The torque profile of anemometer must be greater than drag torque to spin. Once friction is overcome, motor becomes a speed controlled current source which can be regulated by monitoring speed to maintain adequate speed. As slip occurs between cup velocity and wind speed, eddy current losses increase rapidly so a second unloaded windspeed sensor can measure the slip by some speed ratio to mach the impedance presented by load to available no load speed by some slip ratio like 75%.
The anemometers dont need to be identical but similar in no load RPM ratios .
Then SoC can be monitored by coulomb counting current pulses with 50% efficiency in lead acid with maximum voltage for charging during charge pulse using S/H to measure Vp max= 7.1 V
a pulsed load ESR measurement of battery capacity may help and some have correlated thus for a given lead acid battery with AH capacity remaining but this is degraded by aging and cold temps and load current external to battery needs to be counted.
Prop type blades like airplanes or Betz turbines use variable pitch to control loading, which you dont have. Like all fans have a RPM vs CFM and air pressure dependance or load , you will have an electrical load affecting RPM vs windspeed, so pressure can be regulated by the ratio of two props. A small one for true speed no load and the larger one for slip speed controlled current source to optimize current, just like MPPT tends to regulate PV's 75~85% of Voc.
Strictly speaking, SLA's use a lower Vmax for charging than open cell types and are usually temp. Controlled Vmax for wide ranges outdoors.
If your motor is underpowered to battery charge limits and load is unknown at same time, then regulation is more critical.
I have heard prop type anemometers running on a roof top with very loud prop noise running blade tip speeds 10x wind speed charging batteries.