Scotbill
I use the tomtom 300 When updating maps it's recommended that a tt indoor power adaptor is used from the mains electricity. The reason being that updating maps might take some time (depending on your broadband speed) and if the internal battery power is low, it drops the data inputs.
The reason being about the battery power, when the tt is connected via usb to a laptop or pc, the tt does not draw power from the usb only relies on the internal battery or the tt home power adaptor.