Avoid a Dead Battery

I highly recommend having a second battery on your boat.  Then you won’t worry about running the battery down from playing the stereo all day at the beach or cove and the motor not starting when you are ready to go home.

The easiest way to prevent a dead battery is to have one battery to start the motor, and a second battery for the lights, stereo, etc (everything else except the motor).  The problem with two separate circuits is only the starting battery gets recharged by the motor.  The other continues to drain until its either dead, or you take it home and recharge it.

This is where the “Automatic Charging Relay” comes in.  It’s also known as a “Battery Combiner”.  This nifty little device is about the size of a coffee cup.  How it works is very simple.  Whenever the motor is running, it combines both circuits so charging current from the motor recharges BOTH batteries at the same time.  Then, when you shut off the motor, the Charging Relay or Battery Combiner senses the loss of charging current (technically, it senses the small drop in voltage) and separates the two circuits again, leaving the “house” battery isolated from the “starting” battery.  This keeps the starting battery fresh to start the motor when you are ready to leave.

The reason I like this setup is it’s a “wire it and forget it” solution.  It has worked problem free for me for several years now.  The best thing is I don't have to worry about the motor not starting at the end of the day.
 

I use the Blue Sea SI-ACR Automatic Charging Relay.  See photo below.

Below is a picture with the cover off.  Note the two large terminal posts marked A and B.  You connect the A terminal to the + terminal on the starting battery, and the B terminal to the + terminal on the house battery (the one that runs the stereo and everything else).  You connect the - terminals of each battery to each other (not through this relay, just ground both batteries).  You add a small wire to the ground terminal shown on the charging relay through a 15A fuse to one of the negative terminals on the batteries.  This is only used to make sure the internal relay operates.  Again, both negative terminals of both batteries need to be connected to each other.  And that's it!  The other two connection tabs are for extra features explained in the instructions.  You just bolt this somewhere to your boat near the batteries and wire it up.  Use heavy gauge wire for the A and B terminal posts.  This relay can handle up to 120 Amps.  They have a couple models and you want the one rated for more amperage than your boat motor alternator can output.  

Now you have one less thing to worry about while enjoying a relaxing day on the lake!
 

By the way, they sell kits that include a manual switch.  In my simple setup, I decided I didn't need the switch.  The switch can add another $40-$50.  And, adding a switch puts more holes in your boat!