Tesla Model S in Albuquerque's 'snowstorm' during NY-to-California road trip [photo: David Noland]
In both cases, I used precisely 36.0 kWh, which implied a 100-percent capacity of 72 kWh.
Just six months later, after 10,000 miles of mostly Supercharged driving, my apparent capacity was down to 68 kWh—a loss of 6 percent.
Coincidence? Maybe, maybe not.
Full capacity: a mystery
Every electric car battery has built-in “buffers” that prevent it from draining down to absolute zero or charging to absolute maximum capacity, both of which can cause permanent damage over the long run.
The BMW i3, for example has a total battery capacity of 22 kWh, but only 18.8 is usable. My 2011 Chevy Volt used only about 10 kWh of its battery’s 16.4-kWh capacity (among the most conservative of buffers).
Tesla Model S lithium-ion battery pack in rolling chassis [photo: Martin Gillet via Flickr]
Unfortunately, Tesla has never published a figure for the usable battery capacity of the Model S.
This has led to feverish speculation among owners about the real capacity available from the 85-kWh battery. Some owners claim there’s a 5-kWh “secret” reserve capacity, good for about 17 miles, that doesn’t show up on the dashboard display.
In his book Owning Model S, Tesla guru Nick J. Howe makes the case that the magic number for usable capacity is 76 kWh—plus that extra 5 kWh of secret reserve after the battery meter hits zero.
I’ve not had the nerve to test the 17-mile theory, though I've read a few accounts by Model S owners who did. They ended up with their cars on flatbeds. Tesla, of course, won’t confirm the secret 5 kWh.
So, lacking the facts from Tesla, I’ll stick with Howe’s number for non-emergency usable capacity of a brand-new Model S battery: 76 kWh.
Overall, it looked like I’d lost about 12 percent of my battery capacity, from the presumed 76 kWh down to 68 kWh.
2013 Tesla Model S in winter, Hudson Valley, NY [photo: David Noland]
That was not good. A recent study from the Netherlands, based on data from 90 cars, concluded that the typical Model S will lose about six percent of capacity after 50,000 miles.
But here I was with double that loss after only 34,000 miles.
Balancing the pack: salvation?
In researching my problem, I came across yet another rumor triggered solely by Tesla’s policy of keeping useful technical information from its owners: balancing the battery pack.
The idea is this: some lost battery capacity may be the result of unbalanced charge levels among the 7,000-odd individual battery cells in the Model S pack.
Lost capacity may be restored by “balancing the pack”—that is, charging it to 100 percent to make sure that each of the cells is fully topped off.
Tesla, of course, is mum on the subject; I’ve never seen any official word on pack balancing, and the company officially recommends against charging to 100 percent unless maximum range is necessary.
The Model S fan site Teslarati, however, recommends charging the pack to 100 percent every three months or so.