Charge-off 90kW vs. 120kW

I was “recruited” from the Tesla Motors Club Forum to do a 1:1 time test between my car and a 90kW battery A car.  Three cars were present at the charge-off:  one 90kW (Battery A) car, my own 120kW (Battery B) car and a second 120kW (Battery B) car.

A number of customers who have these earlier A batteries have expressed discontent that they cannot charge above 90kW.  Also on the forums there have been many data points and charge curves for individual 90kW and later 120kW batteries.

Tesla was pre-informed and invited to this event and received an advance copy of this post in order to comment.  All statements from Tesla are in red.  All three testers were very pleasantly surprised at the performance of the 90kW battery and felt that Tesla’s statements are in-line with reality and could potentially be met under ideal circumstances.

To provide a quick preview of the results, the second 120kW battery tester said “One of the most impressive things to me was that the taper from a 90kW charging rate on the A battery car started at the same time as our 120kW capable cars passed through a charging rate of 90kW.  This is HUGE!  This shows that there was no difference in taper after ours got to 90kW.  It also meant that his car paralleled ours in miles gained until his kept on trucking. His technically was out charging mine for almost half the time.”

The First Charge-off In Vacaville, California

The First Charge-off In Vacaville, California

The goal of the charge-off was to address the validity of the official Tesla motors statement in regards to the two different batteries:

“For a customer charging from 20% to 90% (more than enough to go to the next supercharger station), the difference in charging time between an early car and a current car is less than 4 minutes”

Label on Battery A

Label on Battery A

We decided that the best comparison was really to test from 10% to 90%.  We used that data to validate the above claim.

On April 17, 2014 three cars performed a charge-off at the Vacaville supercharger.  The weather was a very pleasant 77 degrees Fahrenheit (25 degrees celsius).  All cars had the climate control off and the windows down (in order to communicate screen data).  The 90kW car did not have any fan noise, my car had it for a few minutes, but the other 120kW car had large and extended fan noise.   It seemed that at the lower charge rate, the 90kW car Battery A did not need as much cooling – as the cooling fan did not seem to kick in – but he also had kindly waited for our arrival. Whereas at 120kW, the B battery cars’ fans ran noisily during portions of the fast charge rates.

Question for Tesla: what is this large “fan noise” we hear during charging?

The likely sources of this noise during supercharging come from the thermal management system of the Model S (= fans), which works to optimize the thermal conditions inside the battery pack during supercharging.

I was the last to arrive (confession – I was quite late and had not planned nearly enough time for heavy Bay Area traffic), so my battery was probably slightly warmer.

The first car with the battery A was an early Vin with approximately 41,500 miles already on its battery.  Interestingly enough, this battery in a full range charge with 5.9 reports a range of 260 (was 267 brand new), a degradation of only about 3%.  This car charged in stall 1A.  No car charged in its sister stall 1B.  This charging station had no issues providing 90 / 89 kW of power for about ten minutes.

A 120kW car was in stall 2A.  This car had around 18,750 miles on its battery.  This car was also running 5.9 and has a full range of 248 miles.  No car was charging in the paired stall 2B.  This car began its cycle at a full 120kW of power.

I charged my 120kW car in stall 4A.  I had around 14,500 miles on my battery.  I recently did a full range charge and my battery only had 247 miles of range – significantly lower than the range of the much higher mileage A battery!  I charged in stall 4A, and no other car was charging in the paired stall 4B.  My charging station began only with 112kW of power but near the end of the test, I was receiving more kW from the station than the 120kW car in stall 2A.  Likely my charger was not 100% operational but my car was the first to finish charging.

As you can see from the above statements there are quite a bit of variables:  mileage, expected range and complete supercharger operation.  I was a little surprised that my car finished charging before the second 120kW car when I was started with 112kW from the supercharger and he had a full 120kW.

We started at 26 miles because that number is 10% of the full range of the two 120kW battery B cars.  We also felt this test is more interesting because most supercharger users note that it is more efficient to start closer to empty.  We effectively tested the full 10-90% range between the three cars.  We decided to start at 26 miles and end the official test at 223 miles when the 120kW cars stopped at the 90% mark.  The 90% mark on the 90kW charge continued on a slow taper to 232 miles.

My car reported an increase of 65kWh in the battery at the end, the other 120kW car reported an increase of 59kWh in the battery at the end, and the 90kW car reported a 63kWh battery increase.  These numbers seem like clear estimates and not extremely accurate.

Here is a curve of all three cars comparing mileage gained over time from 10% to 90%.  The three curves have almost identical behavior.  The A 90kW battery continued to beyond the 223 mile test for another 6 minutes with a final mileage of 232.  This part of the data is not shown  in the graph.

Mileage Curves Over Time for the 3 Batteries

Mileage Curves Over Time for the 3 Batteries

The second chart shows the kWh energy being supplied to each of the cars over time.  Although the 90kWh battery was slower for the first ten minutes, after 20 minutes it was charging at a marginally faster rate than the 120kWh cars.  Its hard to see from this chart, but the 90kW battery was charging faster than the other 120kW battery for at least half the time.

Mileage Charge Curves for 3 Batteries Over Time

Mileage Charge Curves for 3 Batteries Over Time

The third chart shows the progression of rated miles on a minute by minute basis between my car, which was slightly faster than the other 120kW car.  Yes, my car charged faster, but after about the twenty minute mark, the 90kW battery cars difference flattened out. The highest difference between the two cars was only 17 rated range miles.  This chart includes the final slow taper of the 90kW battery A to its personal 90% mark, which was not really part of the official test.

Mileage Difference Between Battery A car and My Tesla

Mileage Difference Between Battery A car and My Tesla

Question for Tesla: Why would a particular battery have this slow taper?

The charging rate between different design and age batteries will show up in various ways, as your testing has identified. It’s hard to speculate on which factors would cause changes to different parts of the charging session, especially given the many factors that could not be controlled in such a test. However it’s very clear from your data that you have captured the most significant difference between these two versions, which occurs at the beginning of the charge session, at low states of charge.

Conclusions:

1. The 20-90% comparison test as stated per Tesla was a difference of approximately five minutes.  The Tesla statement is less than four minutes, but there are so many small variables between the cars, the superchargers and testing recording methods that I believe this claim is quite possible in a best case scenario.

   On your methodology for determining a 20%-90% charge time from your 10%-90% test: Did you log the time points when each car reached its respective 20% range? (52 miles for the original design, 49 miles for the newer design). The charge slider would naturally stop the cars at their respective 90%, and give you the end times.

Post script for Tesla’s question:  Yes, we did log those time points when each car reached its respective 20% range.  Minute 7 for the original design, and early in Minute 6 for the newer design.  So the difference was at most two minutes.  But the long taper on the original design took 13 additional minutes.  So analyzing the data in this fashion shows an 11 minute difference.  This was noted below in #4.

  The time for the newer design battery to charge from 26 miles to 49 miles would take longer than the original design to charge from 26 miles to 52 due to the extra 3 miles of range, but more significantly due to the 30kW advantage provided in the initial minutes.

2.  The 90kW car achieved the same mileage range of a 10-90% charge just six and a half minutes later.

3.  A 90kW car to achieve the same typical mileage gains needs to wait approximately 10% more time than a 120kW car.

To make this statement, you should also acknowledge the mileage difference between the cars, and that the comparison is being made from 10%-90%. It might be interesting to look at differences between other start and end points as well.

4.   If you compare the numbers slightly differently.  Battery A’s 20-90% based upon its rated range vs. a fixed number as we did, the time comparisons are not as good due to the long taper.

We try to focus on the amount of energy delivered into the battery and relate that to our customers by displaying the distance they could travel. Comparing on this basis should be acceptable to your readers.

5. The 90kW car has a slow taper to 90% that is not particularly useful to wait for in most supercharging driving situations adding another 6 minutes for only 10 miles of charge.  Our tester often interrupts his charge before this final taper.

Indeed the taper is important, it affects most of the charging time, and grows to me more significant as the battery approaches a full charge. The more of this region that can be avoided the less time will be spent at the supercharger and more time can be spent driving (for both designs).

6. At 103 miles of rated range, both 120kW battery cars were receiving 90kW from the supercharger.  So any Model S regardless of battery type will be charging at the same rate with approximately 100 miles of rated range.

7.  All three testers were quite pleasantly surprised at the results given the significant variabilities between the three cars and charging stations.  We feel Tesla will be happy with the results of the first “charge-off”.

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2 thoughts on “Charge-off 90kW vs. 120kW

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