http://www.powerstream.com/BP-23.htm
I have had a couple of these before that i used with my ham and scanner radios, they have larger 20 Ah also, just depends on how long you may need it, but they work great for some portable power and phones pull such low current it will keep the phone charging many times before needed to be charged back up itself and you can charge it in the car or from a wall-wart....