Charging SLA battery is a good challenge for hobbyist.
I am trying to make one UPS, that requires to build inverter and Charger.(I guess it will publish in forum later 
)
I done the inverter but programming the charger unit now...
You needed to build one Voltmeter to detect battery voltage which is too easy to build one.
Grab 4x20K resistor and build voltage divider of 1/4 since resistors are same (that produced in same time) they are act like (or better than) precision resistor.
After you can control a Power Darlington (like TIP145 or BDX33) via PWM. I choose darlington transistor for small base requirements. And you needed to amplify PWM out voltage to charge voltage to feed BDX33. (You can use opamp(LM358) or optocoupler (pc817)).
Since you are sensing voltage of the battery, you can limit charging voltage to 14.40. But if battery is empty, you cant reach that voltage at start of charging...
Problematic part is sensing charge current. You needed to have constant charge current. How to sense current? I can't make it proper yet. But there is much of IC that senses current. You can use shunt resistor. It's possible to use OPAMP to amplify voltage drop/difference of shunt resistor and reference it to ground to read via ADC of arduino. But I have some problems in action, sensing current properly... Better to use circuits like MAX4172 will give you better results.
Most important part is using FUSE to avoid short circuiting battery. There is too much amperage that will molten any connections and wires in case of short circuit. Quite possible to start fire!.(I am almost ;D) I placed a 40Amp fuse to Battery connector directly...
Also read here to learn how to charge battery...
Good lucks.