If you want to use lead acid and can accept float charging only then 
the total charger (apart form the actual energy source) can be a 
resistor and a zener diode. If you want it slightly safer an eg LM317 
in voltage mode will do the same job. By switching one resistopr in 
the sense circuit leg to ground you can step the '317 to boost charge 
as well.


        RM

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