The frequency that is used (at least from the so called PWM chargers I know of ) is either 60 or 50 HZ, depending on where you live. They are simply using a half wave rectifier and regulating things from there.

This is from a battery manufacture of VRLA batteries. Same thing as SLA. You can not have a true SLA as it would blow up .

http://www.panasonic.com/industrial/...ingMethods.pdf
The referring page is here. http://www.panasonic.com/industrial/...eal/index.html

As you will see, a recommended frequency is not given as PWM is not one of the recommended ways of charging these batteries.

If you are looking for a boost circuit for those cloudy days, I am using one built around a LM2588T. http://www3.national.com/pf/LM/LM2588.html