> From the sensors point of view, the 12V could be between 9V
> and 16V, but I have also some comparators which work at 12V
> and if the voltage drops bellow, the result isn't accurate anymore

Make a local, stable 12V just for the comparators. To guarantee
12V on battery power, use a simple voltage doubler (should give
you around 20V when battery is flat) followed by a regulator, for
instance, as the comparators presumably won't need many mA

> I would also need a charging circuit for a lead-acid battery, can
> you indicate me a good circuit ?

For best care of the battery, there are many charger/monitor ICs
around. For example, as Byron suggested, Unitrode's UC2906
or UC3906

http://focus.ti.com/docs/prod/folders/print/uc2906.html

Application notes include background theory and practical circuits

You'd find others to compare in catalogues or at retailers with on-line
product guides like Radiospares or Digikey

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