Embodiments of systems and methods for providing cathodic protection to a fluid-containing vessel include measuring two distinct voltages at a junction box exterior to the fluid-containing vessel. The first voltage is measured across a shunt in the junction box such that a current magnitude through the shunt can be determined using Ohm's Law that is representative of a current output of one or more sacrificial anodes disposed within the fluid-containing vessel. The second voltage is measured by depressing a pushbutton of a momentary switch to open the switch and interrupt current flow through the shunt. The second voltage is representative of a voltage difference between the one or more sacrificial anodes and the a wall of the fluid-containing vessel.