Arduino gsm on battery, how to drive it and recharge.

Hi! i need to make a gsm alert system.
The real problem i have is that it needs to run on line power and as a backup on some sort of battery.
Since the gsm shield and the other parts, i can't use the Lipo rider pro, since it only gives 1A output (i need a peak of 3/4A since the Gsm shield alone draws 2A also if only in brief bursts).

Probably i can still use the Lipo Rider Pro if i make some switching circuit to run directly on battery when needed avoiding the Rider's converter, but probably would be better to use a lead acid battery, it has more juice, more voltage (i guess it's easier to step down from 12V to 5 that to step up from Lipo's 3.7V and drawing considerable power).

However i guess that also with PB battery i need some sort of switching from one power source to the other, i imagine that i can't simply hook the whole arduino to the charging battery and it's circuits.

Anyone has some advice and/or electronic circuit to do that?