[Q] Not sure how charger works on this phone?

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Anyone know what needs plugging in to get this phone to charge please? I have always used it with both cables plugged in. Suddenly I am trying not to bother using the USB cable and it only charges in the cradle for a minute or so before the orange light goes off. With the phone plugged in OR with the battery plugged into the slot on the cradle. (therefore this doesnt seem to be to do with the charging circuits in the phone itself). I dont think it has always done this.

So I tried plugging the USB cable back in and hey presto, my brand new battery starts to charge properly.

Does anyone know if this means that the ac adapter connected to the cradle is malfunctioning and not putting out enough power? Or enough power constantly enough to continue putting out the necessary 5v? Or is it only supposed to work with both cables plugged in? When I unplug the ac adapter, the battery stops charging so I cant figure out where the cradle is getting its 5v to charge! Other have reported, worryingly, that the unit should charge with the USB cable alone.

I did look at http://forum.xda-developers.com/showthread.php?t=282983 but they all talk about after market chargers not working, whereas I am pretty sure mine is the original Siemens sx66 charger. Does anyone think that the adapter is failing and eventually BOTH the power cable AND the USB cable wont put out enough power to charge the unit?