Quote from:
Pacific RV Parts
THE RELAY - How it Works
The Battery Disconnect Relay is a mechanically latching switch that operates by the momentary application of battery voltage to the coil terminals in one direction for latching (closed) or the other direction for unlatching (open).
To close the relay, +12 volts is applied to the ""I"" terminal and ground the ""S"" terminal of the relay. When this is done, the plunger is pulled into the coil and the contacts are connected. While this happens, the rod magnet suspended above the plunger is attracted (opposite poles attract) to the top of the plunger by the magnetic field.
When the voltage is removed from the coil, the plunger gets pushed upward by the return spring, but cannot move because the rod magnet is in the way.
To open the relay, +12 volts is applied to the ""S"" terminal and ground on the ""I"" terminal. When this is done, the plunger is again pulled into the coil. However, since the magnetic polarity of the coil is reversed, the rod is repelled (like poles oppose), and swings out of the way.
When the voltage is removed from the coil, the plunger gets pushed upwards by the return spring, breaking the connection between the two large terminals.