It’s no secret that a tethered dslr battery will discharge faster than when it is not tethered.  In fact, the discharge can be quite pronounced, depending on the body and task.   So, what drains the battery? Live view – The camera cpu works hard processing the video stream that it sends to the computer. Communication – One would think that the computer would provide all the power necessary for communication, but in fact it is the body that provides most of the power.  Many USB ports cannot