Low cost BMS

[P-hack]

BMS AE-LMD17 REV A1

Seen a 24 cell capacitive balancer for less than $200. 60 amp discharge, Capacitive is a lot more efficient than resistive, it basically uses a capacitor between adjacent cells and mosfets to shuttle charge from higher potential to lower potential:
and it should be able to operate while the vehicle is driven or charging (or just sitting, but that might lead to excessive discharge over time, lotsa leaky caps). IIRC it is like %50 efficient, depending on the distribution of imbalance of course. If you could alternate weaker cells and stronger cells it would work very well
but the cells on the end still don't get refreshed at the same rate.

There should be an inductive version, that takes pack voltage to bring up the low cells too.

[Astro]

@P-hack That is an interesting circuit. It would do the job nicely and if another capacitor and switch set was added then the last cell could swap charge with the first cell so that all cells received the same refresh rate. As i am looking at it i see the number of wires required and can't help but wonder if some sort of combined power transfer/communications could be achieved so that cell top BMS modules could communicate to the BMS master module along those wires and the power transfer capacitors could be integrated into the BMS master controller. Actually seeing as the BMS master controller has a micro controller maybe there is a way for the charge to be moved in a more controlled manner. The BMS master module just having one capacitor that it switched across the cell with the highest state of charge and then delivered that charge to the cell with the lowest state of charge. The BMS master could keep track of each cells performance and warn if a cell was going bad (high self discharge). This would avoid the scenario of one bad cell slowly taking charge from all the other cells and discharging the pack.
Just need a couple of high current switching arrays (does such a thing exist?). It would also mean that the charge swapping would be stopped once there was no more imbalance.

[P-hack]

I did some more investigating and that link above actually talks about inductive balancing. The parts for the power section look pretty cheap ($2.50/cell?) but driving those "floating" mosfets could get interesting. But you could use a 328 for starters (also floating?) with 6 cells per cpu, plus i2c communication between 328s or something. I don't really know what I'm doing just trying to figure it out. They discuss the losses in capacitive balancing in that page somewhere. You can see from the schematic below that one can "chain" power sections for any sized pack in an inductive setup too.

schematic from: http://www.ti.com/lit/ds/symlink/bq78pl114.pdf (discontinued).
power section (8 amps?):
2x 3300pf $0.60
2x 22uf $0.10
2x 20k $0.10
1x 2k $0.10
1x FDC6327C $0.33
2x MA21D3800L $0.17
1x 4.7uh $0.23
Total ~$2.53 per cell for the balance power section.

[Astro]

@arber333
Not sure if anyone else is having the same issue but renaming that pdf to a rar doesn't want to work.

Originally Posted by arber333

...
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...

[arber333]

Originally Posted by Astro

@arber333
Not sure if anyone else is having the same issue but renaming that pdf to a rar doesn't want to work.

What seems to be the problem? I tried the link and it downloaded .pdf fine. Then i right click to the file and select rename. Then just replace *.pdf with *.rar. System asks you if you are sure....yap! Then browse trough newly formed .rar archive.

It worked for lots of people untill now. I have to correct some errors in documentation and add some project files. I will add rev4 as soon as i rig my EMW charger to BMS fully. This is taking my whole time now and garage is not heated .

Anybody has any ideas about android app to show BMS values trough bluetooth?

For now i intend tu use arduino nano with 1,8" display to display only status during load AND save data to a file. The latter is more important for analysis.

A

[arber333]

Originally Posted by Astro

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.
.

I see the logic behind this...
For now my BMS uses either one large 3W/5W resistor or two of the variant for maximum balancing 4A if the mosfet would hold. BUT in reality i see some glitches while balancing for prolonged time at 2A. Possibly heat is interfering with PIC12F... I recommend single 5W 3R3 resistor or dual 3W 4R7 for 1A balancing.
I see that now when cells are equal even 0,5A would work ok.

If you want to balance early you would have to change the code for charger off condition and program PWM so you would only partially open shunt at 3,5V and then at 3,65V balance fully.
Though it is of no use; now i charge at 45A and i see cells riding at 3,5V almost all the time. Only in the end one or two runs ahead. It would be very difficult to balance at high amps without measuring internal resistance. But this is simple BMS... I opted to just reduce charger down to 10A when one module starts to balance - o/p1 of the master. It works for me now...

A

[Astro]

Originally Posted by arber333

What seems to be the problem? I tried the link and it downloaded .pdf fine. Then i right click to the file and select rename. Then just replace *.pdf with *.rar. System asks you if you are sure....yap! Then browse trough newly formed .rar archive.
It worked for lots of people untill now.
A

Arber333, finally got the .rar to decompress. Had to use a WinXP virtual machine running RarZilla. The .rar file wouldn't play nice with any of the Linux based archivers i tried.
Had a look through the files and there is a lot of stuff in there.
Most of this stuff is beyond what i want to have to learn. I am more a software person. I was hoping that harlequin2 would have revisited the forums. I had hoped he would have continued developing his BMS into a product/kit. I am sure i would have had no trouble soldering up the board and programming the software but going from a schematic or a pcb file to the collection of files that the PCB manufacturers want is daunting. I could just see myself ordering $100 worth of pcbs to find them all messed up when they arrived.

I think i will just wait and see if harlequin2 returns and is willing to order some extra celltop/master pcbs and sell them to forum members. Or even better sell a kit of pcbs and components.

I just want to get a working hardware base that i can then get stuck into developing software for. harlequin2's setup was nice and simple. All the smart bits were in the software. This is why i think it would make an excellent starting point for me.

A thought i had as i was looking at your pcb was the idea of an infra-red link between modules. I think you (maybe someone else) mentioned it earlier in the thread. Setting up an infra-red receiver and transmitter on each module. If you placed the transmitter and the receiver side by side and left enough length on the devices legs so they could be bent over to face one side of the pcb or the other depending on the modules orientation. Then i think you would get good alignment with only few cm or so separating transmitter and receiver. You could even put a sleeve (tube) from transmitter to receiver to eliminate all crosstalk and outside interference. If you used a tube between modules you could even use visible light rather than infra-red which would make trouble shooting easier as you would be able to see the flashing led of the transmitter. The tube wouldn't have to be anything special, i am imagining something like a drinking straw but of a material that would cope with the heat it may experience. Get a few meters of tube and cut the lengths required to join the modules. Automotive vacuum line would probably do and is very cheap.

[arber333]

Originally Posted by Astro

Arber333, finally got the .rar to decompress. Had to use a WinXP virtual machine running RarZilla. The .rar file wouldn't play nice with any of the Linux based archivers i tried.
Had a look through the files and there is a lot of stuff in there.
Most of this stuff is beyond what i want to have to learn. I am more a software person. I was hoping that harlequin2 would have revisited the forums. I had hoped he would have continued developing his BMS into a product/kit. I am sure i would have had no trouble soldering up the board and programming the software but going from a schematic or a pcb file to the collection of files that the PCB manufacturers want is daunting. I could just see myself ordering $100 worth of pcbs to find them all messed up when they arrived.

I think i will just wait and see if harlequin2 returns and is willing to order some extra celltop/master pcbs and sell them to forum members. Or even better sell a kit of pcbs and components.

I just want to get a working hardware base that i can then get stuck into developing software for. harlequin2's setup was nice and simple. All the smart bits were in the software. This is why i think it would make an excellent starting point for me.

A thought i had as i was looking at your pcb was the idea of an infra-red link between modules. I think you (maybe someone else) mentioned it earlier in the thread. Setting up an infra-red receiver and transmitter on each module. If you placed the transmitter and the receiver side by side and left enough length on the devices legs so they could be bent over to face one side of the pcb or the other depending on the modules orientation. Then i think you would get good alignment with only few cm or so separating transmitter and receiver. You could even put a sleeve (tube) from transmitter to receiver to eliminate all crosstalk and outside interference. If you used a tube between modules you could even use visible light rather than infra-red which would make trouble shooting easier as you would be able to see the flashing led of the transmitter. The tube wouldn't have to be anything special, i am imagining something like a drinking straw but of a material that would cope with the heat it may experience. Get a few meters of tube and cut the lengths required to join the modules. Automotive vacuum line would probably do and is very cheap.

Well OK, but i already made two sets of BMS modules and hardware works also i know at least one guy made his BMS on my files basis. I got a second one asking...
My files are jus a tad bit different than Nevilles. However i am now improving on software to suit my application if you like to play with source code then PM me.

Regarding IR link check this:
DIY Fiber Optic link - RC Groups

I would use twisted wires troughout battery box, since box is usualy enclosed with sheet metal (faraday cage) i would use only first and last module as receiver and transmitter for optic cable. That way my three boxes and master would be connected by light - no EMI trough cables...

If you still want to use tubes, i would reccomend 4mm al tubes used for model airplane commands - perfectly level and stiff. Carbon tubes are difficult to cut...

A

[4Springs]

Hello Everyone,
I have an electric car and have built the Low Cost BMS. Many thanks to harlequin2 for creating this and sharing it for everyone!

I've created a SourceForge page here: sourceforge.net/projects/low-cost-bms/

The page is intended to be a place where anyone can download all you need to get started with this project. I have uploaded all of my files and I'd encourage anyone else involved with this project to do the same - either join up and become a contributor or PM me and I can upload the files for you. I'm hoping that someone browsing the page will be able to select from several different versions, depending on their cell size, compiler etc.

[mora]

Hello 4Springs.

Did you include your version of master board with TCcharger control circuit in the package? I don't have Designspark, at least yet, so I couldn't open any of those files to take a look.

This got me interested. I could try and build it with for my 12V battery (4x 90Ah LFP) which currently doesn't have any BMS. Has been so for years already. And same for other 12V pack which I use as trolling motor battery.