The Perko Battery Switch
One of the biggest problems with fishing at night on the river is making sure that you have plenty of power, normally my fishing can be 10 hours long and mainly at night. running out of power is no fun. Been there done that, tough to hand start a 125 Mercury pulling 150# of compression on all four cylinders, but it can be done.
My boat has three batteries in it, one is dedicated to the starting of the main motor, it is a 525 amp starting battery the other two are looped together positive to positive ,negative to negative with #6 welding cable both are 925 amp deep cell batteries making them 1850 amp since they are tied together and are dedicated to everything on the boat at night, both livewells, both livewell lights, all the recirculation pumps for the livewells, all the compartment lights, all the interior lights, my light pole and the running lights on the boat, my electronics, the radio, a bunch of stuff to run.
If the fog moves in then your stuck in a spot for hours until it lifts, I had read on several sites mainly sailing sites that they use these battery switches to isolate there power because this is a major problem for them, I learned that I could convert my system over and it wasn't hard at all.
Here is the Perko switch I purchased for about $30.00
It is mounted against the back wall in my battery compartment so that it will stay dry and is easy to get to. It's not hard to wire one of these things up, on the back are three terminals, #1 ,#2 and a common, you need to make two jumper cables, mine I made out of #6 gauge welding cable.
You run a cable from the positive side of the battery you want to start the motor with, to the terminal marked #1 on the back of the Perko switch.
You run a cable from the positive side of your second battery to the terminal marked #2 on the back of the Perko switch.
On the common you run the main cable from the engine and connect it to the common terminal on the back of the Perko switch.
When I'm traveling down the road I'll keep the switch in the off position, when I arrive at the ramp I flip it to position #1, I start the motor and I can use anything I want on the boat, while motoring the river the alternator is charging this battery, #2 is being left alone. When I arrive where I want to fish then I flip to #2, all power is being pulled from the two deep cells that I have linked together, #1 is not being used so I have a fully charged battery waiting.
When I go to move I just flip over to #1, start the motor with a fully charged battery and I'm ready to go, now I can elect to flip it back to #2 while the engine is running then the alternator on the boat will charge up the two deep cycles. I have a 40 amp alternator on the boat so it's plenty big enough to handle any load. Works extremely well and I don't have to worry about running out of juice to start the motor.
One nice part about this switch is it remains in contact from one battery to another, so it makes before it breaks when you are switching it over, no loss of power at any time.
You notice that in a bad situation you can flip it to all and you have all the batteries to fire up the engine, this I won't recommend mainly because if you have one battery that is fully depleted and another fully charged when you flip it to all the depleted battery will pull power from the charged battery's till they all equalize and then you could be stuck with very little power to start the big motor. I would only use this in dire case of an emergency.
One word of caution, if the big motor is running do not flip the switch to off, you will cook all the relays and diodes on your motor, not a fun or cheap fix if that happens, they do have one that will disconnect if you flip it to the off position but I elected not to go that route.