Can I Connect the Solar Controller Directly to the Battery Switch?

Question:

I connected my solar controller output to the battery switch, so I can select which battery bank to charge, the house or engine. I do remember not to use both; I do remember to switch the main DC off so the circuit will not be powered by solar when the battery switch is off (should be using load output in this case); I do remember to disconnect (remove a fuse) the solar power at the same time when I disconnect the batteries; I do remember that one day I will forget to remember all this... :) So why not connect the solar to the battery switch?

Answer:

There is a big difference between a power supply and a charge source.  On a boat there are many charge sources: battery charger, solar controller, alternator, wind controller, etc… all of these devices are meant to be connected to a battery.

A power supply is a device that can be connected directly to loads without a battery in between the loads and the power supply. A power supply is meant to hold a steady voltage as loads are either added or removed from the device. A steady voltage is essential as many loads can be damaged if the voltage spikes beyond their tolerance. Without a good power supply, suddenly removing a large amp draw (i.e. large load) would potentially cause the voltage to spike up.

A charger is NOT meant to hold a steady voltage as loads are added or removed. The battery is supposed to act as a “spring” keeping the voltage relatively steady.  When a load is directly connected to a battery, I.e. a solar controller connected to the same post as a loaded with the battery switch off, the solar controller is in effect trying to act like a power supply. Unfortunately, the solar controller cannot react quickly enough to changes in amperage as loads are added and removed and the voltage output from the controller will potentially fluctuate too much and therefore potentially damage some of your DC appliances.  Therefore, all charging sources need to be directly connected to the battery. - Jeff