Re: Battery Selector Switch
I think it is important to note the reasons for having a battery switch. I have read many posts and responses in these forums. It is common that the original reasons for having the switch gets muddled, or even completely overlooked.
A battery switch obviously serves as a convenient way to turn on and off main power to the boat. It also serves as a means of selecting a second battery. Having a second battery has obvious benefits. Other reasons for having the switch are less obvious but no less beneficial.
One very important reason for having a main power switch is that it provides battery isolation in the event of a battery failure. Boats vibrate and pound around allot more than cars. This makes batteries more prone to internal damage. If one of your batteries fails you can simply switch to the other battery. If a major failure occurs and one of your batteries starts to get hot, you can shut off all power drawing from it and possibly prevent a fire. This is also a good reason that your batteries should not be placed close together or in the same box. Separate them if you have the room. This safety feature also holds true for failures you may have down stream of the switch. If a short occurs, and a fuse fails, any wires on that circuit will overheat and burn. This can burn into other wires causing shorts in a domino effect. Without a battery switch there would be no way to kill the power and stop the short. So safety is a good reason for having the switch even on a single battery setup.
Another reason for having the main switch would be to prevent parasitic drain. Most people think that if all the boat's switches and the ignition is off the battery should have zero drain. The truth is, all electrical systems will have some degree of parasitic drain on the battery. The moist marine environment is enough to cause problems. Couple parasitic drain with radio memory and your boat's battery will die much faster while the boat is in the slip or sitting on the trailer for weeks at a time. All of the boat's power should route through the switch so that it is completely isolated. The only exception would be an electric bilge pump for boat's kept in a slip. In that case, the bilge circuit should be direct to the battery and independently protected with its own fuse. To keep the battery charged, and the bilge working, an automatic onboard charger will also be needed for a boat moored in a slip. A main battery switch provides an easy way to shut everything down without having to dig around in the battery box disconnecting cables.
Another question concerned how many circuits should route through the ignition switch. Most marine ignition switches do not provide a accessory position like the one in your car. One reason for this is that it provides a passive safety feature by isolating the start/ ignition circuits from the rest of the electrics. In fact, it is also a good idea to physically route the start and ignition wires away from other circuits through the boat. In the event of a short leading to burned wires it is far less likely that the essential start/ ignition wires will be damaged and far more likely that the engine will start and run. Isolating the start/ run circuit will also help to prevent damage to sensitive computers and circuitry in modern marine engines. Even small outboards have some form of electronic control built into them these days. It is important to plan ahead for possible failures in your electrical system in all cases. Think in terms of what can happen instead of what simply will work. You will be safer and more likely to prevent a problem as big as this:
