Does the negative terminal of a battery always have to be at 0V?
labelling a battery terminal negative probably comes from it being connected to the cathodes in the cell. (the positive terminal connects to anodes) electrons have negative charge, and so flow from negative to positive. Electron flow is the opposite way to conventional current- but that wasn’t known when the direction of flow was initially guessed, and it’s opposite to the direction voltage is built up in. As you know it’s potential differences between points that drive currents. Defining one point to be @0V simply makes it easier to take measurements without needing to do sums to get the results. When taking voltage measurements is also easier to clip a lead on to a convenient fixed point, and then move the other probe around to the points where voltage needs measured. A batteries negative terminal is usually fairly convenient.
