iphone 3g charging problem

i am having problems with my iphone, when the phone runs out of battery , i stick it on the charger and the phone turns on , but there is no picture on the battery bar on the screen indicating that it is charging , but there is obviously power coming in from the charger as the phone turned on, i have replaced the old charger dock and still no change ......

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I would check the cable. Does it do the same thing when plugged into a computer vs the wall adapter?

I guess I've got the same problem. But it's wired that sometimes it happens, sometimes it's good to go.