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Monday, May 19, 2014

Three Years of Nissan Leaf Driving Part 2 of 3

In part 1, I told you that I have been driving a 2011 Nissan Leaf since May 18th 2011. When I bought the car, my biggest concern was how well the batteries would hold up.

So far, I've driven the car just over 26,000 miles and fast charged (via the CHAdeMO port) 26 times. These fast charging sessions can be up to 500 Volts and 100 Amps and can cause wear and tear on the batteries.

So how could I monitor the batteries? The car has 12 capacity bars. These give you some indication of the battery health, but they are a crude measure. Just after two years of ownership, I found the Leaf Spy app. If you own a Leaf, this is the app for you. It lets you see all 96 battery cell pairs. You can see the battery state of health (SOH), capacity, many other things; it has logging features... Here is a screenshot:

Leaf Spy Pro App Screenshot
You can see a lot of information on this page and this is only one of 5 or 6 screens available. Starting at the top AHr=56.55. The Leaf started with nominally 66 AHr. The SOH (State Of Health) is currently at 86%. Next is Hx. From what I could find, only the Nissan Leaf engineers know for sure what this value is. It is reported with the other data that Leaf Spy reads so it is displayed. The current hypothesis is that this is an internal impedance value.

On the next line you can see that as of early May when this was captured, I had 26 QCs (Quick Charges) and 3006 L1/L2s (level 1 or level 2 charges).

The 3006 charges seemed high. I have only had the car a little more than 1000 days. How could I have charged it 3000+ times? More on this aside later. The battery health is what I want to focus on first.

When Leafs in Arizona started losing capacity, this quickly became a major topic in the My Nissan Leaf forum and other EV discussion sites. Several Leaf owners reported their capacity data. This dataset along with information about the battery chemistry was used to create a battery aging model.

The battery aging model shows that degradation starts out quickly. As the batteries age, the rate of degradation slows. So I should lose less capacity over the next 3 years than I did in these first three years.

I have charted my data along with the model's prediction in the graph below:
Some important notes about this graph:
  1. I didn't start collecting data for my car until just after the 2 year mark. This is when I found Leaf Spy. Data before this is assumed.
  2. There is a sudden spike in range at about the 2.25 years mark. That is when the car had a firmware upgrade. This spike in reported capacity was common and just an anomaly from the firmware change. The actual range didn't temporarily increase.
  3. I am reporting range. This could be Amp-hours or some other jargon value. However, I selected range because that is what matters to most people. "Can the car get me from here to there (and back) on a charge?" is the question. And to answer that, range is what matters.
  4. This is the "EPA Range", not my actual range. The EPA rated the 2011 Leaf as having a 73 mile range. I simply used this as a scale factor. My capacity is 86% so the range is 86% of 73 miles or 62.6 miles. My actual range is better than the EPA rating. The EPA range is nice to know since it is a conservative estimate. 
Looking at the graph, you might notice that the range has actually been increasing over the past three weeks or so. Battery capacity temporarily decreases in cold weather. Now that it is springtime, the days have warmed up and that range is returning. As the hot summer days arrive there will be a similar, small decrease in range. These seasonal effects will be a small annual sine wave overlayed on the general battery aging pattern. Once I have a full year's worth of detailed data, this should be visible.

GOM & The 12 Bars will be final act in this three part post.

Part 3 - Capacity Bars