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Old 03-10-2013, 06:52 AM   #101 (permalink)
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BMS Help

Hi,

I am currently experimenting with Nevilles BMS, the low balancing version. I have two issues now. I have 40 cells and 130VDC system.
System works very well and shows all parameters untill i press the pedal and drive. At that time master shows over 300V and over 60 cells!!!
I have three banks of cells and like 3m of 7wire shielded cable between them. I didnt put the shielding mesh to ground, should i? Would this be the problem here? Clearly EMI is causing comms go wild. BUT comms do not drop out!

Another capital problem is when i try to balance cells in bulk. Display normally shows "balancing 40" and then after a minute or so comms drop out! I mean i get "no comms" sign. I use 3R3 5W wirewound resistors with ceramic core. Should shunt cca 1A from 3V.
After i disengage balancing switch the comms return after a minute or so! Weird no? Maybe master optocoupler is giving trouble...hm?

Link to my build: mazdamx3ev.wordpress.com/tag/bms/
This site is in Slovenian, i intend to translate in the near future... as the time will allow.

Does anybody have any experience with this or suggestion what could cause this? I consider second problem more critical, as it means i cant even reliably balance my cells.

edit: When i connected one cell to master it was balancing ok for cca 1hour, hm... i have to try individual banks and determine where the error is.

A


Last edited by arber333; 03-10-2013 at 08:22 AM..
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Old 04-05-2013, 03:51 AM   #102 (permalink)
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Problem solved

Since some of you are working on your BMS i guess it is good to explain.
I changed comm lines to soft UTP computer cable. The thing is thin! But it works very good. I now have comms stable through all RPM and up to 200A (havent tried it faster ). However when i start to take off from standstill and just after i stop the comms fall out for a second and then resume. Basically i dont mind this as long as the system restarts. However it shows that some powerful EMI is going on at start and stop.
When i read the ACe5 inverter documentation i found out it injects stoping (countering) current into motor just before 0RPM. Hm...

The other problem just went away by itself . After i changed comm cables all three banks are balancing normaly. I tested for 1/2 hour, each module is stabilised at cca 800mA current, but transistor is still hot. I/O port disconnects charger at 3,6V as it should, so bottom balancing works.

I noticed something else though. If i have master connected to some cells and i add another bank to the comms in hot swap, the first modules start to balance without indication. I noticed this by smell of hot resistors. However if i restart (unplug 12V) master, everything shows normal, no balancing. A glitch in software maybe?

A

Last edited by arber333; 04-05-2013 at 04:12 AM..
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Old 05-16-2013, 04:37 PM   #103 (permalink)
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Resistors?

I have been tinkering with BMS for some time now, and i found it very precise when not driving. When underway i find the EMI disturbing voltage reports. However it rarely happens that the system locks up. Probably watchdog is not even aware of the EMI. A friend of mine suggested the interference is normal for AC systems wheres for DC they dont happen. Well maybe the brushes throw something out...

So i decided to play with optical link. I will use IR LED from last out terminal and IR transistor on the receiving end. I designed modules that simply fasten on existing terminals on the cell boards. Of course the will require + connection to the battery to power the module, but it should be simple to put one more terminal to the Batt positive. Then 1mm plastic fibre will carry signal undisturbed to the master module which is protected inside Alu box. Also master powers outgoing module. Should work without major changes. I got the idea here: http://www.rcgroups.com/forums/showthread.php?t=500234

Regarding 5W resistors, why not use 5W C5W car bulbs?
They are relatively cheap, a socket can be used to replace them if necessary and when you would want to perform regular inspection, your battery box will be all lit up .
Last time i used one 55W bulb to quickly bleed one cell to 2,8V. It took like 15minutes and it was with the others, requiring only a little bleeding in the end.

A
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Old 06-04-2013, 04:45 PM   #104 (permalink)
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Problem solved

Hi guys

I solved my problem of the jumpy comms. After msg with NevilleH about his input RC filter i tried as he did.

I got one 0,01uF(103) ceramic cap and soldered it across input resistor R1 (2,2K) pads with R1 in the middle. The first try was a dud. The disturbances were even higher. After brief msg with NevilleH he reccommended using scopemeter to identify the cap value where comm signal tops start to get rounded. That got me thinking; i was actually using different opto than he was. He used Vishay VO615A series but i got SFH618A. However the first couple cell boards i made according to original BOM. I went and cannibalised those boards and switched optos. Hurah! Success the first time. Comms are stable and glitches are gone. It looks like it wasnt the lines that were saturated, but master had its filtering set too wide.

I guess i will stop working on optical comms for now and focus on bluetooth serial connection and android app to show every cells voltages for the full pack. Finaly i can drive with confidence!

A
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Old 06-21-2013, 03:52 PM   #105 (permalink)
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Zero glitches...

OK now we re talking. I made the RC filter, and connected the charger off O/P as failsafe. But now i got constant glitches while driving. Once every 2 seconds, regular. I tried to disconnect the charger off line and sure enough - no glitches. I then went on and layed a new cable trough the cabin so the charger off line is totaly independent from comm lines. I guess it was 12V that was buzzing the comms while driving. Now that i have done this i had no glitches in two rides to work and it is the first time i trust BMS to work independently.
I will transition now to top balancing with shunts 3R3 5W and PMV30 mosfet instead of DNBT bipolar transistor. It will drain 1A insted of only 0.6A that DNBT allowed. I guess there was not enough drive current from PIC port to open bipolar fully. That mosfet is tested on the batteries and can be a direct replacement on the board. In theory it could drain 2A, but that would require 2R 7W resistor and the heat is already a lot.

A
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Old 10-22-2013, 08:15 AM   #106 (permalink)
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BMS Files

Anyone willing to make their own BMS. I prepared my iteration of files and documentation on Harlequin2s BMS on my website. Files are meaningfully named, though user manual is still written in Slovenian language. For english speakers, you should check Harlequin2s original manual.

http://mazdamx3ev.files.wordpress.co...avo-bms_r3.pdf

File is named .pdf due to server restriction with .rar files.
Download the file, rename it to .rar and browse. You should get idea of purpose from the file names .
There are gerber files for pcb manufacture as well as .pcb files for designing your own board dimension.

Regarding functionality, all works. You have to be precise with BOM. Specially optocouplers are critical. Now the 3.6V/cell charger cutoff works good and comms are stable. I also added failsafe loop to cutoff the charger if any cell goes beyond 3.8V and if comms fall off.

I am now working on communication with PC or Android device. So you could perform a sort of regular servicing, checking each cell value and spot irregularities. Shouldnt be too complicated... .

Have a great time constructing...

A

Last edited by arber333; 10-23-2013 at 01:47 AM..
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Old 11-10-2013, 11:29 PM   #107 (permalink)
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@harlequin2 have you considered listing the cell top modules and master modules on eBay?
I would be interested in either a PCB only or if available a kit where you get the PCB and components and then build it yourself. Even selling pre-programmed pic chips would probably help a lot of the less technical EVers that would like to use you system.
This looks like a very nice BMS, very elegant. I like that it is simple enough to be easily understood yet complex enough to do the task. Well done.

Last edited by Astro; 11-13-2013 at 03:49 AM.. Reason: Add pre-programmed PIC suggestion and fix typo's.
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Old 11-18-2013, 07:20 AM   #108 (permalink)
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Just had a thought that i would like to run past you all.
When top balancing a battery pack, if i understand it correctly. The charger puts out full charge current until the first cell reaches a set voltage. Then the charger goes to low current mode and the BMS modules switch a load across each cell as they reach the top balancing set voltage. This load effectively keeps them in a holding pattern until all the other cells catch up.
My thought was, why not have the BMS module switch a load across the cell with the highest voltage right at the start of the charging process. That way the charger could stay in high current mode. The cell with the highest voltage effectively gets charging current minus whatever the BMS module load absorbs.
As the other cells catch up another cell may become the highest voltage and so its BMS load is activated. So you are constantly reducing the charge rate of the cells with the highest state of charge.

Basically the cells are all in a race and you want them all to reach the finish line at the same time. So instead of letting the fast cells get to the finish line and then have to wait for the others. Simply reduce the fastest cells charge current allowing the slower cells to catch up mid race.
This way the charger stays in high current mode the whole time and you avoid the long wait at the end of the race as the slowest cells are slowly brought to the finish line with the reduced charger current.

I am new to this BMS stuff but i am slowly learning so please be gentle if this question is a total noob question.
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Old 11-20-2013, 04:08 PM   #109 (permalink)
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Principle is right. Charger will only see pack voltage (in most cases) and switches between full current/reduced current based on voltage information. It doesn't know if some individual cells have different voltage than others. When pack voltage reaches certain level charger will start to reduce current on its own. BMS might tell charger to ease a bit if certain cell voltage is reached or even tell charger to cut off completely.

If main goal is to have all the cells at same voltage in the end of the charge one should perform top balance separately for each cell. Ideally none of the shunts (load across the cell) shouldn't get activated during charge process and charger should finish the complete charge on its own. Only if some cells go out of balance and try to reach unwanted voltage levels should shunt resistors activate. And this should happen during recuded current charge. Pack requires manual balancing if BMS trips charger before it starts reducing current.

Shunt resistors will get hot when they are in use. If you want to remove more charge from a cell you need more powerful resistor. Then it gets even more hot and requires a big heatsink. It is not desirable to keep them on for a whole charge. It is all wasted energy there. The less you need to use them the better.
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Old 11-21-2013, 08:19 PM   #110 (permalink)
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I started thinking about using the cell top BMS to load the higher voltage cells at the start of the charging process after i read post by harlequin2. Harlequin2 mentioned that some people had balanced their cells by leaving them in reduced charge current state for a few days. That was probably an extreme example but it made me think, the low current period of cell balancing is burning up kwh's whilst possibly waiting for just one cell to balance.

Quote:
Originally Posted by harlequin2 View Post
...
I balanced all mine with an individual cell charger made from an old 5 V/ 20 A computer power supply modified to put out 3.6 V instead of 5 V. Might take a while if you have a few hundred cells!
Also, weber and coulomb on the AEVA balanced theirs with only 1/2 A just by leaving the charger in balancing mode for a few days - they have about 220 cells.
1/2 Amp * 220 cells * 72hrs = ~28.5kwh

I would imagine it would depend on how out of balance the pack is as to whether holding the charger at a reduced current until all the cells caught up would use more power than reducing the charge current only to the highest voltage cells for the whole charge process.

The total charge time would be reduced and i get the feeling that it would actually use less power to load the high cells during high current charging. Tthe calculations to work it out hurt my head.
But say your high current charge period was 8 hours then loading the highest voltage cells during that period would at most consume 8hrs * cell count * cell voltage * BMS load current.
For the example quoted from harlequin2 and assuming the high current charge period is 8hrs then if the cell balance was achieved during high current charging. Then it would be an absolute maximum of 3.16kwh rather than 28.5kwh burned up by the cell top BMS modules. As not all 220 cells would be loaded by the BMS, only the highest voltage ones, it would actually be considerably less than that.

This is all purely guess work so it may be very different in practice.

Another thought i had was that as more and more people take up electric cars it is likely that the cars will be treated more and more like a regular appliances. The users will be able to drive the car but have no idea of the mechanicals. Eventually it wont just be enthusiasts using them.
At that time having a reduced current balancing period at the end of the charge cycle may cause some very short lived battery packs as the users just plug them in and then when they are almost fully charged they unplug and drive. Because what difference does 98% full to 100% full make? So cell balancing may never actually occur and the pack drift more and more out of balance. If the balancing was done during the high current period of charge then no matter how short the charging period was cut, the pack would end up more in balance than it was before the charge.

I am hoping to build some of harlequin2's cell top bms modules and then play around with the software and see how it works in practice.

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