We can't regenerate enough energy to overwhelm any of our current battery chemistries. The X1 will go 0-100-0 in eleven seconds without using any friction brakes (until the last few mph)! All regen energy is just stuffed back into the Li-ion batteries. Me thinks that we don't need more than that!Are those the batteries which have about half the energy density of lead acid? They're somewhat useful in cars as their high charge rate can capture energy from braking... but I didn't think that was a big problem for big batteries.
Here's a chart that's a few years old that'll show some energy density values for various chemistries.I remember someone saying recently it took about ten years for the last doubling of lithium battery capacity.
Just imagine if energy density for gasoline would double every five or ten years!