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Twin battery systems

A sometimes misunderstood system on boats.

The primary purpose of a twin battery system is to provide backup power in an emergency.

However, there are a few ways to do this.

  • Carry a loose spare battery on the deck with jumper leads. (ad hoc, a temporary solution at best)
  • carry a battery jump pack. (good)
  • install a twin battery system. (the best option)

A marine, twin battery system consists of a motor (to use and produce power), a start battery (battery number 1) a four-position battery switch, and a reserve battery (battery number 2). 

The critical thing to remember in this system is that you only have your battery switched to either battery one, or battery two at any one time. Sometimes people with this setup will mistakenly run their vessel with the battery switch in the 1+2 position (both batteries connected). When the battery switch is in this position, you effectively have only one battery. If you experience a fault with one battery, it can flatten or damage the other battery, rendering the whole purpose of a twin system useless.

In a simple twin battery system, it is essential to charge each battery individually.

Each battery has a different amount of charge it can hold, and if charged together, will only "fill" to the capacity of the weakest battery.

Therefore it is an excellent habit to get into running your vessel, for a time, on each battery, not only to charge it but also to discharge it a bit. This process is called cycling.

Connecting both batteries should only be used to start your motor in an emergency or put an initial charge into a flat battery. Think of the 1+2 position of the battery switch as jumper leads.

When your day on the water ends, the cleaning, the packing away, and bulldust are over;  ensure you use the battery switch's final function to isolate the batteries. It will ensure that the motor alternator or any devices on your boat can not flatten the battery for any reason.