# How does the FFE Charge?

This question could be answered in many ways depending on how you interpret the question: There is a technical answer (it uses a 6.6kW charger inside the car), simply (you just plug it in and it charges), how much current is used, how fast does it get to 100%.

In this case I was interested in the last answer above: Does the FFE’s % battery full “meter”/value increase linearly when charging? Li-Ion batteries themselves don’t charge up linearly. As the battery gets closer to full the internal resistance of the battery increases making it harder to force more electrons in and thus the charging rate of the battery slows down the closer to full it gets. If you were to plot this you’d get a curve where charging is quite fast at the beginning then as you approach 100% it flattens out slowly approaching 100%.

Having done a little reverse engineering on the API that the mobile app and the website use I was able to make a program that logs the % battery value and the range over time. Now if I were to log these values during a charge cycle I could produce a graph to see the charge curve. The graph could be a line as the internal software in the FFE could normalize the % value to a line while charging (to simplify it for users). Frankly I don’t think they would go through that much effort–there would be no benefit to anyone (other than geeky people like me who write blog posts about graphing the car charging! LOL). So I give you my FFE’s charging plot from about 35% to full on a freezing Friday evening:

The blue line is the % battery value and the orange line is the estimated range value (only topping out at 70 miles here due to the cold days we’ve been having and the fact that I’ve been using the heater).
What surprises me the most about this plot is how flat it is. I was expecting it to be much steeper with a pronounced knee near the end as it flattened out. Note that there are a few gaps in the data as the API timed out or otherwise gave an error. There is also an unexpected turn faster around 10:00pm where it charges slightly faster. I wonder if this is where the temperature management system kicked in to cool the battery a bit allowing for a slightly faster charge..

I have another idea for this logging: What if I were to let it log while I was driving the car around? That plot may also prove interesting as the % battery value would decrease in a predictable manner but the estimated range (the guess-O-meter) value would be all over the place depending on if the heat was on or off, if I was on the freeway, etc. Some more experimentation is in order here…

## 4 thoughts on “How does the FFE Charge?”

1. WattsUp says:

Hmm, isn’t this just a function of how you’re chosen to scale your graph? How about simply compressing your horizontal scale? I think you’ll see the knee more pronounced then.

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2. jamiegeek says:

Interesting thought. Just scrunch up the horizontal axis..
That doesn’t seem to call out the curves any more than the above graph does just seems to make the lines steeper:

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3. WattsUp says:

Yeah, steeper and you can more clearly so the roll off at the end of the charge (there is a little bit of a roll off). You complained the graph as “flat” — or did you just mean “linear”? That is, that the charge rate just seems fairly constant and then pretty much just turns off at the end.

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4. jamiegeek says:

Ah yes terminology problem: Meant linear.

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