Unless you want the down rigger/house battery to be able to be used to start the engine(s) in an emergency, you don't need a battery switch. The positive wire from the starter battery goes to the engine only, and the positive wire from the down rigger/house battery only goes to the down riggers (and most likely some bilge pumps, lights, electronics, etc). The only common connection of the two battery systems is the ground wires.
If your boat is a day boat (goes to shore every day at the end of the trip) get a house battery with enough capacity so the it does not need charging while on the trip; charge it with a shore charger. The voltage regulators on all stock engines (both auto & marine) are designed to charge a starting battery not a deep cycle battery; they will overcharge a deep cycle battery leading to premature deep cycle battery failure.
What is the point of the battery switch? I have been told that after draining battery #2 that I should not put the switch on both as #2 will drain the battery and damage it. Also the two unequal batteries should not be connected in parallel, can damage one of them or damage the charging system. Also have read never switch when the motor is running
What you have been told is true.
The problem is that this requires a lot of thought by the operator, who is trying to relax in the first place. Sooner or later a crew member may leave the switch in the BOTH position with the engine off, resulting in 2 dead batteries. Statistically, dead batteries are one of the most common reasons for calling a towing service. “I can’t crank my engine!” is right up there with “I’m out of gas!” for ruining a day on the water.
This is very true and is why automated systems exist.
If you use my proposal, make sure that your shore battery charger can properly charge the type of deep cycle battery you end-up getting. Common types of deep cycle batteries are Lead-Acid, Gel, & AGM.
Also, make sure that the capacity of the deep cycle battery is enough so that the amount of battery charge you use on a typical day on the water does not exceed 50% of the total amp-hour capacity of the battery to ensure long battery life.
You will need to estimate how many amp-hours of battery capacity you use while on your trip. Your electronics will be on all the time & their power consumption in amp-hours can be found in their respective manuals. If your electronics use 5 amp-hours, (5 amps per hour) and your trip lasts 8 hours, then their total power consumption per trip will be 40 amp-hours. If each down rigger consumes 10 amp- hours, you have 2 of them, you raise them up 20 times per trip and it takes 2 minutes to raise each down rigger, then their total power consumption per trip will be 2 minutes X 20 X 2 down riggers X 10 amps per hour = 13 amp hours. Total power required per trip = 53 amp-hours, requiring a 106 amp-hour battery.
You actual figures may differ. A group 27 lead-acid battery would fit the above example, but barely. AGM batteries can be discharges deeper that 50%, so for a safety margin either a group 27 AGM 0r 4D lead-acid battery would provide a better safety margin.
You could still install a switch so that the deep cycle battery can start the engine if the starting battery fails, but this switch would probably never get used.
