It would be good to hone in a little further on the stated topic of this thread, and consider why ultracapacitors (or supercapacitors) have been studied so intensively for electric vehicle use: because their electrical properties are complementary to those of lithium ion batteries, not directly competitive with them. You might have noticed that your Model S has a peak motor output power of 240 or 300 or more kW when you really step on it, but that your maximum regen capability on a steep downhill is far less - 60 kW if the battery is warm and even less if it is cold. That's unlikely to be a limitation in the motor itself, since it performs as a motor to provide torque when you supply it with electricity and as a generator to provide electricity when you supply it with torque. Therefore, its ability to accept input power should be about as large as its maximum output power.
We also know that, with the newest superchargers, Model S batteries are capable of accepting 90, 120 or in some cases even 135 kW of input power, at least when the batteries are fairly empty. Why not allow the battery to accept more power during regen if it's available?
One thread I read suggests that it could be a software issue, and that Tesla didn't want to make regen too jerky if you take your foot completely off the pedal:
http://www.teslamotorsclub.com/archive/index.php/t-9614.html
That may be true, but I think the more interesting and provocative reason is that the batteries and associated power electronics may not be intended to absorb instantaneous spikes of power that high. And yet, that's exactly what ultracapacitors thrive on. They are not intended to store large amounts of electricity for long periods of time, but they are absolutely perfect for absorbing and releasing very large power spikes over short periods of time. In fact, Maxwell and others are currently selling ultracapacitor-based products to do exactly that in buses and large trucks.
The other key desirable quality is their ability to continue performing well at very low temperatures, which is a condition lithium ion batteries are not nearly as happy about. In fact, ultracapacitors last longer if operated at low temperatures. So what does that all mean?
Imagine if Tesla offered an optional upgrade of an ultracapacitor module that would fit in the supplemental storage space below the hatchback area. That space is perhaps 5-7 cubic feet. Would it be possible to use such a device to provide four key benefits?
1. Recover more power from regenerative braking without making the handling jerky, by letting the additional power flow first into the ultracapacitor, and then more slowly into the battery thereafter.
2. Allow even greater instantaneous bursts of acceleration and/or lower vehicle weight and cost and longer battery life, because the battery pack would not have to be designed to handle such large power flows. It could instead be designed to keep some electricity in the ultracapacitor for that purpose, filled slowly from the battery but drained quickly by the motor during periods of rapid acceleration.
3. Allow faster range supercharging, by serving as temporary storage for large amounts of electricity that could be fed into the battery more slowly as the battery is able to accept it.
4. Allow the car to perform more normally at very cold temperatures, rather than limiting regen capabilities until it warms up.
Here's Wikipedia's chart showing energy density and power density for various types of energy storage devices:
The latest supercapacitor designs are based on graphene, and offer even more promise:
http://www.technologyreview.com/vie...-vehicle-energy-storage-say-korean-engineers/
As more research unfolds, the possibilities for Tesla to utilize the best of lithium ion batteries' capabilities and the best of ultracapacitors' capabilities in a combined system start to look really interesting. This thread could be a good place to house a running discussion of those opportunities.