Life With Tesla Model S: Does Supercharging Cut Battery Capacity? Page 3


Tesla Model S in Albuquerque's 'snowstorm' during NY-to-California road trip [photo: David Noland]

Tesla Model S in Albuquerque's 'snowstorm' during NY-to-California road trip [photo: David Noland]

When I asked at my local Tesla Service Center about “pack balancing”, I was told they did it all the time.

“Discharge it as close as you can to zero, and then charge it as slow as possible all the way up to 100 percent,” I was told. “You’ll probably get some capacity back.”

It works!

So, after a week-long abstention from charging and a bit of judicious planning for the last few miles, I pulled into my driveway with the battery meter reading 1 percent and the projected range readout at 2 miles.

I plugged into my regular NEMA 14-50 outlet, but set the charging rate to 20 amps, half the normal level. The full charging process would take about 18 hours, the longest I could manage to do without the car.

2011 Chevrolet Volt and 2013 Tesla Model S [photo: David Noland]

2011 Chevrolet Volt and 2013 Tesla Model S [photo: David Noland]

Long story short:  During those 18 hours, the dashboard readout told me that I’d pumped 74 kWh worth of electrons into my battery.

I had apparently regained not only the capacity I’d lost from the Supercharger binge, but also an additional 2 kWh that got me all the way back to 97 percent of Howe’s number for full new capacity. 

Not so fast….

Was this apparent 6-kWh capacity gain after balancing the pack real or imaginary?

During the subsequent week of local driving, the numbers didn’t add up.

When the battery hit 50 percent, I had used only 31 kWh and had traveled 105 miles. That implied a full capacity of just 62 kWh and a max range of 210 miles. I seemed to be losing even more capacity.

2013 Tesla Model S at Supercharger station on NY-to-FL road trip [photo: David Noland]

2013 Tesla Model S at Supercharger station on NY-to-FL road trip [photo: David Noland]

Oddly, though, the “rated range” display told me I had 125 miles remaining from the last 50 percent.

I chalked up this 20-percent discrepancy to “vampire” power drawn while the car was parked, which Tesla puts at about 1 percent per day.

Long-distance test

Fortunately, I had another long trip coming up, this time to visit friends in Maine. It would be the true test, untainted by any vampire losses. 

The round trip had four legs—two each of 160 and 200 miles.

On the first leg of 160 miles, I started at 100 percent and finished at 32 percent, using 48.0 kWh in the process.  That worked out to a theoretical 100-percent capacity of 70.6 kWh.

Results for the other three legs were similar: 70.3, 69.9, and 70.0 kWh.

2013 Tesla Model S at Supercharger station on NY-to-FL road trip [photo: David Noland]

2013 Tesla Model S at Supercharger station on NY-to-FL road trip [photo: David Noland]

Call it 70 kWh.

That was better than the 68 kWh before the pack-balancing, but well short of the apparent 74 kWh capacity immediately after the pack-balancing process. 

It still looks like I’ve lost about 9 percent of my battery capacity after 35,000 miles—a lot more than the projected losses in the Dutch study.

Conclusions

Based my experience, it seems to me that Supercharging probably does somewhat degrade the long-term battery capacity.

And as for balancing the pack, it seems to work ... a little bit, sort of.

But how long does the pack-balancing benefit last? That remains to be seen.

Tesla, of course, could easily clear up all this confusion  by simply revealing a few basic facts that it certainly knows:

  • What is the usable capacity of the battery when new?
  • Is there any reserve capacity after the battery meter hits zero?
  • What is the effect of pack-balancing?

(Its failure to reveal such basic information continues to baffle me.)

Finally, I'd also like to see Tesla provide some actual data to back up its claim—in my opinion, now rather dubious—that Supercharging does not affect long-term battery capacity.

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