Charging in High Temperature environments 13A vs 32A vs 70A: cost of air conditioning

Tetsous raised a question to me at a recent meet-up. He charges at 13Amp 240volt and is seeing dramatically more energy being used to charge the car than being consumed by the car on the road (according to the info the car provides). He asked me if I was seeing the same as I charge at 70Amp 240volt. The suspicion is that due to the high temperature (around 30celcius +/-5celcius), the air conditioning used during charging to cool the battery pack is consuming perhaps as much power as the car itself (doubling the cost of a charge).

I got three data sets. The outside temperature was not the same on the three days, but was dropping over the week (perhaps 28celcius at 70A down to 24celcius at 13A) - so that should favor the 32A and 13A charges.

All charges were in standard mode.

First day: I drove about 44km on my commute. Car says I used 7.4kWh (rounded to .1 kWh). Recharge was 50minutes at 70Amp, using 8kWh (rounded to 1 kWh). P.S. This puts my commute at about HK$4 each way (about 1/4 the Prius I replaced, and 1/10 the Land Rover the Prius replaced).

Second day: 10.0kWh usage (commute plus a couple of school runs) took almost 2 hours and 12kWh charge at 32Amp.

Third day: 9.2kWh usage. Charging at 13amp (using HPC dialed-down by the car) needed 4hours 25minutes and 15kWh energy.

I'm not sure if the net energy usage figures include regen (I assume regen has already been taken away from the figure). For example, for day 3 the car showed 9.2kWh usage and 1.9kWh regenerated. On average, the regen numbers are about 20% of the 'net energy used' numbers (or about 17% if net energy used already includes regen).

IF (big if) the aircon used about 1kWh, the numbers would work out. The 70A charge took 50 minutes (and the diff is about 600Wh), the 32A charge took 2 hours (and the diff is about 2kWh), and the 13A charge took 4 1/2 hours (and the diff is about 5.8kWh).

The power to run the aircon doesn't seem to affect charge time.

Does this make sense? At 13Amp, we seem to be paying about 50% extra for the charge.

On a related note, what would the impact of the aircon be on charge time?

13A @ 240v is 3.1kW
32A @ 240v is 7.7kW
70A @ 240v is 16.8kW

If the aircon is 1kW, presumably that would use 1/3rd of the available 3.1kW @13A, and extend charging time appropriately (but have negligible impact at 70A)?

I have the logs for these, if it helps...

tetsuos said:

I am still waiting on some feedback from Tesla Motors on their own experience.

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Tesla Hong Kong started in the autumn last year, right? I think they also charge at 70A at those HPCs in Hopewell.

As the summer heat up, I think we're all in for a learning experience ;-)

At least the AirCon seems to be holding up. Even at 28celcius (a week or so ago) I've only got it on the first click to blue and it is still damn cold.

P.S. Really glad I got that HPC. Noisy fans and AirCon on run for an hour or so at night. 32A doesn't seem too bad though, and easy to install.

If 240v @13A is 3.1kW and the AirCon really takes 1/3rd, the situation at 110v must be appalling. Charge duration 3 times the already long normal.

I agree that AirCon while charging is a good thing. At 70A it has minimal impact on charge duration or energy used. But, the issue is at 13A (which is why tetsous raised it).

richkae said:

This is a great source for information on charging the Roadster: Tesla Roadster Charging Rates and Efficiency - Tom Saxton's Blog
Thanks Tom!

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Richkae,

Thanks for the link. I hadn't seen that before, and it makes fascinating reading.

Tom says "all charging was done overnight in cool weather" and:

Charging in a hot environment definitely changes energy consumption during charging because the fan and A/C will kick on to cool the battery pack. It's harder to control for ambient temperature across multiple charges, but it would be interesting to collect data and see how things change. I would not be surprised to see a significant penalty for charging at higher current if that pushes the temperatures high enough to require the A/C during the charge.

I guess we need to see how much current the aircon draws and that should give us an idea of its impact.

scott451 said:

Mark,
Can you and tetsuos send me a log file and I'll take a look?

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Scott, I'll send you mine tonight. I've got the data points for 13A, 32A and 70A (albeit at slightly different ambient temperatures).

I suspect that the roadster has different 'tolerances' for battery temperature, depending on whether it is charging or not.

At 30celcius, when charging the aircon is on pretty much continuously. Once charging has completed, the aircon turns off within a few minutes. But, just sitting there plugged in all night, I don't see/hear the aircon coming on.

The original report was that charging was using 2kW from the socket to put 1kW in the power pack at 13amp 220v. I'm not sure it is that bad, but it does seem to be about 1.5:1 at 13amp 220v, from my own experiments.

I've sent the logs to Tom and Scott, to see what they come back with. It will be interesting to see.

tomsax said:

I took at look at Mark's log file and calculated Wh per Range Mode %, then adapted the results from my blog to compare.

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Thanks Tom. I gave a copy of the logs to Scott, and hopefully he can get ambient temperature readings from them. A 20% hit is not too bad. The original numbers suggested at something more like 50%->100%!

Are you sure your figures are wall-to-battery, not just charger->battery?

The corresponding figures from the car screens I got for the three charges are:

• 7.4kWh usage. Charge was 50mins at 70amp, using 8kWh.
• 10.0kWh usage. Charge was 2hours at 32amp, using 12kWh.
• 9.2kWh usage. Charge was 4hours 25mins at 13amp, using 15kWh.

Did some work with Tom on this, trying to check logs vs energy usage screens on the car.

There are two screens that show usage: "Trip" and "Energy Use History".

I did a 40.4km run and monitored all the screens. I am running the 'latest' firmware (installed just a few weeks ago, containing the fix for the J1772 adaptor). I was not using air conditioning, heat or cabin fans (although other fans were most likely on).

The Trip screen showed 40.4km, 53minutes drive time, and 5.81kWh net energy used (resulting in 145Wh/km).
The Energy Use History screen showed Net Energy Used Today 7.5kWh and Regenerated Today 1.6kWh.

Both Tom and I have seen that going down a hill, the Trip screen Net Energy Used number gets smaller. The resolution (0.1kWh) on Energy Use History is not enough to see this affect.

5.81kWh + 1.6kWh = 7.41kWh (~ 7.5kWh).

It seems that the "Trip" screen Net Energy Use figure is the energy taken out of the battery minus the energy put back in by regen. It is also seems that the Energy Use History screen is not as accurate, and the Net Energy Used Today figure does not include regeneration.

These figures look ok to confirm energy use. I still need to confirm that the trip meter also records aircon usage (which is simple to do - just run the car idle, with the aircon full blast, and hopefully the energy use number will increase at a faster rate than without aircon).

On the charging side, we still can't match-up the charge duration and energy figures shown on the charge history screen with the logs. They are way off (but Tom's previous tests in cold climates had them very close to what a meter on the wall showed). I'm going to get (and install) a meter on the HPC so we can record actual usage and compare against what the car is telling us, because the figures at the moment just don't make sense.

Ongoing...