Impedance Track cell balancing issue

Qmax will not update if the conditions for an update is not met. In your scenario, Qmax will update during that rest/relaxation period. In response to your question, yes if there is an update, the value can be trusted.

-Onyx

Hi, Thanks for you response. What if a Qmax update happens while the charger is holding the pack in a float voltage thus raising each individual cell voltage?  In the customer installation the charger will hold the pack in float indefinitely during which the current will decrease to near zero and the voltage will be at a steady state thus the Qmax update conditions will be met.  The fuel gauge may think it is in relaxation, but in reality it is being held at a float votlage.  How can the fuel gauge accurately update Qmax if each cell voltage is being artificially inflated by the charger float voltage.  And if Qmax updates during this float period could this have adverse effects on the cell balancing calculations?

Are you saying that the voltage which the charger holds the pack at is not the termintaion voltage? How does this happen and what's the reasoning behind doing that?

Bear in mind that when Qmax updates, it does so for each of the cells so the cell balancing calculations will not be affected.

-Onyx

These are LiFePO4 cells so the charger goes to the termination voltage of about 3.65V per cell then drops to a nominal float at 3.45V once the taper current is reached.  This is to offset any parasitic loads and keep the pack SOC at 100%.

Please expand on your second statement.  I know Qmax is updated for each cell, that is my concern because if there is charging voltage applied how can an accurate OCV measurement be made?  Also, the applied charging voltage exaggerates the voltage differences between the cells and therefore the calculated capacity differences will be exageratted as well, correct?

What I am trying to understand is if the IT technology is even applicable to this type of charging environment where the cells are never really in relaxation because there is always some charging voltage applied.

Hi Nick,

The bq20z65 is not optimized to work with  LFP batteries. The better alternatives are the bq30z55 and bq34z100. If you must use the z65, certain modifications need to be made to the .encr file to ensure proper guaging accuracy, Qmax and Ra updates occurence. This is due to the very flat OCV curve of LFP cells.

Now as regards your application, when the charger drops to 3.45 V, the pack voltage will obviously be higher than the charger voltage and there will be no current flow. This means that there is the possibility that your cells get to be sufficiently rested and an OCV reading will be taken. On the other hand, if the current during the OCV reading is non-zero, then an IR correction is done. The first iteration of DOD (depth of discharge)  is found from the uncorrected OCV reading; then the cell resistance value is found from the internal R(DOD) table and used to correct the OCV value as OCV'=OCV- I xR. The corrected DOD is then found from OCV'. You obtain the best accuracy if the current is below C/20 rate.

In summary, Impedance track should work with your application.

thanks

Onyx

Hi please answer the below:

1.  Where do I find out more info on what specific changes need to be made to the .encr file?

2.  Please answer if Qmax update is accurate under these exact conditions: charger is holding each cell voltage at 3.5V, there is no current flow, It has been 5hours since last current flow, if charging voltage is removed actual cell OCV would be 3.4V.  How can Qmax calculation account for this?

No answer?  Still trying to understand what happens to Qmax if charger is holding the cells at a higher voltage than the OCV??

Qmax will be calculated based on what the DOD is set at that voltage at which it rests.

The parameters in the .encr are hidden and you do not have access to them.

I have sent you a friend invite. Lets take this communication offline.

thanks

Onyx