kiev wrote:The internal resistance of 2.8 mOhms is good and shows slightly higher than the first car due to the colder temperature.
I have a strong suspicion that the BMU doesn't actually measure the internal cell resistance of individual cells (even though it could) and that this figure is purely calculated from a lookup table based on cell temperature sensor readings. In other words it assumes the cell resistance based on how the cell should perform at a given temperature.
I've monitored this figure a lot and it only seems to correlate with differences in cell temperature. Prior to replacing some cells in my pack I had three cells that had obviously higher than normal internal resistance due to them going to maximum voltage well before other cells during a rapid charge, (reaching 4.105 volts even at 20% SoC after less than a minute charging) and yet there was never any spread indicated between minimum and maximum resistance, with warm cell resistance being 1.5mO.
After replacing the suspect cells the "new" cells now maintain a significantly lower voltage than others during rapid charging (suggesting they actually have lower internal resistance than the remaining original cells) and yet the minimum and maximum resistance values quoted by the BMU have not changed and still just relate to cell temperature.
So I don't think any useful information about the true health of the cells can be gleaned from the internal resistance figures if they are just derived from cell temperature with a lookup table.
There are only three useful stats for looking at the condition of the battery pack in my opinion:
1) Reported overall Ah figure.
2) The voltage balance between cells at 10-30% SoC after a full balancing charge can be used to identify weak cells with low capacity. (The voltage balance above 40% is more or less meaningless, and no inference should be taken from a good balance at a high SoC)
3) The voltage balance between cells at a low SoC a couple of minutes into a Chademo rapid charging session can be used to identify cells with high internal resistance as these will quickly go to the maximum 4.105 volts at high charge current long before other cells that are healthy.