More supercapacitor and ultracapacitor nonsense

Well my post about the absurdity of NASA’s ultracapacitor powered drill is bringing out the fanboyz so I feel the obligation to demonstrate a few more limitations on using capacitors, even super or ultra ones, to power cars. I was horrified a few weeks ago to read this idiotic CNN article abut how a patent on an ultracapacitor will soon end the age of internal combustion engines. Even the producers of really great ultracaps like Maxwell Technology are skeptical of this patent and company and the claims it can be used to power cars. I have mentioned to my friend Bob Pease the absurdity of some of these claims about using supercaps for cars. He looked at EDN editor Margery Conner’s latest article on ultra caps. Note Margery, being an electrical engineer, does not claim this cap will run cars, but points out the very valid and useful things it can do like power flash units and provide low impedance for cell phone audio. Anyway Bob used the values of the supercapacitor that Margery wrote about to do some quick calculations. Bob writes:

These caps can go up to 0.55 Farad, at 5.5 volts. I fired up my SLIDE RULE, a good old Pickett. I’m not going to spend a penny of battery on a calculator to compute a joke like this. For this $2 capacitor, we can assume C x V-squared/2 is 7.5 Joules. (Assuming a high fraction of capacitor energy recovery.)

A VW   Beetle can run on 5 HP (or 3600 watts) at 55 mph. Assuming a good electric car with excellent tires can beat this by 2x, 1800 watts x 9 hours x 3600 seconds can get you to 500 miles with 60 million Joules. So you would need EIGHT MILLION    $2 supercapacitors. Uh, yeah. Am I gonna abandon my VW Beetle?   Not bloody likely.

I don’t care how you series them or parallel them, you will find it hard to get that far. As Pogo used to say, " I don’t care how you slice it, it is still a lot of Baloney."

If the other new Super-capacitor guys can beat that figure of merit by 10 x, they will only need 0.8 million super-capacitors.   I’ll believe it when I see it.

Now back to that CNN article, the BS starts early when they claim you will be able to charge their car for 5 minutes and then drive 500 miles. OK, dreary engineering math time. Bob Pease pointed out his Beetle uses about 5 HP to go 55mph. I remember larger American cars needed 15 hp to do 60 mph. So lets use 10 hp, a nice round number. So if you drive 500 miles at 60 mph that takes you 8.33 hours. Since you are using 10 hp that whole time your energy use is 83 hp-hr. A handy online converter calculates that those 83.3 hp-hrs is the same as 223.7 megajoules. Mega. As in million. OK, so we have to put that energy into the ultracaps in 5 minutes. A joule is a watt-second. Five minutes is 300 seconds. So the 223.7 MJ being charged in 5 minutes means you have to deliver 745kW into the car for those 5 minutes. That would be 7,450 amperes at 100 volts. See what I mean about mendacious marketing grant-sucking venture-plucking liars? Now, if you want to charge it from a 1500-watt wall socket it would take 149,133 seconds, or 41 hours.

OK, OK, so maybe I am wrong and it does not take 10 hp to push a car down the road at 60 mph. Maybe we can do it like Bob’s tiny VW Beetle. So that is a factor-of-two wrong in my assumptions, so then it takes 3725 amperes at 100 volts in 5 minutes or you could charge up in 20.5 hours from a 1500W wall socket. So maybe you insist we use rock-hard polyurethane tires and a 1000-pound car, well, that probably still takes 2.5 hp so then you need 1862 amperes at 100 volts for five minutes or 10 hours to charge from a 1500 W wall socket.

I’m not saying you could not make a tiny commuter vehicle and charge it up overnight. I am saying that the CNN article is technically absurd. The charging in 5 minutes is absurd but you can look at Bob’s comments for why ultra capacitors are a poor choice compared to lithium batteries. As to the other comments in the CNN article, you can see they are hinting that the internal impedance will be lower than batteries. Well that is doubtful if you have been following the amazing progress of RC modelers using lithium polymer and other technologies. They routinely discharge the li-po’s at 20-C and even 70-C momentary rates.

Not even hinted at in the article is the one conceivable use I might admit for ultra caps in electric vehicles. It is to accept and buffer the energy from regenerative braking. See, all lithium chemistry batteries have a current limit on charging that then turns into a voltage limit about 4.2 volts. This means you cannot stuff charge into them the way you can with lead acid and ni-cad batteries. This would mean that the feel of a regenerative braking system might be limited to a very slight drag, always the same, that happens when you touch the brakes. If you want to use a more natural or harder braking force, the energy will have to be dissipated as heat since the lithium pack will not accept the charge. You could use ultra caps to take in a bunch of energy at variable rates and only use the brake pads for the very hardest applications of force. Then you could empty the ultracaps into the motor or the battery and wait for the next braking cycle. The only problem with this scheme is that regenerative braking does not seem to be paying out for American drive cycles. For a taxi or a buss it is great, but we drive too many highway miles on average and it does not pay to have all the added complexity of a regen system.

Ultracaps are great for many applications but running electric cars is not one of them, even with magical breakthroughs. The reason we have magnetic motors and not electrostatic ones is that charge is a pretty bad way of transferring force and energy. Keep checking out Margery’s blog for the latest news on ultracaps from reputable manufactures like Maxwell and Cap-xx.