Circuit that will allow uniterupted automatic switchover between batteries.

The batteries are 14.4V 4.4Ah Lithium Ion packs. I'm powering a system off one cell at a time but i need for the system to notice the charge level is becoming low on that loaded pack and to cut automatically over to the other fully charged pack without any break in supply to the output to load. The now discharged pack can be removed (to be recharged later) or another fully charged one installed into its place. This is pretty basic stuff but working with Lion, the internal protection keeps shutting down outputs momentarially. The Packs are Sony BP-GL65. Any ideas appreciated...