[theltrj]

supercaps need waaaaay more energy density before they are going displace batteries.....there is a lot of published research into battery/supercapacitor hybrids, the benefit is primarily seen in short transient charge/discharge moments

In a car the use case would be driving with a lot of hills or a ton of start/stop (urban city) driving

Once you get out on the highway where there are long transients discharge moments, you won't see much benefit from the supercapacitors. Depending on the circuit topology it could also be a drain on the overall efficiency of the circuit.

Also, supercapacitors are very expensive....plus you need the high efficiency power converter ($$) between the two to manage the different power sources