If you have anything on your boat that uses electricity, whether fishing gear, a trolling motor, or even appliances, odds are you’ll need a battery bank to keep those devices running. For some, a single deep-cycle battery is enough to keep your needs met.
For others, a larger power source is needed that isn’t available in a single battery. To meet those power consumption needs you’ll need to create a battery bank, which is wiring up 2 or more batteries to double the voltage or capacity to meet your requirements.
Battery technologies evolve overtime - be sure to speak with a professional before attempting any build.
How to Connect a 12V Battery Bank System
A 12V system is by far the easiest and most common DC voltage to use. Batteries are commonly manufactured in 12V and are plentiful across all types. For this setup all you need is one. For example, if you purchase the battery below, all you need is to connect the positive and negative terminals to whatever it is you want to run.
Connecting two 12V batteries in parallel
- Keeps your battery bank at 12V
- Doubles the usable capacity
- Requires a special charger
If you want to increase the capacity of your battery bank system, but still keep it at 12V, you’ll want to wire the batteries in parallel.
Only connect batteries of the same age, type, capacity, and preferably model. Connecting different batteries can be dangerous. Do NOT wire Lithium Iron Phosphate batteries in a series. Ensure batteries are close to the same voltage when wiring together.
Looking at the diagram below, this means connecting the positive terminal from your first battery to the positive terminal of the second battery. Then connecting the negative terminal from your first battery to the negative terminal of the second battery.
To charge these batteries, you’ll want to connect a deep-cycle specific charger to the red or black dot terminals as shown on the end of each connection above.
How to Connect 24V Battery Bank System
24V deep cycle batteries are available, however they are becoming more difficult to find. As a result, the price of a single 24V deep cycle battery is sometimes more expensive than purchasing two separate 12V batteries.
Connecting two 12V batteries in series
- Doubles the voltage, with the capacity remaining the same
- A 24V charger is required
To reach 24V, you will need two 12V batteries of the same capacity. Connect the positive terminal from the first battery to the negative terminal of the second battery. Then connect whatever device(s) you are powering to the negative terminal of the first battery and the positive terminal of the second battery as seen in the diagram below.
To charge this battery bank, simply connect the charger like you would whatever electrical device you are powering.
Connecting multiple parallel battery banks in series
- Doubles the voltage AND capacity for longer run time
- A high amp 24V charger is required
If you are running 24V in your system, you are likely running appliances or other devices that may have a higher energy draw, or simply want the benefits of a 24V system.
To wire this system you will need 2 sets of the system described in “Connecting two 12V batteries in parallel.Connecting two 12V batteries in parallel.” Ensure these batteries are all of the same capacity, voltage, age, type, etc. Wire two of the batteries as described above, then do it again with the other two batteries.
Now connect the negative end from the first bank to the postive end of the second bank. Connect your device to the two remaining terminals.
Charging the system is the same as powering a device. However, you will want to purchase a high amp, 24V charger for this setup. You cannot use a 12V charger on a 24V bank. A lower amp charger will suffice, however you may be waiting days for the batteries to reach full charge.
How to Connect 36V Battery Bank System
To create a 36V system you will need to wire 3x 12V batteries in a series, similar to 24V series wiring with an additional battery. 36V systems are uncommon outside of an industrial or commercial application. To charge a 36V system, you will need to buy a charger that can charge three 12V batteries individually, or spend money on an industrial level charger.
How to Connect 48V Battery Bank System
Creating a 48V system will require 4x 12V batteries in a series. Again, this is the same as the 36V and 24V series where you are connecting the ends of each battery together. The wiring diagram below explains how to do so.
Charging a 48V system is possible through a 4-bank battery charger.
Deciding on System Voltage
Now, you’ll need to decide on the voltage you want to run. This can become complicated, so let’s run through a few scenarios.
A single-battery, 12V system is ideal for you if:
- You only plan on taking short trips that last a day or less before you’re back on shore.
- Your power consumption needs are low, less than 50Ah.
- Wiring around sharp corners or in tight spaces is not an issue.
- You are working on a budget
A multi-battery, 12V system is ideal for you if:
- You have higher power consumption needs, but will not be away from shore for more than several hours
- Your electronics can only operate on 12V DC power.
A multi-battery, 24V+ system is ideal for you if:
- You will be away from shore for extended periods of time
- Your power consumption needs are on the higher side
- Especially if you plan on using appliances or electronics that draw more energy
- Wiring space is limited or your boat will require longer lengths of wiring
This is not a black and white issue as there is some overlap between those scenarios. Let’s walk-through the differences in voltage, the benefits of a 12V system vs. a 24V system, and more.
Be sure to double check the specs of your devices. Some devices can run on both 12V or 24V, while some can only run on one or the other. Failure to do so can result in damage to the device.
Benefits of a 24V vs. 12V Battery Bank
A 12V system is typically more cost efficient, however if you will be away from shore for extended periods of time and will be using larger devices, a 24V system is the way to go. Here the main benefits of upgrading to a 24V battery bank:
- You can use a smaller wire size.
- Smaller wire means it can fit into tighter spaces and around corners easier.
- In larger applications, there is a significant savings in weight due to decreased wire size
- Greater efficiency in electrical draw
The main reason for upgrading is power consumption and wiring. Although there is an increase in cost, a 24V system is twice as efficient as a 12V system of the same capacity and you can use less wiring.
For example, Dometic’s electric cooler/freezer combo operates on either 12V or 24V DC power.
A screenshot from the product manual shows us lesser energy use at 24V vs 12V.
For the CFX3 55 model, at 12V it draws around 8.7A. With a 24V connection, that draw is only around 3.7A. If you want to learn the science side of why that works, you can find that info here.
Additionally, you will typically use about 50% less wire by using a 24V system over a 12V system. This is advantageous if you need to wire in tight spaces (greater flexibility), or there are greater wiring distances from the battery bank.
Calculating Your Power Consumption Needs
Arguably the most important step, calculating your power consumption needs will provide important info on:
- How many batteries you will need to buy
- How much wire you can expect to use
- Whether to wire in a 12V, 24V, 36V, or higher configuration
- How many Amp Hours your system will need
- And more!
You can download an excel template here (coming soon!) where you can plug in the values for all the items you plan on bringing. Be sure to follow deep cycle battery best practices such as:
- Keeping the depth of discharge 50% or less
- Using only all new batteries of the same type and capacity
- Using a deep-cycle specific charger
To start, make a list of all the electronics you plan on using. A list of common electronics is below for each type of boater. Keep in mind these are electronics that you want to run while the engine is off if they’re connected to the main 12V system.
Be aware that any items you want to bring from your home that normally plug into the wall will require an inverter to be used. The items mentioned below are commonly wired to run on DC, rather than AC power.
- Trolling motor
- Fish finder
- GPS plotter
- Navigation lights
- Bowfishing lights
- Marine refrigerator
- Marine air conditioner
- Navigation lights
- Instrument, anchor, and interior lighting
- Communication radio
- GPS Plotter
- Stereo system
- Misc small appliances
- Electric cooler
- Decorative lighting
- Navigation lighting
- Misc small appliances
Note the amp draw of each. For items listed in mAh, divide the number by 1000 to get the draw in Ah. These are usually listed somewhere on the item itself, a tag, or you can google the model number to find the specs. Otherwise, find a similar model and estimate it.
Next, estimate the time usage of each in hours while it is on the boat. Multiply the time usage by the listed amperage and you have the daily estimated use for each individual item.
This sheet also takes into account additional energy sources such as solar power. If you do not know the values for these or do not use them, leave them blank.
Sum those numbers together and you have your estimated daily use. Double your estimated daily use and that is the ideal capacity for your battery bank. While you can safely discharge a deep cycle battery to 20% of its capacity, it’s recommended to not discharge below 50%.