Old BMS General Thread

[GregsGarage]

After driving the Fiat to work this morning, went to connect the charger and noticed that the high voltage cut-out led on my management module was on. A quick check with a DVOM showed that the fault was in my left rear battery box (instead of daisy chaining all the optos together I run the wires from each of my 4 battery boxes to my management module and connect them together there). Lifting the lid, I could see the culprit by the illuminated load led.

The moment of truth, was it a software lockup or a hardware fault. Reseting the pic with the reset jumper made no difference, so I removed the board and had a closer look. Found one suspect solder joint, resoldered and tested with bench supply, all back to normal.

P.S. Peter, got the screw terminals today. Thanks.

[retepsnikrep]

Found one suspect solder joint, resoldered and tested with bench supply, all back to normal.

Greg, which solder joint exactly? That will useful if any further issues.

Must have caused pic to read high my guess is the lm385 v ref somewhere?

Phew, glad it worked ok once you spotted it.

[GregsGarage]

Sorry Peter,

I wasn't paying attention, one of the resistors, but now its back in the car I really can't tell you which one it was. The boards are simple enough that any problems shouldn't take too long to trouble shoot with the schematic, a DVOM and adjustable bench power supply. I found a few faults while building the slaves and testing on the bench, mostly poor solder joints but one faulty led. Hopefully that will be the end of any problems.

By the way, the power supply can be a simple unit like this one that I used.
SkyTronic 0-20v 2amp. The seller is top notch and based in Goole, Yorkshire.

[mikep_95133]

Peter,

BTW, do you use a long threaded stud for anchoring the interconnects and the slave boards to each cell?

Mike

[retepsnikrep]

Mike, I used threaded plated mild steel stud cut to length.

Just reporting on my first balancing charge which has been going on for 36hrs at 150ma. 49 cells are full at 3.60v and the load lights are cycling on and off nicely, all within 40mv of each other.

I cell is stuck at 3.36v for some reason. So it may be just very out of balance and will catch up eventually if I give it long enough. Checked it with meter and Slave is reporting correct voltage to Master. Or perhaps Slave has a fault and load is operating unseen bleeding off the power, meter and infra red thermometer do not suport this though at the moment I'll give it another 24hrs (3Ah charge) and see if it pulling it's socks up. I'll bung it on an individual charge later if still no joy. might just be a low spec cell we shall see.

Bizzarely when I turn on my 8x 80w fluorescent lights in the garage during charging it trips out the Master and cuts charging, obviously picking up the emi through the charger somehow and giving a bad cell V reading. To that end and to reduce errors I have re-introduced 10x oversampling back into the Digital Slaves.

I'll report on that later, hopefully one spurious reading won't then trigger an error.

I've also added an interrupt driven low power sleep mode for the digital slaves so consumption drops from 1ma (operating) to <200ua (sleeping). They wake up whenever the Master is turned on, and go back to sleep when no data requests for > 1 minute.

I haven't loaded the slaves with this new software yet but will do Monday. Hopefully that errant cell will be upto spec by then.

[arnolde]

Hi everyone, I just wanted to let you know I finally got a demo board of the LTC6802 BMS chip (mentioned somewhat further up). I had to go through the whole NDA procedure so I couldnt talk about it, but just a few days ago, the datasheet was released: http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1037,C1134,P86662,D26880

I've also been promised some more sample chips in 4-6 weeks, enough to finish my 600V battery pack. So thank God that will save me almost 1000 EUR in BMS costs (200 slaves...)! I plan to make some boards with several LTC6802 chips on them, and see if we can interface those to Peter's master controller - now that would be cool combining cheap slave hardware with peter's open source project

[retepsnikrep]

Ethan

Good luck with that, nice to have a different chip and direction, the more the merrier IMO. Not sure how it would interface with my Master that could certainly be a long term project.

[retepsnikrep]

Sorted my earlier problem above, cell 30 was just badly out of balance, charged it seperately to get it in line and now all 50 are within 40mv of each other, It would have done it at 150ma but would take days!

Going to re-wire two of the data plugs to get battery block data connectors down from three to two, don't know why I didn't do that in first place Oh well all good fun. I want everything prepared and well tested before install day so I can just drop it in and plug and pray

[GregsGarage]

Peter,

Glad that last cell charged up o.k. Mine took a while to balance as well, first time around, but some of that was due to my poor attempts at manually balancing the cells. These cells will sit at around 3.4 volts (depending on charging current) almost to the point when they reach a full charge and then the voltage climbs quite quickly. The temptation would be to put a larger load to reduce balancing times but if you have a problem as I did were one load got stuck on then it could drain the cell quite quickly.

Interesting to see how yours behave after a few cycles.

[retepsnikrep]

Greg.

My few years experience with the 200ah TS cells and Cedric's 250ma balancing load cell protectors leads me to believe that in regular use our 250ma load will be plenty for balancing, especially as my cells are only 40ah. I do have the luxury of course of being notified by the Master of any cell problem and I can see the individual cell voltages.

Your boards still working OK?

Peter