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How to Add a Second Battery to Your Boat: Step-By-Step Instructions

BougeRV’s 12V marine batteries LiFePO4 for boat

There’s nothing like the open water. That’s why millions of Americans own boats. But when you’re out on your boat, you need enough reliable power to meet your energy demands. If you don’t have enough power, you may consider adding a second battery to your boat.

That’s why we’ve made this guide. It takes an in-depth look at the process of adding a battery to your boat with step-by-step instructions, tips, and more. So, without further delay, let’s get into it.

  • How Do Boat Batteries Work?
  • Why Would You Want to Add a Second Battery to Your Boat?
  • How to Add a Second Boat Battery?
  • Replacing an Old Boat Battery With a New One
  • Getting the Most out of Your New Boat Battery
  • Summary of Adding a Second Battery to Your Boat
  • Adding a Second Battery to Your Boat FAQs
  • How Do Boat Batteries Work?

    The first thing you should know, if you don’t already, is that boats tend to have separate batteries for their starters and on-board energy demands. This is done to ensure that you don’t accidentally drain your starting battery and get stranded.

    If you’re thinking about adding a second battery to your boat, it will likely be to increase your onboard energy supply. There are times when you may need to replace your starter battery, but that’s not the purpose of this guide, so you may need to look elsewhere if that’s your goal.

    Why Would You Want to Add a Second Battery to Your Boat?

    A boat

    It makes sense to increase your boat’s power supply by adding a second battery in two different situations. 

    The first is if your current setup has only one battery that’s shared between starting the engine and running your gear while you’re on the water. As mentioned previously, you don’t want that, so adding a second battery would be a smart idea.

    The second scenario in which you may want to add another battery to your boat is if you have a dedicated battery for onboard power, but it’s not meeting your energy needs. In that case, you could consider either replacing the battery with a new one or adding a third battery to your boat and wiring it together with your existing house battery.

    One of the key advantages of adding a second battery to your boat is the potential it offers for solar energy. You can connect a new battery to solar panels so that you generate free energy while you’re out with your boat. This can increase the overall power you have access to by quite a bit when you boat on sunny days.

    How to Add a Second Boat Battery?

    Now that we know how boat battery systems work and the scenarios in which you might need to add to your current setup, let’s dive into specific instructions for the process. 

    We’re going to split these into two categories based on the scenarios outlined in the previous section. Follow the first set of instructions if your boat currently only has a starting battery. Follow the second set of instructions if it already has a second battery for onboard power but it’s not currently meeting your needs.

    Adding a Second Battery to Your Starter Battery

    If you only have one battery on your boat, your goal in adding a second battery is to give yourself a separate source for onboard power. 

    The first step in that process is turning off all of your electrical systems. This is really important because you risk seriously injuring yourself if you try to work with active electrical currents running through your boat’s battery system.

    Once you’ve done that, you can get started with the following steps.

    1. Choose a battery switch

    First – you need a battery switch. This should be at least a dual battery switch, which essentially allows you to easily switch back and forth between each battery on your boat. That way, you can use the starter battery for starting your engine and the other battery for onboard power once you get going. You can hold off on installing this for now.

    2. Choose a second battery

    BougeRV’s 12V solar battery for boats

    Now you need to choose the second battery that you want on your boat. It should be powerful enough to meet your energy demands. If you don’t know what those are yet, consider the appliances and electronics that you want to be able to use with it. You can add up the watt-hour demands of those devices and consider the number of hours that you want to be able to use each of them on your boating expeditions.

    For example, you might want to be able to power your radio for at least 8 hours. The radio should have a watt-hour rating. This tells you how many watts it takes to run per hour. 

    You can multiply that by eight to figure out how much energy the battery would need to provide to power the radio for eight hours. If you don’t know the watt-hour demands of your appliances, you can look them up online by entering the model number into the manufacturer’s website or with a standard Google search.

    3. Add the battery switch to your boat in a convenient location

    Now we’re ready to get started with our installation. We’ll begin by installing the battery switch on the boat. 

    Choose a convenient location for this so that it’s easy to switch back and forth between batteries on demand. You typically need at least four to six inches of space.

    Typically the battery switch that you purchase will come with instructions for adding it to your boat. Follow these, and don’t worry about hooking up the wiring just yet. You’re going to need to install the second battery before you can finish that part of the job.

    4. Connect positive cables first