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Thread: BMS based on ltc 6804 and arduino

              
   
   
  1. #21
    Senior Member tomdb's Avatar
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    Another set back, this time again pcb troubles. Turns out they want the Vref1, not allowed to be used still to be capacitieve coupled outside the chip. Need to add two caps to solve this.

    However the software works, but gives wrong values due to the missing caps.

    update: new boards order, loss of 30 dollars worth of pcb, plus some components. I will salvage as much as is reasonable from the three old pcb's. Made the changes in a way that I can still use the solder stencil.

    Lets hope this is the last design revision before a installation on a actual battery pack.
    Good thing I view this as a learning experience so the few bucks here and there do not bother me too much.
    Last edited by tomdb; 29 February 2016 at 1157.

  2. #22
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    Tough break, but sounds like great progess on the whole. Good job.

  3. #23
    Senior Member tomdb's Avatar
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    Been a little distracted working on cracking oem inverters and getting a brushless motor from a hybrid working. Also got a charger from a Opel Ampera/Chevy Volt, which I can canbus control.

    So got myself nearly a complete vehicle with canbus control. Now just the vehicle.

  4. #24
    Senior Member tomdb's Avatar
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    An update:

    Got my time per board now down to roughly 1.5 hours for the total soldering time. Due to my stencil.

    It flashed the bootloader fine, then uploaded the software.

    All thats is left to check is the canbus side of things and then to test out the additional caps. This should be done tomorrow. If all goes well I know what im doing this weekend.
    Got roughly enough stock of components to make atleast 5 more boards.

    Besides making progress on the boards I got distracted by my DIY inverter build, have to get the test motor ready for mounting on a small self build electric moped. Besides this build im also working on an even larger BLDC motor for which I have to make a casing so a long term project. Also got a little carried away and am now building an semi automated spotwelder using my diy transformer spotwelder using pneumatics to control the force.

    As for the BMS side of things, if the testing goes well I will build my first test pack of 68 cells with 6 slaves monitoring them (due to module sizes). This way I can write bms master software that can control the charger and balancing.
    This is due to the fact that the Volt/Ampera charger will need atleast a set point of 200V to have a self shut-off feature.
    Attached Images Attached Images
    Last edited by tomdb; 05 April 2016 at 1425.

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  6. #25
    Senior Member tomdb's Avatar
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    Update;

    Its alive and well. Yesterday I only had 30 minutes of tinker time. Resulted in a slight depression, the isolated canbus side did not work however the LTC6804 was working perfectly. Only component that was from a different vender or exchanged were the optocouplers. Scoured the datasheets turns out there is a slight difference in voltage reaction. So that should not be the problem. Pondered quite sometime at work, ordered some just incase optocouplers.

    Turns out after some more troubleshooting, measuring the working canbus on the old board with the not working on the new one. Signals correct everywhere except the TX pin on my transceiver was a cold joint. Even my digital multimeter measured no connection. Little solder application and she chooches like never before.

    So tommorow evening Ill get all my parts sorted and organized so I can decided how many board I can make/want to make now. Should have enough parts for 5-6 units. Will get some video once I get one more board made and a little test setup.

  7. #26
    Senior Member tomdb's Avatar
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    Batch of 3 more boards is done.

    I believe that doing them more then 3 at once will result in a too long worktime for the solderpaste. Towards the end of component placement it was already becoming quite dry. However my turn-around time per board is now roughly one hour, starting with solderpaste application through component placement smd reflow, mounting the through-holes and finally troubleshooting and programming the chips.

    DSC_0351.jpg

    This means I can start testing of two of my 17s A123 amp20 modules in series. I will start with writing some code for the temporary arduino master to control the "balancing" (point of shunting) voltage and will use the internal transistors of the ltc6804 to start by driving some leds. Got add-on boards made that can shunt more current with separate transistor per cell.

    During testing I might start to run into the issue of an Arduino Uno has too small of an overhead to function as a master. Will be switching to a Teensy 3.1 later but might need to rig up a temporary board to get through testing the slaves.

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  9. #27
    Senior Member tomdb's Avatar
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    Regrettably one of the boards is quite dead on the canbus side, very strange. Will hook up my oscilloscope tonight to measure the spi bus and the canbus signals. Lucky I got three working boards which are the number needed for 2 series 17s modules. So i can start to do some 100v+ testing.

    Update:
    Scoped the board, after ofcourse trying to rush it using my multimeter and guessing. Turns out after replacing the canbus controller mpc2515 and the transciever (wrongly so) nothing was fixed. Got bummed out, it was late yesterday. So said screw that ill look at it tommorow.

    Got my scope out checked the spi bus, thaught i saw capacitance, so to double check it check a known working board. Same rise time. Damm not it.Checked the crystals, not it.

    Started checking the optocouplers. Transmit side not it, was seeing signals on the canbus so yea go figure. Measuring the recieving side opto, transiever to opto measured fine. Opto to controller, nothing.

    Shit, it was the opto all along, maybe i fried it during reflow.

    So the leason here is, have fait in your layout if some boards work and some dont.
    But most import, a multi meter is good for voltages, but for signal always use a scope. (best 400 euros spent so far on electronics)
    Last edited by tomdb; 11 April 2016 at 1344. Reason: Update

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  11. #28
    Senior Member tomdb's Avatar
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    Update.

    It looks like i got a winner this time. Messed around with my code some more. Wrote an dumb program to blast the control can message that the master generates on the canbus. Built a setup using an arduino with canbus shield, measuring a potiometer converting the value to my desired shunting voltage and inserting it in to my control message.

    Then tweaked the slave code with some more debugging lines and some fine tuning of the states. Inserted a led and resistor between the headers ment for the balancing add-onboards. And turning the pot made leds turn on and off. The test pack is not exactly balanced so this was a good scenario to test. Now I can populate the balance add-on boards and have a slave board run to balance a 12s section of the pack. With me monitoring the serial data ofcourse.

    Seems that I will be needing a compentant bms master soon, then comes the fun part of writing a suitable coulomb counter / voltage curve SOC algorithm. Planning on making it three step;

    Threshold voltage for low range (so reset to a fixed low value once number of cells dip below this voltage, and then decline with the voltage)
    Mid section above the threshold voltage in which coulomb counting is used to derive the SOC (need to figure out if an inductive sensor is good enough for this)
    High section above a set threshold (same as the low section, once certain amount of cells pass this voltage SOC is reset to a value and then rises with the voltage)

    Guess im at the stage where I am willing to share my pcb's and code if anyone is interested. However no guarantees on fault free functioning and ofcourse some assembly required.

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  13. #29
    Senior Member SplinterOz's Avatar
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    Quote Originally Posted by tomdb View Post
    ...
    Seems that I will be needing a compentant bms master soon, then comes the fun part of writing a suitable coulomb counter / voltage curve SOC algorithm. Planning on making it three step;

    Threshold voltage for low range (so reset to a fixed low value once number of cells dip below this voltage, and then decline with the voltage)
    Mid section above the threshold voltage in which coulomb counting is used to derive the SOC (need to figure out if an inductive sensor is good enough for this)
    High section above a set threshold (same as the low section, once certain amount of cells pass this voltage SOC is reset to a value and then rises with the voltage)
    Great news. I know I for one would like to have a look at the coulomb counting code and logic, I would also be really happy to help test and debug that part o f the project.
    SplinterOz
    From Australia'a Capital
    http://rgelectric.wordpress.com/

  14. #30
    Senior Member tomdb's Avatar
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    You built any kind of coulomb counter before? Right now just doing some research into how accurate do i need it. Plus looking into what current measurement would be the easiest/safest/accurate.

    The biggest problem I foresee is that if the pack is not charged to the top or " compeletly" discharged large drifts may occur, however I believe that worries like these will most likely never happen in real life use. Who does not charge their vehicle when done with riding it

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