Beware, The circuit shown above could get you in trouble and ruin your battery. To properly charge a SLA or any lead/acid battery, two requirements exist to prevent damage. 1. Limited current (there is a maximum current you can put into the battery without harming it)
Yes limited current. Thats why YOU CHOOSE A LOW CURRENT transformer (or multitap primary to select current charge) or set up a current limiting resistor. I choose the low current transformer method
and 2. Constant voltage must be maintained to prevent overcharge and again ruining the battery. That value is somewhat temperature dependent but a relatively safe value is 13.6 volts. The circuit shown does not include either of these safeguards.
Umm the final output voltage if you know what math to apply to center tap full wave rectification is at .636(15v RMS/(Sqrt 2/2) - .7v) =
13.0464 volts CONSTANT
FYI
Also guess how a damn Sears charger works? THE EXACT SAME WAY. THERE IS NO SPECIAL CIRCUITRY AT ALL TO PROTECT THE BATTERY. THE CURRENT SELECT IS DONE BY SELECTING DIFFERENT TAPS OF THE TRASNFORMER AT THE PRIMARY SIDE WITH A SWITCH. THE SECONADARY SIDE IS A CENETR TAP WITH TWO RECTIFIERS FOR FULL WAVE RECTIFICATION THAT CONNECTS TO BATTERY CABLES.