Wednesday, May 20, 2015

Battery capacity estimate

I can estimate battery capacity by taking measurements from state of charge (SoC) percentage and energy taken in kWh. Since DCFC is found to be over 90% efficient (see my past blog on efficiency), the accuracy of the battery capacity estimate obtained should be in that ballpark. Only if SparkEV would report kWh instead of (or along with) miles remaining, this would not be necessary. Alas, I must use this round-about way to figure out if what I'm paying for is what's advertised. I could probably use this technique in the future to find battery capacity as it gets diminished from use and time. I wonder if other EV can use similar technique?

I recorded 4 miles remaining reported by SparkEV and 4% SoC from eVgo at the start of charging. Yes, it's from "4 miles remaining and loving it" episode. After 22 minutes 54 seconds (1374 seconds) and 14.68 kWh, SparkEV showed 84 miles remaining and 89% SoC.

Knowing the SoC percent and kWh used, one can estimate the actual battery capacity.

(89%-4%) / 14.68 kWh = 5.79%/kWh
100% / (5.79%/kWh) = 17.27 kWh

Since SparkEV has 19 kWh battery, this is pretty close with 1.7 kWh for "reserve" or for saving battery health.

Another fun thing to find is DCFC rate at kWh/second.

14.68 kWh / 1374 sec = 0.0107 kWh/sec

Compared to Leaf at 50% SoC, SparkEV charges twice as fast. Another is how much money I spent for the drive.

(84-4 miles) / (14.68 kWh) = 5.45 miles/kWh

Wow. That's pretty good driving! (hint: some traffic jam). My MPGe table was based on 4 miles/kWh, but 5.45 miles/kWh is 35% better! Well, not really. 4 miles/kWh was mostly using on vehicle charger, L1/L2, where as these are based on DCFC, so it's not apple-to-apple comparison. Maybe it's closer to Fuji vs Granny Smith comparison.

In another DCFC session,

SparkEV showed 21 miles remaining and 25% SoC at eVgo at start of charging. After 14 minutes 51 seconds (891 seconds) of charge showed 10.05 kWh. At the end, SparkEV showed 74 miles remaining and 82% SoC at eVgo.

Finding 100% battery capacity as before,

(82%-25%) / 10.5 kWh = 5.43 %/kWh
100% / (5.43 %/kWh) = 18.42 kWh

It's not extremely accurate, but it's too close for comfort for usable battery SoC and battery longevity if you ask me. I hope SparkEV engineers know what they're doing. They have impressed me so far, so I have confidence in them.

DCFC charging rate is

10.05 kWh / 821 sec = 0.011 kWh/sec

Let's bash Leaf again. That's still twice Leaf's charging rate at 50% SoC! And the amount of money I spent for the drive is

(74-21 miles) / 10.05 kWh = 5.3 miles/kWh

That's not as good as before, but it was driven at higher speed with less traffic. Still, that's 30% better than 4 miles/kWh the MPGe table is based on.

This shows 2015 SparkEV has 19 kWh battery, and not 21 kWh battery like the 2014 model of SparkEV. For given EV range, it's better to have lower capacity battery so the replacement cost will be lower. I am curious what the 2014 SparkEV show if same kind of measurements are made. What would Leaf show? Or Tesla? And other EV?

Update 2016-06-23:

I don't monitor the charging at DCFC much, but the last one turned up as follows:

Start 13%, 12 miles remaining
End 85%, 12.09 kWh
12.09 / (85-13) * 100 = 16.8 kWh

But % reading are not accurate. It could be that the reading was taken just at the cusp of going to next value. Then the best case / worst case are as follows.

12.09 / (84-14) * 100 = 17.3 kWh (best case)
12.09 / (86-12) * 100 = 16.3 kWh (worst case)

Now this may seem bad, but it's not really. Few times I did monitor a year ago, I hit 16.5 kWh. It seems the number is not constant. Even the two examples above vary between 17.3 kWh and 18.4 kWh. Indeed, the range shown on GOM (guess-o-meter) has not changed since the car was new, and I haven't driven it any more economically than before.

Then the question becomes, what is the battery capacity when new? This was asked in forum, so I'll put my findings here.

According to commenter from Insideevs (probably also the same guy at forum) named "WopOnTour", capacity is 54 Ah for two cells in parallel and 96 in series.

Without better source, we'll go by 54 Ah rating to estimate the battery capacity when new. But without knowing the voltage, we cannot find the energy capacity. EVPump DCFC at San Clemente shows voltage and current while charging. Below are screen shots at 48% and 84%. (those numbers were chosen after careful analysis!)

Note that the current is only 99A, which makes 373 * 99 = 37 kW rate. It seems EVPump maxes out at 100A while ABB chargers max out at 125 A (373 * 125 = 47 kW). Indeed, I've seen 48 kW on ABB as well as the label on side of the unit stating 125 A as max DC output.

We know that the charger would have to be higher voltage than the battery. How much higher is unknown, but it's probably safe to estimate that about half capacity charger voltage is about nominal battery voltage when full. Yes, it's a guess, but probably not far off. Then the nominal battery capacity is

373 V * 54 Ah = 20.14 kWh

Out of this, we see the usage to be about 18.4 kWh. Somehow, this figure is made popular, but not 20.14 kWh. Forum post says it's actually 19.44 kWh from some GM guy, and I've read somewhere long ago about 19.2 kWh. Regardless, it seems the usable capacity on 2015 (and 2016) SparkEV seem to be about 18.4 kWh when new, which represents 91.4% of full capacity.

2014 SparkEV

For 2014, I don't have voltage data from actual charger. But it is known that it's 3 parallel at 60 Ah and 112 series from "WopOnTour" comment. Voltage for individual cell is found to be 3.32V by bicycle guy using OBD in different forum.

Then the capacity is

3.32 V * 112 * 60 Ah = 22.3 kWh

Out of this, experiments showed about 19.5 kWh used, which is 86% of full capacity. But Chevy advertised 21 kWh for 2014 SparkEV. Well, it's close enough I guess. Maybe the number I have is peak capacity while published figure is for derated for some factors with the derating factor to be 21/22.3 = 94.2%.


  1. Do you know how wide the operating range of temperature is for your battery? The ambient temperature + driving/charging profile can significantly change the amount of energy I have available from my Leaf throughout a day. I might start with an estimated 19kWh available, and after some driving/charging events with my battery "warmed up" (shown as going from 5 battery bars to 7 in my Leaf, or say +5C in the AM, up to +40C later on - see for data), I might have a little over 20kWh available!!

    As to total available capacity, on my 1 year old Leaf, with ~45,000 km, at a charger in June where a technician was on-hand with capability to view more data in the DCFC unit, he reported to me that on that day, my battery capacity was 20.7 kWh (reported with 5 battery bars and ambient temp of ~20C when I plugged in). I believe it is generally accepted that a brand-new Leaf, in ideal conditions (eg. 70F) has about 22.5 kWh available of the 24 kWh ( So for myself, I have experienced about 20.7/22.5 = 92% of original capacity, or about 8% degradation. For the amount of km's etc, that isn't too bad for a Leaf. Degradation should slow down now. I'm hoping to get a report like that again next year for comparison purposes.

    1. I didn't know Leaf shows you available kWh. That certainly makes it easy to find batteryy capacity.

      I don't think ambient temperature in SoCal in spring would matter much for SparkEV with liquid cooled battery. It's always sunny at 20C degrees. But I can see how that can complicate matters for Leaf, especially in Canada.

      I think 8% degradation depends on profile. If it's linear, it's seems bad. But if it's inverse exponential, that could be pretty good. If you're driving so many miles and using L2, you might be doing good for the battery. LiIon tend to degrade quicker if it's sitting at high SoC, so your driving much would mean you don't let it sit at high SoC. You also don't use DCFC often (twice?), so that's another benefit to the battery.

      I've driven the SparkEV less than 5000 miles, so I don't see any degradation in range even with all the DCFC I do.

    2. Sorry I wasn't clear enough. The car only shows soc %, it doesn't show capacity. You can get an app and a OBDII wireless adapter and it will read a unit called a "GID" from your car's computer, which is a pretty close estimate of your battery's capacity (I do not have this though). The DCFC unit itself has some sort of additional information screen that can only be accessed by a technician - that is how I found out that on June 18, 2015, my battery capacity was 20.7 kWh (usable).

      As to my degradation, it is definitely not linear, that much has been established. And you are right, I avoid leaving my car at high soc %; I have the charge timer set so that it doesn't start until the early hours of the morning and finishes just before I leave on my commute. I have DCFC'd about a dozen times, but real-world data on the Leaf has shown that whether you use DCFC or L2 charging has less of an impact than was originally expected.

      Ambient temperature your car is used in, stored in, and what level you leave the soc at seems to have far more influence. Hopefully with your thermally managed battery pack you won't have to worry about these things!