Basically there is a chip inside that responds to a certain radio frequency. I do not know about a battery, there may be one, jsut the speedpass's output is so low it barely gets drained, but from how long they last I'd say there is no battery. In that case, the RF energy from the receiver is resonated in the device, then is routed through the chips internal circuitry as electricity which in turn goes out a small antenna to send a signal back to the receiver. Data transfer is limited, so all that is transfered is a ser. no. which is checked against Shell's database and then bills accordingly.