[BrianG]

A higher kv will generate more voltage for a given rpm, so I would go with that. And, I wouldn't worry about stalling the motor because charging a battery is not nearly as much load as shorting the motor leads. The battery charge current is "regulated" by the current battery voltage and any inline resistance or circuitry. And, you might have to gear up the motor because a windmill does not spin very fast and these motors will have more output at higher rpms. Maybe an outrunner would work better in direct drive?