Vectux BMS - Feedback, please!

Batteries, Chargers, and Battery Management Systems.

Vectux BMS - Feedback, please!

Postby Mr. Mik » Wed Oct 08, 2008 8:27 am

I am continuing to test and re-condition the 102 NiMH cells in my Vectux.
It has taken 3 1/2 weeks so far, several more to follow at least....

The results of reconditioning the cells are very promising so far. And VEEEERY SLOOOOW .

I am pondering options for a BMS = "Battery Monitoring/Managing System" or rather a " Battery Manatoring" System.

This BMS is just a plan so far.

It would in it's current form be utterly dependent on (educated) human input every step along the way, a human/machine hybrid system so to say.

It integrates a single Paktrakr8 into the system, but more could be used.

The human input (once the hardware has all materialised) will consist of turning knobs on over 30 switches...

It is definitely a geeky device. I am not sure if it is moronic or ingenious, but I'm sure it has potential!

At least my soldering will improve due to all the practice!

The cable harness might end up too thick to leave the battery compartment, or the cumulative resistance of the 6-way rotary switches is too high, or their current rating too low. Lots of potentials SNAFU's.

Here is what I think it might be able to do:

A) Monitor voltage of each individual cell if selected. But not all at the same time.
B) Log minimum and maximum voltages to the DMM for review just after riding or charging.
C) Display real time voltage to help avoid reverse charging and overcharging when the the cell most likely to experience this is switched to. The PakTrakr8 can continue to monitor any cell or blocks of them even if another voltage is shown on the DMM display.
D) Allow to find the lowest cell by systematically "zooming in" on it, even during riding.
E) Allow to recondition individual cells without taking the scooter apart.
F) Allow recharging of individual cells, maybe even from a switchmode power supply powered by the entire string; this would dramatically increase range if one or a few cells have reduced capacity.

Could you please have a look at the attached PDF schematic and let me know if the design appears functional to you.

I would also like to hear about potential pitfalls before I start to build it.

The green line from the center switch is the output to the DMM or CBA2 battery analyser or the input from the charger.
Attachments
Vectux BMS Schematic expanded.pdf
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Last edited by Mr. Mik on Wed Oct 08, 2008 8:08 pm, edited 1 time in total.
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Wed Oct 08, 2008 7:22 pm

The input named RETAMPI in the schematic is a device which I have used on the Vectux for quite a while now.

RETAMPI is an acronym for REal Time AMP Indicator. It shows the percentage of the maximum possible current going through the battery. I do not know the absolute current yet. It produces negative voltages up to 100mV for load conditions, and positive voltages for charging, including regenerative braking.

Because the RETAMPI voltage output is going to be monitored by the PakTrakr8, I will be able to see in the Paktrakr log under what load (or regen current) the Voltages were measured. Without the need for the optional PakTrakr current sensor.
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Wed Oct 08, 2008 8:01 pm

I just realised that I made a mistake during conversion of the schematic from a "Mindmap" to a PDF file.

A part of the Map was "collapsed" and this is not obvious to people who are unfamiliar with this software. On the PDF conversion that part is just missing. It is the part for cells 1 to 17.

Therefore the schematic might not have made much sense to you!

Attached is the expanded version of the schematic.
Attachments
Vectux BMS Schematic expanded.pdf
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Re: Vectux BMS - Feedback, please!

Postby lawsonuw » Thu Oct 09, 2008 10:45 am

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=451-1038-ND would give 24 positions and two poles. Less switches needed. (this switch has non-shorting contacts too)

Personally I'd expose a tap for each cell and wait till the Linear Tech chip discussed in THIS thread comes out. I've also seen a similar chip from Maxim, TI, or etc. mentioned on the board. At a minimum this chip should make monitoring each cell simple.

Anywho, good luck with all this. :D

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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Sun Oct 12, 2008 9:22 am

Thank you, Marty!

The switch you suggest has a much better current rating (1A) then the ones I can source locally (150mA), but is very pricey.
I'd have to spend big bucks just to experiment for evaluation purposes.

As for using microprocessor switching: I might consider it once there is a chip available that has been tested and is recommended by the EV community, including "how-to"'s.

The "taps" you recommend are just cables connected to each cell, right? Would you include fuses?
That will be a main part of any BMS and would be able to be used in a variety of ways.

I have started to build a prototype for testing purposes:
Image
Image

If you click on the thumbnails you can see more detail. I have included a fuse in each cable and made the cables longer than they will be once installed in the Vectux.
The Sushi dish is large enough to hold all 34 switches if the testing shows that it works.

So far I have had success charging at 3A with the switches heating up to 25dC on the surface (20dC ambient) for a few hours, as well as discharging a cell through the CBA2 at 0.3A and at 0.1A (currently running).
The cut-off voltage measurement is of course very incorrect with all the resistance introduced between the cell and the CBA2, namely a fuse, long cable, connector, 4 switches in series, then clamps to the CBA2.

The charger had to go to approx 2.6V to charge at C/10 = 3A for a single cell at the start of charging. Usually about 1.46V.

But at least it looks like the switches can be connected the way I imagined / designed it.

What would be the right size fuse to use for each individual tap?
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Re: Vectux BMS - Feedback, please!

Postby lawsonuw » Sun Oct 12, 2008 11:02 am

Mr. Mik wrote:The switch you suggest has a much better current rating (1A) then the ones I can source locally (150mA), but is very pricey.
I'd have to spend big bucks just to experiment for evaluation purposes.


Yea, Digikey is likely not the cheapest place to get rotary switches from, it gives you a start for further web searching though. Even with the cost that one switch would select between a 24 cell block, replacing about 5 of your current rotary switches with just one. That's got to make the system easier to use, and less error prone. You could also just make your own switch.

Mr. Mik wrote:The "taps" you recommend are just cables connected to each cell, right? Would you include fuses?
That will be a main part of any BMS and would be able to be used in a variety of ways.


Yea the "taps" are just a wire to the junction between each cell. This gives easy access to each cell in the future for use with any battery management system you'd like. Right now I'm thinking that using wire with high temperature insulation, and carefully securing the wires is a better idea. Specifically the insulation should stay not allow a short even if one of the wires is blown or overloaded. I'd also tend to under size the wires a gauge or two to insure the wires blow during a short instead of just glowing. I'm not too hot on fuses. First 100 and some fuses with holders will get expensive fast. Second, for the fuses to do the most good they should be right at the cell junction. Third, the fuses add a fair amount of resistance, they'll get hot and restrict the maximum current that can be used.

Marty

P.S. bla! way too many cells it that battery pack!
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Mon Oct 13, 2008 7:58 am

In order to allow shunting of all current during charging the cables / fuses / switches would need to be able to take 10A continuously. That's probably unrealistic.
But the first part, from the inter-cell-connectors to outside of the battery, should be able to conduct 10A or 15A continuously in order to be somewhat "future proof" (preferably without even getting hot).
That might allow switching currents via relays or a whiz-bang new chip once it is available.

As long as I can get the whole system to accept 1.5A for deep discharging / reconditioning the cells and 3A for recharging individual cells at C/10 it will be useful.

I am running 1.5A through the prototype BMS at the moment, reconditioning a cell. I have set the cutoff level to 0.01V to allow the CBA2 to continue to work despite the current drop of 440mV.
0.68V at the CBA2 when at the cell contacts it's 1.110V.
The switches only heat up by 1degC, from 22 to 23degC, under that load.

If I can get the CBA2 to continue to discharge at 1.5A until the voltage at the battery terminal gets down to 0.4V, then I will probably be able to replicate the results of reconditioning the cells on the bench, but without significant downtime and with little effort. And I am talking about a 40% increase in capacity here! That's a whopper, considering that the weakest cell determines the overall capacity of the pack.


And with cables thick enough I might one day even be able to shuffle electrons between cells whilst riding.

In the meantime I am wondering how to best label or mark 103 cables emerging from 90kg of battery cells. Different colours might help, of course, but there are only so many, and I might get cheaper cable if I buy in bulk.
Maybe some sort of coloured bar-code - any ideas?
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Fri Oct 17, 2008 5:07 am

Mr. Mik wrote:
I am not sure if it is moronic or ingenious, but I'm sure it has potential!


Well, fortunately I realised the moronic part of the design before building it:

It has the potential to kill!

This rather unwanted effect could occur during the making of all those connections, or during handling of the finished product, or during an accident with damage to the scooter. It could even hit an unsuspecting rescue worker trying to clean up the mess after an accident.

The problem is that the design in it's above form would export up to 150V DC out of the relatively secure battery compartment, with capability to deliver much more than the 500mA needed to kill.

I missed this fact initially because I fell for the misconception that the largest number of cells handled by deliberate switching in the design is 34 cells, meaning 51V max during the end of charging.

But of course, the switches are receiving cables from cell 1 to cell 102, so there is the full voltage range in there, even if it cannot be deliberately "switched" to the DMM display. Picking up the connected device and ending up with a finger of each hand on the wrong rotary switch connectors, or touching them during the hundreds of soldering connections needing to be made could well be lethal.

Back to the drawing board!
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Re: Vectux BMS - Feedback, please!

Postby TylerDurden » Fri Oct 17, 2008 8:04 am

OK now I know why you want to add fuses (other thread)....

For cable identification, you can get sticky numbers from the electronics store; designed for that exact purpose. (They are like bird-bands.)
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Fri Oct 17, 2008 8:32 pm

TylerDurden wrote:OK now I know why you want to add fuses (other thread)....

For cable identification, you can get sticky numbers from the electronics store; designed for that exact purpose. (They are like bird-bands.)


Thanks, I'll check out the labels option!

Re: the fuses: I do not expect them to stop electrocution at all.

500mA is enough to kill a human, and I need much more amps to flow through those cables for charging and balancing.

I might however end up putting low-mA rated fuses between the part of the BMS inside the safe compartment and the cables leaving it. They can then only be used for monitoring what is happening. If I need to intervene (recondition a cell for example) I could open the battery fan housing to get to the tabs allowing to exchange 10-20A currents with individual cells.

At this stage I believe I might just connect up some of 35 cells (in series) after re-arranging the cells so that all the worst ones are in that sub-string.
As long as those weak cells are monitored and remain within safe limits, all the others should be OK.

And the voltages exported out of the safe compartment would remain below 50V max, non lethal, and therefore I could wire it in such a way that I can balance / recondition / charge / discharge the cells in this part of the whole string without taking the scooter apart.
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Wed Oct 29, 2008 7:34 am

The results of capacity testing and reconditioning of the 102 NiMH cells of my Vectux are such that 13 cells are significantly lower in capacity than the others. These cells are also showing some swelling.
The damaged cells were almost exclusively located in the lowest of three layers, and the damage done to the cells is in my opinion probably due to exposure to heat radiation from the hot road surfaces, during riding and parking/recharging in the "Australian Sun".

I have made a schematic showing the arrangement of the 102 cells in the two Vectrix batteries and the locations of connectors, temperature sensors and the two voltage sensors of the Stock-Vectrix BMS.
It also shows the planned Vectux BMS for the first 27 cells. (This will keep the voltages safe enough whilst allowing monitoring of all "at-risk-cells).
The diagram also shows how I plan to re-arrange the cells within the string so that the stock-BMS can "see" the imbalance much more clearly....
Low capacity cells are numbered in red; good cells which have been moved (to make room for concentrating the low capacity cells) are labeled in green. A high resolution .pdf file is attached to show you the necessary details if you are interested.
Image

Image

As you can see in the diagram, the stock BMS only monitors the voltage at V1 and V2 and GND. It would therefore be "aware" of the cumulative voltage of the first 27 cells, the middle 48 cells and the last 27 cells in the string.
As you can also see, I plan to locate all of the low capacity cells (red) into the 27-cell sub-string monitored as "V2" by the stock BMS.
The diagram below shows a different view of the same planned distribution of low-capacity-cells (red dot) and swollen cells (black dot) after re-shuffling them.
Image

An integral part of the planned Vectux BMS is the ability to recharge individual cells within the 27-cell segment which is being managed.
I hope to do this with a switch-mode power supply similar to the set-up shown in the diagram below:

Image

The main questions I would like you to comment on, speculate about etc. are these:

1) How do you think the Stock-Vectrix BMS will respond to the concentration of low capacity cells into one voltage-monitored segment?

2) How do I choose the right Switchmode power supply for the job?

Thank you!
Attachments
Vectux Cell reshuffle BMS.pdf
Unconventional schematic of vectrix battery structure, Vectrix stock BMS, planned changes to cell sequence and planned partial Vectux BMS. You'll work it out if you try...;-)
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Re: Vectux BMS - Feedback, please!

Postby fechter » Wed Oct 29, 2008 8:16 am

Mr. Mik wrote:
The main questions I would like you to comment on, speculate about etc. are these:

1) How do you think the Stock-Vectrix BMS will respond to the concentration of low capacity cells into one voltage-monitored segment?

2) How do I choose the right Switchmode power supply for the job?

Thank you!


Hard to say for sure, but I'd expect the range to be significantly reduced if the BMS senses low voltage on that segment and goes into shutdown. It may prevent a full charge on the other segments during charge also if one part comes up too early.

Not sure what you want to do with the power supply. First figure out what voltage/current you want.

One other thing, if the cells have vented a lot there's pretty much no way to restore their capacity. They will need to be replaced.
"One test is worth a thousand opinions"
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Sat Nov 01, 2008 9:37 am

fechter wrote:
Hard to say for sure, but I'd expect the range to be significantly reduced if the BMS senses low voltage on that segment and goes into shutdown. It may prevent a full charge on the other segments during charge also if one part comes up too early.

That would be a good outcome if the range limitation is not to extreme.
The good cells would last very long that way and the lower capacity cells would be protected from repeated damaging over-charge and over-discharge.
A bad outcome could be if the electronics just show me the battery warning symbol and the "Wrench of Doom", as another Vectrix rider termed it, i.e. the "maintenance required" telltale, and refuse to move the scooter an inch...
I hope to avoid this by placing all cells back into the scooter in fully charged state. There are basically no voltage differences obvious at full SOC and the voltage drop would gradually grow as the battery empties.
The incomplete charging of the good cells you mention might be a problem if I install a charger as described below. The ability to recharge low capacicty cells form the strong cells depends of course on the presence of "excess" charge in the good cells.
Not sure what you want to do with the power supply. First figure out what voltage/current you want.

I think I'll use what I already have: A 100-240V 50Hz to 15V DC at 5A switch-mode power supply from a dead laptop computer, and the IMAX B5 charger. The IMAX can accept input voltages from 11V to 18V and could therefore get powered straight from a string of 12 cells inside the 102 cell string; but that would introduce more imbalance.
So I hope to power the switch-mode supply from the 89 stronger cells in the pack, and the IMAX B5 gets powered by the switch-mode power supply and charges all (or some) of the 13 weaker cells. It can charge up to 14 NiMH cells in series and can charge at up to 5A (50W max).
One other thing, if the cells have vented a lot there's pretty much no way to restore their capacity. They will need to be replaced.

The bad cells improved about 12% to 18% with reconditioning, similar to the good cells. So the absolute difference in capacity is maybe even greater now than before reconditioning! But the overall range and power should be improved anyway.
I have come across a nugget regarding the venting: It might be possible to determine if and how much the cells have vented by weighing them! I will tackle that in the near future and find out if the low capacity, slightly swollen cells are also lighter.

In regards to my safety concerns about exporting lethal voltages out of the battery safety housing: I now plan to place the 13 low capacity cells adjacent to each other and wire them up for voltage measurement during riding and charging as well as for individual or group charging and discharging/reconditioning.
Image
The rest of the 102 cell string will be wired so that I can measure voltages of modules of 9 or 8 cells. I will also be able to recharge or discharge these modules of 8 or 9 cells after taking off the battery container cover, but the wires leaving the battery container from these cells will be originating from in-line 150kOhm resistors and therefore the maximum current leaving the safe container through those cables will be 1mA.

I am still not certain about how I will fuse the tap wires (coming from the cells to a junction inside the battery compartment.)
This affects the wires planned for the weak cells as well as the good (modules of) cells; the tap wires for both are probably going to be rated at 30A.
I have been unable to melt "20A Fusible Link Wire" at 40A so far. And several attempts to find good explanations on how to use Fusible Link Wire have yielded little. The length of the wire appears to be important, I guess it's because if it is too short then the heavier gauge wires soldered to it will act as a heat sink and prevent opening of the fusible link.
The CBA2 battery analyzer is only capable of maintaining 40A discharge current for NiMH cells for a short time. It turns itself off after about 30s at 40A (from 2 of the NiM cells.) Supposedly it works better for Lithium cells.

After several fruitless attempts to blow the "Fusible Link" I attached a 20A blade type fuse instead. It blew in under one second (at 40A.)
Image

So unless I get lucky somehow and find out more about fusible links, I'll have to manufacture a bunch of custom (holderless) fuses....
Last edited by Mr. Mik on Wed Nov 19, 2008 4:25 pm, edited 1 time in total.
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Fri Nov 07, 2008 7:55 am

The work on the battery pack is continuing to progress slowly. In a few days all cells will have been re-conditioned at least once, but there is still much to be done to install the BMS.
Like designing it... :wink:

But the design is coming along nicely, I think, here is the latest schematic which I have made to get clearer about what I need to do etc etc.


Image

The design might however contain beginners errors, like really nasty design faults that break stuff or make it unsafe.

I am still a beginner with electronics design and would appreciate it very much if the more experienced forum members could have a good look at the diagram (high resolution PDF attached) and let me know if this could work.

In particular I would like to know if anyone can see a way out of the battery safety container for any potentially lethal currents. I hope that I have placed 150kOhm resistors in all the right places, but it is a bit complex!

I hope I have included enough labels etc. to make it self-explanatory.

The general aims I hope to achieve with the design are:

1) The red (=low capacity) cells are all located adjacent to each other so I can charge them via the on-board switch-mode power supply, using the "surplus" charge in the good cells, and they can also be discharged if needed and re-conditioned without taking anything apart. The maximum voltage leaving the safety container through the 10A rated cables (to the boot under the seat) would be 13x1.5V=19.5V. (The single red cell in the top left corner shows no swelling and has recovered very well with reconditioning, but it can be individually monitored by the BMS to detect it early in case it deteriorates again).

2) The good cells can all be voltage monitored in Modules of 8 or 9 cells, during both riding and charging. No lethal current should be able to escape due to the 150kOhm resistors.

3) After opening the safety container the good cells can be recharged or discharged in modules of 8 or 9 cells. This can be done after disconnecting the (Stock) Andersons connector between the batteries, which limits the voltage to about 60-70V maximum.

4) The weak cells and the modules of good cells all have 30A cable with a 20A fuse right at the cell connector. This should allow for inclusion of a relatively powerful active and automatic BMS later on, without taking everything apart again.


Something I have not included at all yet is a temperature monitoring system under my control. All the 12 temperature sensors in the above diagram are controlled by the Vectrix Stock BMS and do not share their information very freely so far....

I would like to implant a few temp sensors of my own before putting it all together again - any suggestions what could work safely in this pack? Ideally some sort of sensors that will later on allow automatic programmed analysis of temperature gradients between ambient temperature and battery temperature to determine when and how long to run the Auxiliary Battery Cooling System.

Maybe I could even tap into those temperature sensors without messing up the info transmitted to the Vectrix electronics. I'll have a closer look at that soon.
Attachments
Vectux BMS Preliminary Design 2008-11-07b.pdf
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Tue Nov 11, 2008 8:00 am

Attached is the newest version of the Vectux BMS schematic including the final order of cells in the string - they are all back in modules of 8 or 9 cells now, phew! Big adventure.....Done!

Cell 103-099 is being reconditioned now, tomorrow or the day after they will all be done.

I have also attached the schematic for the switch array, lots of soldering to be done....

And I have cables and connectors and fuses ready to be soldered - but still no temp sensors.

Still not sure if I will stabilise the holderless fuses at the cell level with hot glue or epoxy resin - both work well, hot glue is much easier, but might melt in a catastrophic fashion when things go wrong badly.
Attachments
Vectux BMS Preliminary Design 2008-11-11b.pdf
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Vectux BMS Switch Array 2008-11-11b.pdf
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Re: Vectux BMS - Feedback, please!

Postby TylerDurden » Tue Nov 11, 2008 9:50 am

Mr. Mik wrote: Still not sure if I will stabilise the holderless fuses at the cell level with hot glue or epoxy resin - both work well, hot glue is much easier, but might melt in a catastrophic fashion when things go wrong badly.

Silicone sealant is tough but pliable and good at high temps... easier than epoxy too.
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Tue Nov 11, 2008 2:54 pm

Thanks for the hint!
I tried one type of clear silicone so far, but that particular type is too pliable and might allow the fuse wire to get bent when the cable gets yanked somehow.
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Re: Vectux BMS - Feedback, please!

Postby TylerDurden » Tue Nov 11, 2008 10:52 pm

Mr. Mik wrote:Thanks for the hint!
I tried one type of clear silicone so far, but that particular type is too pliable and might allow the fuse wire to get bent when the cable gets yanked somehow.


The art-car folks glue toys and stuff to autos with silicone... seems to hold pretty good.

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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Thu Nov 13, 2008 8:50 am

I had a great result trying to tap into the existing temp sensors of the Vectrix battery pack today:

I measured the resistance of the Vectrix battery temp probes and found that it is about 10kΩ at 25°C, reducing with increasing temperature.

Then I asked (again) at an electronics shop how their thermometers work - no luck, no useful answer. They had told me in the past that the temperature probes on their indoor /outdoor thermometers cannot be cut off or switched - I tried it out anyway and found that it works very well!
So I used the thermometer linked here together with a small switch and an old probe from a previously water damaged thermometer to measure the air temperature in the Vectux inlet and outlet. This allowed me to calculate the approximate battery temperature and worked well for several months.

Today I took this a big step further: I measured what the probes of the digital thermometer do, and, guess what: 10kΩ at 25°C !!! Yeeehah!

And when I soldered cables to the Vectrix BMS PCB (pictures without modifications below) I got almost identical temperature measurements from the two thermometer probes and the one Vectrix probe hooked up so far. I have tested it at 22°C and 35°C so far and am convinced that it will be useful over the entire measurement range needed. A further test with a plastic bag and glasses of hot water and ice cubes will hopefully confirm this.

So I will most likely be able to measure cell temperature at 12 locations on the cells and additional locations, like underneath the bottom layer to determine if heat is the culprit for the uneven capacity reduction distribution within the pack, and inlet and outlet air temperature to help decide when to run the fans.
And I will be able to display the temperatures with the AU$19.95 device on the dash board, using a few extra rotary switches in the (planned) array to allow selection of the temp sensor which I want to check.

I'll probably measure above the battery pack, giving me the average temperature of the cells in one of the batteries, below the batteries (small air space shared between both batteries) and in one of the intake air conduits.
And directly on 12 cells inside the pack!

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Mr. Mik

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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Fri Nov 14, 2008 7:50 am

Very favourable results of the Vectrix stock temp sensor testing with the AU$19.95 digital thermometer mentioned above:

Between 25°C and 53°C the two probes which came with the digital thermometer (and it's predecessor) and the stock Vectrix temp sensor cause the display of virtually identical temperature values on the digital thermometer (within 1°C). (Big sentence but you'll work it out if you are interested!)

At -18°C the stock Vectux sensor reports a balmy -13.5°C, though! I'll have to be sooo careful on my next trip to the Antarctic...;-) so I do not inadvertently overheat the batteries...

Just to satisfy the obsessional part-timer in me I'll continue to throw ice-cubes into the jar and find out what happens between 25°C and 5°C - not that it matters much at all, because in that temperature band the batteries would be as happy as pigs in mud, anyway!

Of course I won't be able to tell if the Vectrix CanBus gets confused by the measuring current that the digital thermometer will put through the sensors until I have put it all back together again, but I think it will be no problem.
Mr. Mik

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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Fri Nov 14, 2008 9:22 am

Further testing results:

Down to 9°C the stock Vectrix temp sensor causes the digital display to show fairly accurate measurements.

At temperatures below 9°C the stock Vectux temp sensor does not work well with this particular thermometer/display and shows the temperature to be higher than actual temperature. At 3°C it is about 1-2°C out, at -20°C it is about 6°C out.

I'm glad this measurement error is completely irrelevant for my pleasant climate zone!
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Sat Nov 22, 2008 6:06 pm

Oh, glorious taste of progress....!!

I can now measure the temperature inside a pair of socks in 8 places with some accuracy! ROTFLMAO!!
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Instead of risking to mess up the stock front battery temp sensor PCB by soldering additional cables onto it, I spliced additional cables into the cables between temp sensors and the stock PCB.
1 spliced cable for each sensor and one for common ground, total of 7 cables.
I also added 2 further temp sensors and there are connectors available for 2 more if I do not hook them up with common ground, or 5 more additional temp sensors with common ground.
There are also a further 17 unused cables in the rainbow cable for future extension if needed.



The three thumbnails below lead to photos of the circuit board I made for the front battery.
Initially it had banana-plug connectors on it for recharging modules of 8 cells, but I had to remove them to make it flatter. There is very little space to add anything into a Vectrix.
(The date stamps on the thumbs below are incorrect, the camera had reset itself to factory default)
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Instead I started to make an adapter to allow the module charging, which will be removed during normal operation of the Vectux, below are photos of the incomplete prototype:

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Below is a photo of the very first prototype which was even thicker, and a side by side photo with the latest (and hopefully final) circuit board before the banana plugs were removed:

Image Image
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Fri Nov 28, 2008 2:40 am

I came across a problem with the fuses I have soldered in line with the connector tabs:

The resistance of 2 x 20A fuses and cables and a single cell is too high to allow sufficient current to flow to open the fuse.

I tried to find out at what amperage the 20A fuses would actually open, starting with 10A and gradually increasing the current (using the CBA2). Initially I used a single Vectux cell, connected via two 20A fuses and two lenghts of 30A rated cables of similar length to what would be needed inside the actual pack. I measured the fuse temperature whilst increasing the current stepwise, but when I set the CBA2 to 20A it would only show a current of 18.6A or so.

1.2V/18.6A = 64.5mOhm resistance (about 2mOhm come from the cell itself).

Using two cells in series I managed to increase the current further, until the fuses got to just over 100dC at 26A without opening.

At 28A one of the fuses opened within 5 seconds.

What it means is that a short between neighbouring tabs would short the cell without the fuse opening, potentially destroying the cell. A short between any other tabs would open the fuse (unless the cells are very empty and cannot produce 28A for a short time).

I will need to test if 15A or 10A fuses would make a difference, but I suspect that their resistance might be higher and the same problem might occur - fuses not opening for a single-cell-short....
Mr. Mik

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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Sun Nov 30, 2008 9:03 pm

I have had my next encounter with Ohms law:

After I connected the PCB with the 150kOhm resistors to the rotary switches I did some testing and then did the maths....DOH!

I should have done the maths first.

Having 150kOhm x 2 in series with the 10MOhm of the DMM does (of course) introduce measurement error!

Theoretically it should be 2.91%, but turnes out to be between 2.9% and 3.2% including cables and switches.

So I get a result of 10.25V for an actual voltage of 10.59V for an 8-cell module.

The safety margins are much larger than planned, though: Less than the 1mA I was aiming at will get through at 150V.

I am trying to wrap my brain around Ohms law more thoroughly to decide which resistor type to use instead. Probably 15kOhm (2%) or similar.

That would limit the max current to 10mA (between the direct link to cell 103-001 and the 15kOhm link to cell 103-102).

This would allow touching the mangled remains of the Vectux BMS after an accident even with abrasions and / or wet hands. It would hurt but not kill, even if the rider or a pedestrian was caught underneath the wrecked scooter with the BMS cables touching an open wound.
Sounds dramatic but is very possible.



I'll try to calculate if I can use resistors with low tolerance (1% or 2%) with the right resistance to introduce a measurement error of either 1% or 0.1% to make it easy to correct the error mentally.
Last edited by Mr. Mik on Mon Feb 09, 2009 8:10 am, edited 1 time in total.
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Re: Vectux BMS - Feedback, please!

Postby Mr. Mik » Mon Dec 01, 2008 8:37 am

The front battery is back together with 8 tabs connected to a PCB with 150kOhm resistors and from there via rainbow-cable to the switch array.

The charging adapter works well, it will only be used after removal of the battery cooling cover during major maintenance work, like after a main fuse replacement or motor controller board repair.
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Charging through the adapter, whilst simultaneously measuring voltage through the 150kOhm resistors and temperature through the spliced taps to the thermistors works well on the bench:
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Of course, the 3% measurement error continues so far, and can make detection of neg-delta-V and imbalance more difficult.

I'll leave the 150kOhm resistors in the front battery BMS for now, but have bought 15kOhm (1% tolerance) resistors for the rear battery BMS.
The rear battery BMS will be set up for individual cell measurement and charging or discharging of individual cells, or groups of cells, as needed.
Using 15kOhm resistors will give me measurement results that are 99.7% of the correct value and will still limit the maximum amperage to a safe 10mA.
Mr. Mik

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