BMS and the CellLog

It is tougher than it should be...

The other day I was driving back from work and the battery alarm started going off.  Now this was just after I had rewired the battery monitor cables to CellLogs so I thought there must have been a bad wire somewhere causing the alarm and that the batteries could not be alarm since I had just charged them this morning and it had only been about 3 miles.

New harness setup....

Turns out the alarm was working just fine and the pack had become so imbalanced that a few of the cells were barely charged.

Here is my best guess of what has happened.

1) The CellLog interface board from Electric Porsche opens up the (-)negative on the first cell in each subpack.  This shuts off the CellLogs but there is still some residual draw from the other cells (besides #1).  A quick measurement and measured 30mA on the Battery 1 negative.  Others have reported in this range with about about 0.5mA when off on most of the pins except battery 1.

2) From the factory, the CellLogs treat batteries 7&8 differently and don't pull current from them to power the CellLog itself, only for measurement purposes.

Proof of the problem.

1) The 26 cell pack is divided into 2 six cell and 2 seven cell subpacks

2) On all 4 subpacks, cell 5 is consistently the lowest OCV (open cell voltage) cell and #5 were all alarming during the earlier drive.

3) On the 7 cell subpacks, #7 is the highest OCV.  Using a CellLog 8S (which logs voltages over time), I monitored a recent charging cycle and here is what it looked like.  Clearly near the end of the charge cycle, Cell 7 is becoming full and triggering the charger to shut-off before the others have a chance to top off.  The data at the beginning of the graph is from driving around a bit.  Cell 5 is the lowest line.

4) After draining the 2 #7s a bit, all four #1 cells were reporting at the highest OCV.  Please note that the packs are fairly well balance and there is only a 20-30mV difference between the highest and lowers cells.

Some Math....

No Driving Scenario

1) Cell 1 current draw=0; rest of cells current draw = 0.5mA.

2) Over 6 months (180 days) = 2.2 AH of difference.  In a 60AH battery, this is about 4%.  Not a lot since I am trying to run the batteries between 10%-90% of the operating range.

3) I think this explains why the #1s are all reporting a little high, but not a lot.

While Driving

1) Celllog pulls about 30mA while operating, this is divided amount the first 6 cells, is 5mA/cell.

2) Assume driving for 1 hour/day for 100 days.

3) this equates to about 0.5AH of difference for Cell 7 which is not see a draw.

Plan going forward

1) It appears that simply opening up cell #1 in each subpack is not sufficient to limit the CellLogs slowly pulling the pack out of balance.  

2) Modify the two cell logs to add the D5-D4 jumper to help balance the draw from Cell 7.

3) Create a new CellLog board which completely isolates the CellLogs when power is off.  It will have 8 4PDT relays which will only energize during driving and charging.
4) This will also allow a optimization of the charger cut-off circuit by integrating with the rest of the CellLog interface. 

BLDC Timing Advance Update

The timing advance circuit is working well.  Lots of tuning lately and finally landed on a advance scheme that is smooth yet provides the additional boost above 1,000 RPM.  Noticeable current decline when the shifter is timing phase activated