What does temperature have to do with it?
Pretty much everything, it seems.
Now that we’ve waded through the second 25 pages of posts and visited countless other articles, blogs and images from said posts, the fact that all of our early battery capacity loss LEAFs are in Phoenix, or at least in Arizona, is no coincidence.
It took only 11 days for the first 25 pages of posts to be created regarding this topic. Two weeks covered the next 25 pages, taking us up to June 4. We had two more LEAF owners declare battery capacity loss – both living in Arizona – bringing our vehicle count to seven thus far. All seven vehicles were in service from June through September of 2011. Why is this important? Because for three months straight from June 10, 2011 through September 10, 2011, the high temperature in Phoenix was over 100 degrees every single day. The highest high was 118 degrees on July 2, and there were 34 days at or over 110 degrees with one consecutive eight day stretch with highs of 110 degrees or higher. The lows were typically in the 80 to 90 degree range during this time, with drops into the high 70 degree range near the beginning and end of this time frame.
One of the reasons that we think temperature and not mileage is the deciding factor comes from a LEAF owner in Washington state. In his first year of ownership, he has covered almost 38,000 miles (which is likely considerably higher by now), with no battery capacity loss. There are also reports of other LEAF drivers having driven 22,200 miles in one instance and 20,500 miles in another instance with no battery capacity loss. There are several others with an accumulated 18,000 miles or more that have experienced no loss in other parts of the country that have owned their LEAFs for roughly the same amount of time.
Reviewing some of the reference items in the various posts has us more convinced of the impact of heat on battery life. We will actually start with an interesting bit from one of the owners that has already lost a battery capacity segment. Here is a graph plotted from his car that shows the relationship of capacity over time:
This plot was achieved using an aftermarket tool developed by a LEAF owner that measures the amount of electricity actually entering the battery pack. The red dots in the above graph represent the energy stored after a 100% charging session. From October 2011 through February 2012 it appears that the battery capacity actually increased as the temperature cooled in Arizona and it started falling again as the temperature started moving up in the spring. What we don’t know is this – is this a result of the battery management system (BMS) controlling the amount of energy sent to the battery, or is it actually a change in the internal composition of the battery? Perhaps some combination of the two.
Another interesting graph made its way into the discussion as the impact of temperature on battery capacity loss became more prevalent:
As can be seen in this graph, the impact of the temperature on battery capacity is significant. This evaluation was not performed on a Nissan LEAF battery, but it was performed on similar Lithium ion battery chemistry. There is, though, one thing worth noting in the observation of this graph. At 20 degrees Celsius (68 degrees Fahrenheit) the green line shows the slowest rate of capacity loss. At 50 degrees Celsius (122 degrees Fahrenheit) the red line shows a greater loss, but a slowing rate of decrease in capacity with an increase in cycles. Without seeing this experiment run over a greater number of cycles, it is difficult to predict, but one can extrapolate that the extremely high temperature data set may continue to slow over some period of an increasing number of cycles. It may be wishful thinking on our part, but we see each of these slopes as the equivalent of the right side of a typical bell curve, tapering off quickly from the peak (or slowly, in the case of the 20 degree Celsius curve) with the curve flattening out the further from the peak that it gets. Certainly we would like to see this experiment repeated with a much larger cycle parameter involved. The number of cycles indicated in the above graph is not the physical number of times that a LEAF battery would be plugged in to a charging dock, but the equivalent of full charge to full discharge per cycle.
As we have seen thus far, battery capacity loss in the LEAF does not seem to be a factor of mileage, but rather one of locale. There is much yet to be learned though, as this topic has now reached almost 200 pages in length and we still have 150 pages to go. As they say, stay tuned.