In general, in one aspect, the invention features a method that includes using an optical coherence tomography system to acquire a plurality of frames of a sample where each frame includes optical information about the composition of the sample through a section of the sample. Next, the load power and the environmental temperature when the battery is discharged are measured, and then a life value corresponding to these measured values is selected from the data so as to be set as an expected life value. The rear supporting wall forming a bit support for the ploughing bit slopes down to its outer regions.