Please refer to the following notes as you read along: *Note1 -> Single outboard engines tend to have charging systems that are not capable of charging more than one battery at a time.. If you plan on using your single outboard to charge more than 1 battery you should verify that your outboards charging system can output enough amperage to do so.
*Note2 -> NEVER switch a On/Off or 1/2/both/off switch to the OFF position while the engine is running!! Doing so will destroy the charging system on your boat resulting in downtime & expensive repairs!
Common Boat Wiring Setups & Explanations
This diagram shows a pretty common setup, one engine, two batteries a 1/2/off/both switch. This setup can be used on a boat with a single outboard engine *Note1. Battery #2 could be a trolling motor battery or just a backup in case battery #1 goes dead from running the lights/radio/etc at anchor. The way it works is: The engine will crank & charge whichever battery is selected by switch "A" (1/2/off/both) *Note2. Once we are underway the engine will charge the selected battery. Once we get to wherever we are going to fish/swim/sit/etc we can change the switch position #2 to troll/listen to the radio etc. If #2 goes dead we can switch back to position #1 and be fine.
Using this setup battery #1 and #2 are charged by the alternator. This set up is NOT recommended for boats with a single outboard engine *Note1. Either setting on switch "A" can be used (switch "A" is a 1/2/off/both switch). In the event of a dead battery on setting #1 a simple change of the switch to battery #2 will allow us to crank up and get underway. The isolator will prevent battery #1 from draining battery #2 and vice versa. The BOTH setting on switch "A" *Note2 is not used..
This setup is commonly found on boats with two engines.. Each engine has it's own battery for charging/cranking and there could be an optional 3rd battery for running the boats electronics....
The lead from the alternators on the starboard & port engines are connected to the positive post on their respective batteries. The grounds are all connected. Position #1 on switch "A" (1/2/off/both) is connected to battery #1's positive post. (red) Position #2 on switch "A" is connected to battery #2's positive post. (purple) The common post on switch "A" (1/2/off/both) is connected to the starter of the starboard engine.
Position #1 on switch "B" (1/2/off/both) is connected to battery #1's positive post. (blue) Position #2 on switch "B" is connected to battery #2's positive post. (red) The common post on switch "B" (1/2/off/both) is connected to the starter of the port engine.
Normal operation: Switch "A" in position #1, switch "B" in position #2. Battery #1 is charged by the starboard engine while battery #2 is charged by the port engine.. The house loads normally are connected to battery #1 in these setups.. In the event of a dead battery, say battery #1 went dead. The port engine battery (#2) is still charged it can crank the port engine as is, then switch "A" would be moved to position #2 it is now connected to battery #2, it can also be cranked. Once running the switch can be moved back to the #1 position and resume normal operation..
Optional house battery: The above diagram shows battery #3 set up as a house battery, this typically would be a large deep cycle as it will never need to be used to start the engines. It is connected to battery #1's positive post and there is a simple On/Off disconnect switch "E" to allow us to sit at anchor and not worry about draining battery #1. Switch "E" could also be replaced with a battery isolator or an automatic charging relay to make remembering to flip switches a non issue.
