Just one:  as far as I know there is *no* simple scheme to charge Li-Ion
batteries (like constant current or 
constant voltage) - you have to follow the rules and they aren't that
simple!  Lead-Acid are by far the most 
simple to charge and the most forgiving of abuse.

Two stage, like SLA but with tighter tolerance, is what most of them use. 
TI does some nice simple chips for that.
But, I would not recommend them into this application.


And (OK, two!) if you're using the charging-effort as the fitness element
you will need an alternative energy 
dump (say a big hefty heat-sinked resistor) as well because you can't be
sure the battery will always take the 
energy the punter is putting into the system.

True. A big battery can take a joke, maybe up to 10% of its rated current
for short periods.  You don't want a car battery for this, definitely a
marine deep cycle or similar.

Generically, lead acid batteries do well with a two stage charger, high rate
with one set voltage, till current drops below X, then low rate with a lower
set voltage. 



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