Actually, the idea was keep the battery but since it can only load at, what, 90W or so, if the regenerative breaking produced more power than that, it would be wasted as heat. Of course, I will admit that if the LEAF has a 80kW motor and I assume for the LEAF the regenerative component would simply be turning the electric motor into a generator, I guess there's no way to recover more energy anyway, at least here. But if a higher-capacity generator was used for Regenerative Breaking, then Ultra Caps would definitely be wonderful at capturing the excess charge and releasing it back into the battery more smoothly. Anyway, that would be the idea. As someone said, this may be a better issue for the hybrids, depending on how their configured. It's really a question of how the Regenerative Breaking is configured.