Battery powered application

I am designing my first battery powered device. I am planning to use a Li-ion battery cell pack (10.8V, 4800maH eg. https://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2211 ). I want to design a built-in charger for the product, so that I can simply plug in an AC/DC adapter to charge the batteries. My question is, when the adapter is plugged in, should the portable device run on the external power supply or the recharging batteries? (For example, when using a cell phone that is being recharged, is it running on the batteries or the external power supply?) Would I use FETs to do that? A block diagram would help.......
Oh also, do you know of any suitable battery charger ICs ?
Thanks!!! :shock:

Disconnecting the battery from the load while charging would seem to be an un-necessary complication; but you will want to monitor the battery current so as not to exceed the charge rate.

Google is great!
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http://www.analog.com/library/analogDialogue/archives/31-2/liion.html
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