As of today, October 15th, 2015, commercially available ultracaps have obtained an energy density of 10.1 wh/kg. A standard lead-acid battery, such as the one you almost certainly use to start your car, offers an energy density of 40 wh/kg.
This means that within about four times the weight (and probably the volume) of said battery, you could use ultracaps (and the appropriate control electronics), you could completely — and permanently, as the ultracaps have multi-million charge/discharge cycle capability — replace your vehicle’s starter / power battery. This capability is considerably further along the development and commercial availability road for ultracaps; the last time I really paid close attention, ultracap energy density was down in the 2.5 wh/kg range, and so it would have taken sixteen times the weight (and volume!) of them to do the same job. That’s a lot tougher to justify.
A system like that would probably consist of the ultracaps, some high-power controller circuitry to keep the output voltage of the ultracap array stable, and a small solar panel to keep the charge up when the truck is parked. The solar panel is because most ultracap technologies suffer from a low-rate self-discharge, so you have to gently keep ultracaps topped up if you want them to always be ready to go. Not a problem, just something that has to be taken care of properly.
For instance, I drive a pickup truck. It would be no problem at all to dedicate some of the pickup bed to a system like this, and then forget about ever buying buying batteries again for the lifetime of the truck. For that matter, when the truck dies (it’s a Chevy, it will die, sigh) I could just move the whole thing to my next truck. And sell the battery it came with!
Another use case that is ready right now is low-demand devices.
For instance, a shaver or an electric toothbrush. Both of these are typically used for a few minutes, a few times every day, and then put down. That’s an ideal application for ultracaps. When manufacturers design a battery system for devices such as these, they make sure they take a good charge and can run for quite a few instances. This is so they are sure not to run out of power when you’re using them, because if they do, you’re done — you have to put them down, stop what you’re doing, and wait for them to recharge. So they’re much larger than they really need to be in terms of powering one use.
With an ultracap power supply, that’s not necessary at all. Instead, a small ultracap is designed in that can store enough energy for one typical shave or tooth-brushing episode. If you go a little long, you just touch the device to its base / charger, and within a fraction of a second it is fully charged again, ready to go — and you simply carry on. You’d never have to worry about the battery aging and making the device useless, or be unable to use it because you forgot to charge it.
Ultracaps are also superb for keeping a system alive while you change its battery. For instance, if you have a small computer that has battery operated systems that need to keep running, but inevitably you’ll have to replace the battery, adding an ultracap to the supply design can allow the system to remain powered for quite some time while no battery is present. This use case applies to all manner of things like AC outlet timers, calculators and so on. Any small device that needs to hold on to its settings while it’s batteries are changed.
I’m feeling very optimistic. Ultracaps have finally moved into the zone of practical systems I can make use of in the three areas most important to me: price, energy density, and reliability. And as I can design such practical systems at home, just one engineer working alone, believe me, big companies can do it and do it very well. Consequently I fully expect to see ultracaps replacing batteries beginning very soon now.