- Joined Aug 18, 2017
There is also a controller to provide variable flow in the circuit, which is one reason to use a relay instead of just diodes. With a relay, I can connect the battery to the pump directly. If I do that with just a diode, the battery will supply 12V if the controller drops voltage below 12V. I could connect the battery to the controller, but it is 1 more thing that can go wrong if power is out and 1 more thing drawing power off my battery.