How about a 4 pound "battery"?

[CTScott]

Last week James (Jerkratt) sent me a video via Facebook of a guy demonstrating the use of a bank of supercapacitors as a super light weight replacement for the battery on his Scion xA. I have used supercaps for various engineering projects in the past, but have never considered using them to completely replace a car battery. After watching the video I immediately ordered six Maxwell 650 Farad 2.7V ultracapacitors. These supercaps are 2" tall by 2.5" in diameter. The weight just under half a pound each, so six weigh in at under 3 lbs.

For comparison, the OEM battery weighs 33.2 lbs:


Capacitors behave very differently than batteries because of their low internal resistance, so they can charge and discharge extremely quickly, without the large losses that occur with a battery (due to the much higher internal resistance of a battery). With the supercaps having a voltage rating of 2.7V, they must be connected together in series, so that the sum of the ratings is greater than the maximum voltage of the system. With a car, the alternator can output up to about 14.8 volts, so six of the supercaps connected in series provides a safe margin with a 16.2 V limit. Supercaps can be extremely dangerous, as they may explode if connected in reverse polarity or are connected to a voltage source over their limit.

To experiment with the supercaps that I purchased, I made some strips of bus bar out of aluminum to connect them together and used some fire resistant ABS plastic to make top and bottom covers to prevent accidental shorting.

Here is one of the SuperCaps:



Here are six of them connected in series:



This was my first pack configuration. I was trying to make it as small as possible, but didn't really think about the correct location for the terminals. This configuration weighed just under 4 lbs:



After doing some bench testing with the pack (charging it with a bench power supply and then driving a large 12V DC motor), I was ready to test it in Crashy. I immediately realized that my choice of shape and output connection locations were not optimal, but it would be fine for my initial testing.



I charged the pack to 14.2 Volts, installed the pack in Crashy and turned the key. To my amazement she started with no problem. The voltage sagged to about 11 volts during cranking, but within a few seconds of starting, the pack charged back up to 14.1 volts. I started and immediately shut her off about 20 times, and each time the pack had charged back up so fast that I could have repeated this all day without a problem. A normal battery takes quite a few minutes to charge all the way back up, so this demonstrated one of the advantages of the supercaps.

To test the amount of "reserve" the supercap pack had I then unplugged the EFI fuse (which powers the injectors) to prevent the engine from starting. I then cranked the starter for four seconds and then stopped. I was able to repeat this nine times before the pack had discharged enough to no longer be able to crank the starter.

Impressed with the results, I decided to reconfigure the pack to better match the OEM battery. I just loosened the bus bars and shifted them so that the ends would be along the long direction of the pack. I added two more pieces of bus bar and connected those from the two end caps to 12mm bolts that I decided to use to simulate battery posts. The reconfigured pack measured 9" long x 5" wide x 3.5" tall. With the extra two pieces of bus bar and the two large bolt I increased the weight to 4.4 lbs. Using thinner bus bar and a lighter weight plastic would easily get me back under 4 lbs.



I mounted it back in Crashy to continue my testing, but the new configuration will allow me to easily use the pack on the road in Crashy2. I will leave it in Crashy until tomorrow, as I want to see where it discharges to overnight, with the Yaris' significant static draw of about 150 mA. Once I verify that it holds enough charge after 12 hours of sitting to still easily start the car I will move it over to Crashy2 to begin some road testing.



I took a few videos, and once I get around to editing them I will post them as well.