Automatic Switching to Battery on Power Failure

Hi, I want my arduino and gsm shield to be powered even on power failure. I have one 12V 3A acdc adaptor and 6V 4.5 AH lead acid battery. I'm thinking of using a relay or a diode. which one is better? I already have a 1N5822 schotkky diode for lower voltage drop.

Here is the schematic. The LED represents the arduino and gsm shield.

I like the diodes for simplicity. The voltage gets reduced to 3.3V for the shield anyway?

I thought of that too but with the diode it is still draining a very little ampere on the battery even when not in use. Do you think its okay? yes, the gsm shield has on board regulator.

How can it drain when the diode is reverse biased by the 12V source?
You can also add a battery trickle charger to keep it topped off.

Oh sorry my bad. How to add it? Do you have some sample circuit?

I've not done anything with charging SLA batteries. I would recommend a commercial unit such as this

Charging sla's aren't that bad, they are pretty robust(I toture them often with electrically abusive experiments), probably the most effecient way would be a mosfet, a regular diode would only be worth it at low currents, a germanium diode is better but still has current limitations(as well as heat)
Basically for that size battery you will want to charge it @.3-1amp constant current untill it reaches close to 13.8, then you have it constant voltage @13.8
Easiest way is to use a lm317 although @1amp you will need a heatsink

Those levels sound kind of high for a 6V 4.5 AH lead acid battery.

Oh wow missed that, thought it was 12v battery,

I think i already have one. Its this one.

so i can connect it to my circuit the whole time? no overcharge?

I believe so.