3GS charge

Ok, Ive had this problem with my Iphone 16g, and now its happening with my 3GS. I've noticed that if my battery runs low, and I connect it to my laptop, and continue talking, my phone will still lose charge, and ultimately shut off due to a low battery. It will not charge while I am using the phone while connected to the computer. If you even look at the battery, the little lightening bolt icon, changes to a plug icon. Does anyone else have this issue?