I have a question - how efficient is regenerative braking ?
Does it depend on the power and energy content of the braking episode i.e. is light braking more efficient thean heavy braking ?

Are supercaps better than batteries ?

Are certain battery types better than others ?

Are there any websites dedicated to this topic ?

I have heard that regen. braking is 15 - 20% efficient - in this case, reducing people's speed by 10% would achieve the same for much less cost - or am I raving ?

"Is light braking better than heavy braking?"
Yes, you're off of the gas peddle sooner and friction is helping more with light braking. This is even more true with the relatively light regenerative braking available in the Prius. I really notice a reduced gas mileage with agressive start/stop driving.

"Are supercaps better than batteries?"
No, they're different. Supercaps store energy as electrostatic charge on conductive plates with a non-conductive dielectric material in-between. The energy is stored as an electric field. Batteries store energy in chemical form. A chemical transition is involved. Supercaps can generally be charged/discharged much faster and have an extremely high cycle life. (hundreds of thousands or millions of cycles) They do not have the energy storage density of batteries. (...unless EEStor's supercapacitor storage unit can be manufactured to the last specifications they gave.) A good compromise design is to use both supercaps and batteries. The supercaps are used for start/stop energy storage, i.e. load leveling. They protect the battery from many small charge cycles that might reduce the batteries life. The battery is used for long term energy storage at a higher density.

"Are certain types of batteries better than others?"
Oh yeh. jcwinnie mentions Altairnano. They seem to have the best Li Ion battery. They've run tests showing 15,000 deep charge/discharge cycles, 6 minutes each. This is good for 41 years of use when fully charging and discharging once per day, with 85% capacity left at the end. This is amazing and they may not require a supercap to assist the battery. Other Li Ion battery products are not as good. From information I have:
A123 3,700+ cycles @ 87%
Lithium Tech 3,000 cycles @ 80%
ABT 4,000 cycles @ ?%
MHI 3,500 cycles @ ?%
Most of other Li Ion products and all of NiMH and Pb Acid products are not as good.
Nilar NiMH 2,000 cycles @ ?%
FireFly Pb Acid 2,000 cycles? @ ?%
Axion Pb Acid 1,600 cycles @ ?%
normal Pb Acid 200-500 cycles? @ ?%
Note that all of these will have a very long life, on order of several 100,000 cycles, if shallower discharge cycles are used. Shallow discharge cycles is what is done in all current HEVs.
Deep discharge life is essentially the technical problem they are talking about in developing batteries for PHEVs and EVs. The solution is to use AltairNano battery, or use battery/supercap combined, or come up with your own AltairNano competitive battery, or come up with a higher density supercap like EEStor is trying to do.
BTW Maxwell's BOOSTCAP 390 is a supercap (or ultracapacitor) module designed to augment batteries in heavy HEVs like buses and delivery trucks ...where there is the most to be gained from regenerative braking. Pretty neat, eh?

Regenerative braking and lowering the speed limit are not mutually exclusive. They're both good approaches to reducing fuel use. Why make them into an either or proposition? Do one, do the other, or best of all do both.

Seems like there are three basic approaches:
1. Just have a really good deep cycle battery like AltairNano, or maybe not quite this good but get the cost down farther.
2. Use supercap and battery together to get benefits of both.
3. Use a relative good battery like AltairNano or A123 and don't bother with regenerative braking. If you're already running all-electric, then the regenerative braking increases range and reduces energy use, but does not reduce fuel use. Maybe you don't care. Keeps cost of car down some. Phoenix Motorcars is doing this.
{There will be a fourth approach if EEStor is successful. If they come close to specifications made public, then this one will dwarf the others. Do or die? ...wait for it.}

I don’t claim to be any expert. This is just information I’ve gathered. Hope that helps.

Link on Series Hybrid Bus development work:

http://www.greencarcongress.com/2007/07/uqm-150-kw-prop.html

Series PHEV Bus with Fuel-Cell generator - July 2007.
“UQM 150 kW Propulsion System to Power Plug-in Fuel-Cell Series Hybrid Electric Bus“

Different battery configurations mentioned in two of three projects at bottom of write-up:
(a) Li Ion battery + supercap:
“The drive train will consist of a newly developed motor and controller, a Ballard 60 kW fuel cell, A123Systems Li-ion batteries and Maxwell ultracaps.”
(b) Li Ion battery only:
“battery dominant fuel-cell bus employing Altairnano Lithium Titanate batteries to operate”

sorry - post in error.

Does anyone know the efficiency of regenerative braking

a: Under light braking conditions when the battery should be able to absorb all the energy ?

b: Under heavy braking when the battery will be unable to absorb the energy.

In this case, can we capture it with supercaps ?

Has anyone actually built and tested one of these ?

as opposed to some powerpoint slides.