The backpack energy harvester not only harvests energy to charge portable device but also reduces the risk of orthopaedic and muscular injury. In this paper, a lossless current sensing approach for backpack energy harvester is present in swop fly-back converter. The swop fly-back converter is reconstructed by the input capacitor and transformer in fly-back converter. It is controlled with constant on time algorithm and is switching in boundary mode strategy. The measured digital data is transmitted to smart phone by Bluetooth. And then, as a portable base station, the smartphone sent to the cloud. In the previous test, the linearity between time to digital converter output and output current is good as the reason that the correlation coefficients is 0.9772. The average output current is 0.1686 A for striding and 0.4242 A for jogging. The maximum output current is 0.9972 A for striding and 2.16 A for jogging.