i have a 6V 4.5Ah Rechargeable Battery,And a small custom made 20W inverter hooked up with a CFL (compact Florescent lamp)

and i also have an external charger 7.5V 1A with alligator clips at the end..i have a small switch on the wire going to the inverter(from battery) and i use it to turn it on..and when i want to charge it…i have to reconnect the alligator clips of the ext. charget all the time..

my BASIC question is…i WANT to make this system fully automatic..like..when the pow from the mains is comming..then it will trun off the lamp and charge the batt…when the power fails ..it turns on the lamp using the battery,

NOTE:I DONT CARE for overcharging..i DONT need any over charging circuit..and i damn care if my battery explodes right in front of my eyes..I JUST NEED THE AUTOMATIC THING…please dont tell me circuit for charging and over charging cut-off..

now.. someone told me that i can use a thing called relay..exactly for this purpose…i got one rated at 12V..now..it has 5 Pins…2 are on one side..and 3 are on the other side…now that man told me that give it 12v frm mains via transformer…i thing my charger can be used for this purpose..it gives 7V and ..can it drive the 12V relay?..if so..
where to connect these things..

2 pins are on left side..on the corners..and 3 pins are on the right side..2 of them are in corners..and 3rd in center..is slightly closer to the left pins..i found no pic..on the google..

NOTE: it is NOT a car Relay..it is a PCB mountable small pins relay
what pins among them are the switches..?and where are the 12v voltage input pins to control the switch ?