I need to charge two 4v sealed lead acid batteries of different states of discharge from a 12v battery that is charged from a VAWT turbine. Can this be done with a voltage divider circuit? Two zener diodes?

 

When charging lead acid batteries you can use a unregulated power supply BUT you have to make sure that you do NOT trickle or float charge the batteries. They must be disconnected when they are fully charged. When the batteries are fully charged the current will begin to taper off causing the voltage to increase which will cause damage to your batteries if left connected.

 

SLA batteries need to have their charge current controlled. For longest life, charge them slowly at about 1/20th their AH rating. If you try to charge them more than about 1/8 their AH rating, you'll cause their internal temperature to rise, which drastically shortens their lifespan.

Additionally, if they are gel-cels, rapid charging will cause bubbles in the gel, which will permanently reduce battery capacity.