay in perfect balance with no need for any cell balancing.

But the characteristics are not equal so cells will get out of balance typical 0.5% per month so could be 6% per year but if cell balancing is used the cell imbalance will be forever limited to typical 0.5% max 1% for LiFePO4