Supercapacitors for electric vehicles are gaining traction
Jul 14, 2014 Dr Peter Harrop
- Over 20,000 supercapacitor-based hybrid buses in use worldwide
- Over 600,000 supercapacitor stop-start systems in action in cars
The replacement of batteries means grabbing a chunk of the lithium-ion battery market in particular but also inroads into lead-acid and nickel metal hydride territory. With lithium-ion batteries alone, there is a market five times that of aluminium electrolytic capacitors to go for so replacing aluminium electrolytic capacitors that we touched on earlier will remain a minority sport as will replacement of tantalum electrolytics where the market is even smaller. It was therefore pleasing to hear Dr. Priya Bendale of Maxwell Technologies talk on "Supercapacitors: Gaining Traction in a World of Batteries" at the IDTechEx, "Printed Electronics USA" Supercapacitor stream in November 2013.
In our opinion, Maxwell is the most creative marketeer in the industry, opening up new applications all the time. He focussed on transportation, notably heavy duty vehicles and regenerative braking energy recovery, and combining supercapacitors and batteries for engine starting and start-stop for conventional vehicles.
He summarised the relevant parameters as follows:
Table 1. Supercapacitor parameters compared to batteries
|Calendar Life hrs||2-6 years||8-15 years|
|Cycle Life (without over-provisioning)||~3000||>1,000,000|
|Low Temperature Performance||Poor||Excellent|
|Monitoring & Balancing Complexity||High||Low|
|Charge/Discharge Storage Efficiency||~90%||95-98%|
|Thermal Runaway Potential||Yes (Li-ion)||No|
Source: Maxwell Technologies
Success with hybrid buses
Thanks to Maxwell, there are over 20,000 supercapacitor-based hybrid buses in use worldwide, some in use since 2002. They are the preferred solution because cycle life is 8-12 years, cold weather performance is excellent, weight is less, storage efficiency is better, there is simple monitoring and balancing and no thermal runaway failure modes.
Success with stop-start
Supercapacitors are needed for a reliable Stop-Start "micro hybrid" system because batteries are not efficient in receiving and delivering power (high rate). In cold weather, battery charge acceptance decreases and lead AGM batteries "cycle down" when not allowed to completely recharge in stop-start applications.
Nevertheless, supercapacitors are not yet the preferred solution, being outsold by lead-acid batteries at least three to one in this application, mainly because PbA is cheaper. Maxwell has sold over 600,000 stop-start supercaps for French PSA diesel hybrids and its competitors are racing to copy them. Advantages of the PSA system include up to 15% fuel economy improvement in urban driving, reduced carbon dioxide emissions to comply with EU mandate, 30% smaller battery, reduced cabling and assembly labour, extended battery life and "every time" restarts in all conditions and voltage stabilization for entire electrical system.
A good compromise is a battery with a supercapacitor across it as with the Batscap supercap in the Bollore pure electric car and the Nippon Chemi-Con one across the traction battery of the Mazda pure electric car. Maxwell now offers a supercap/lead-acid battery combination to replace the conventional vehicle's stop start and starter/hotel facilities battery. In forklifts, a combo extends useful life between charges by 33%, we learned.
Increasingly, combos reduce cost up-front. One example is: 2000F +30 Ah motorcycle battery, total weight 13.6Kg (30 lbs.) - a 25% reduction. Higher minimum voltage, lower ESR, longer cycle life, protecting the battery and lower temperature operation are cited. A study by Argonne National Laboratory found that cycle-related battery capacity degradation decreased by a factor of two and cycle-related battery impedance degradation reduced by a factor of 5.9. It is no longer contentious that supercaps across traction batteries increase vehicle range by up to 10%. It is bad news for the battery industry because its market is reduced in a double whammy: the battery can be smaller for the same range and it lasts longer.
Replacing Li-ion in flash chargers
Supercapacitors have even replaced lithium-ion batteries in the flash chargers for the new ABB TOSA bus because only they can reliably deliver the surge of power.
Fig. 1. Supercapacitor-based flash chargers at ABB TOSA bus stops
Photo: ABB Switzerland
Top image: Maxwell Technologies