[mpg_numbers_guy]

IIRC lithium can handle much greater amounts of regen than lead acid. I wonder if a tiny 6ah lithium battery could handle all the regen you could throw at it, and then use a DC-DC converter with a voltage cutoff (to prevent the lithium from discharging too far) to slowly charge the deep cycle lead acid battery from the lithium battery. Then you might be able to regen at a higher amperage - say 300 amps of regen versus just 50 or 100 (which might damage the lead acid battery). One would just need to do something to make it pull that 300 or so amps whenever braking. Obviously like in a hybrid any regen is still an overall efficiency loss compared to EOC, but if you have to brake, why not charge the 12v while you're doing it?

A small, 25w solar panel exposed to 4-5 hours of sunlight a day, running at average efficiency, theoretically should provide 5-6ah of charge per day: 20 watts * 4 hours = 80 watts / 14v = 5.7ah. Even with efficiency losses that's still about half of your electricity use.