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Boat Battery Size: A sizing guide for seafarers

Romain Metaye

Ph.D. in Chemistry, École Polytechnique

May 24, 2022

Not sure how to determine your boat battery size? Then you’ve come to the right place.

Depending on the type of boat you have and what you use it for, it might be wise to invest in a solar PV system, a large enough marine battery bank, and a powerful inverter (only if you wish to power AC devices).

This way, you’ll be able to be out in the sea while still having the luxuries that electricity provides, like cooling your beverages in a portable refrigerator, cooking a meal using a portable electric burner, or pretty much anything you can think of that requires electricity.

In addition, smaller boats — like those used for fishing trips — are usually equipped with a trolling motor powered by the boat battery.

This way, if you have one of these motors, your boat battery size will determine how long you’ll be able to power your trolling motor, so you should size it according to your needs.

In this article, we’ll explain how to size your boat battery bank step-by-step.

Moreover, we’ll present a battery size chart showing what you can power — and for how long — with specific battery capacities.

Finally, we’ll discuss the best battery size for a boat with a trolling motor (24V and 36V).

(Please note, in this article we will assume you use DC appliances in your boat, as this is the most common setup.)

What Battery Size Do I Need For My Boat?

Your boat’s battery bank size is determined chiefly by your energy demands.

Different boaters have different energy demands; fishing boats usually have a battery bank to power trolling motors, fish finders, live wells, and lamps.

Coastal boaters have different needs; they often need power for radars, GPS, and VHF radios.

Meanwhile, recreational boaters often have refrigerators, sound systems, lights, portable burners, and televisions on their boat, and all of these appliances are powered by the boat’s battery bank.

While estimating your energy demand is crucial, there are a few other factors that should be considered when deciding on your boat battery size, such as:

• The size of your boat.
• The number of appliances your wish to power (and their respective wattages).
• How much autonomy do you wish to have?
• The number of people aboard.
• The size of your boat’s solar PV system (if it has one)
• Type of batteries you want to use (Lead-acid, Lithium-ion, etc.)
• The amount of space you have in your boat for your battery bank
• The voltage you need your boat battery bank to be

Some of these points are tricky to determine, such as your energy demand. But don’t worry, we’ll guide you through these trickier ones, and by the end of this article, you’ll be able to determine your boat battery size all by yourself.

So let’s get to it!

Estimating Your Power Demand

Here’s where a bit of math comes in. Some might not enjoy it, but trust us, it’s not as bad as suddenly finding yourself with no power during your boat rides.

Every appliance is rated in terms of power. The rated capacity of an appliance expresses how much power the device requires to function correctly.

Alternatively, appliances are rated according to the voltage and current they require.

The relation between power, voltage, and current is given by the expression:

Power (W) = Voltage (V) x Current (A)

This way, if a certain appliance is rated for 110V and 3A, then the power it consumes is:

Power (W) = 110V x 3A = 330W

When determining your boat battery size, you should check the rated power for each appliance you wish to power in your boat. Your power demand will be the sum of the power for these appliances.

Autonomy: Runtime

Once you know the rated power of each appliance you want to power, you’ll need to figure out how long (approximately) you wish to power them.

For example, let’s say you want to build a battery bank that can power the following appliances:

• Mini fridge, around 65W, for 6 hours
• Portable electric burner, about 1200W, for 1 hour
• Small electric grill, 1500W for 1 hour
• Other smaller appliances like speaker and phone chargers 40W, for 3 hours

We express energy demand in Watt-hours and:

Energy demand (Wh) = Power (W) x desired runtime (h)

Therefore, for the examples above:

• Mini fridge — 65W, for 6 hours. Energy (Wh) = 65W x 6h = 390Wh
• Portable electric burner —1200W, for 1 hour. Energy (Wh) = 1200Wh
• Small electric grill —1500W, for 1 hour. Energy (Wh) = 1500Wh
• Other smaller appliances like speakers, phone chargers, etc. — 40W, for 3 hours. Energy (Wh) = 120Wh

Adding all of them, we get 390Wh + 1200Wh + 1500Wh + 120Wh = 3210Wh of energy demand.

Now you must consider the DoD of your battery.

Depth of Discharge (DoD)

Depth of discharge (or DoD) expresses the percentage of the battery capacity that has been discharged.

The recommended DoD varies according to the battery chemistry.

Lead-acid batteries usually have a recommended DoD of 50%. Frequently discharging a lead-acid battery (like an AGM or Gel) more than 50% of its capacity will negatively impact the battery’s performance and life cycle.

Conversely, a lithium-ion battery (like a LiFePO4) has a recommended DoD of 80% (however, averages do range between 80 – 95%) DoD. Why? Because lithium batteries don’t suffer from deep discharging the same way as lead-acid batteries do. The main components (electrodes and electrolytes) of lead-acid batteries can degrade after a certain point during discharge.

Lithium batteries are also much more stable, so they don’t degrade with use as much as lead-acid batteries. That’s why they last up to 5 times longer than lead-acid batteries.

DOD Influence

So how does the depth of discharge influence your boat battery size?

Let’s say you’ve built a 12V 200Ah battery bank. This system provides a total of 2400 watt-hours of energy:

Energy (Wh) = Voltage (V) x Capacity (Ah) ∴ Energy (Wh) = 12V x 200Ah = 2400Wh

However, you can’t use these 2400Wh, because, for this, you’d be discharging your battery 100%, which is highly not recommended.

Therefore, let’s calculate the usable energy (how much energy you can use from your battery bank) for both types of batteries:

This shows that you get more usable energy from a lithium-ion battery bank than a lead-acid battery bank for battery banks with the same voltage and capacity specs.

So when sizing your boat battery bank, you should consider the recommended DoD for your battery according to its chemistry.

Determining Your Boat Battery Size

In the example from the previous section, we’ve calculated an energy demand of 3210Wh.

If you used lead-acid batteries, which have a recommended DoD of 50%, you would need a battery bank that stores twice as much as your energy demand. Using the example, you need a battery bank that could store 2 x 3210Wh = 6420Wh.

But what battery configuration could store this amount of energy?

Because Power (W) = Voltage (V) x Current (A), it is easy to understand that
Energy (Wh) = Voltage (V) x Capacity (Ah).

From this, we get that Capacity (Ah) = Energy (Wh) / Voltage (V)

This way, there are different ways of achieving a battery system that can store 6420Wh of energy:

Using a 12V battery system: Capacity (Ah) = 6420Wh / 12V = 535Ah

This system can be achieved by wiring three 12V 200Ah batteries in parallel.

Using a 24V battery system: Capacity (Ah) = 6420Wh / 24V = 267.5Ah

This system is achieved by using a 24V 300Ah battery, which is hard to find. Alternatively, you can achieve it by wiring two 12V 300Ah batteries in series, thus converting 12V into 24V battery systems.

There you have it — this is how you size your boat battery bank.

Of course, this is only an estimation; it’s not entirely accurate because a battery’s rated capacity is only maintained in ideal conditions. However, it’s an efficient way to help you determine your boat battery size.

Boat Battery Size Chart

Here we present a boat battery size chart, where you can easily find an estimation of the runtime for a particular appliance/device, given your boat battery bank capacity.

Additionally, we’ve included a trolling motor runtime chart, so you’ll have an estimation of how long you can run your trolling motor according to your battery capacity.

Appliances And Devices Runtime

This table considers the usable energy for each battery (sealed lead-acid (SLA) 50% DoD and LiFePO4 80% DoD) and average amperage ratings of typical boat devices/appliances (to find the power rating multiply the amperage by 12V).

Trolling Motor Runtime

Simply divide the usable battery capacity by the average amperage draw to find the runtime:

Runtime (h) = Usable Capacity (Ah) / Average Amperage draw (A)

To build this table, we’ve considered an average amperage draw of 30A. However, it’s important to note that throttle level impacts the amperage draw, thus affecting the runtime.

This table is only an estimation. Check the user manual to find the amperage drawn by your trolling motor at different speeds.

Best Battery Size For A Boat With A 24-Volt Trolling Motor?

A trolling motor is an electric outboard motor that moves the boat to take you to a different location or keep your boat in the same spot.

Small trolling motors operate using external 12V deep cycle batteries. However, larger trolling motors require 24V or 36V to function. In these cases, two or three 12V batteries are connected in series to boost the voltage from 12V to 24V or 36V.

This is a diagram of how to wire two 12V batteries to achieve a 24V battery system to power a 24V trolling motor:

The right boat battery size to power a 24V trolling motor depends on how many amps the trolling motor draws at a certain average speed and how long you want to run your trolling motor at this average speed using your battery bank.

Calculating How Many Amp-hours You Need For Your 24V Trolling Motor

Here’s an example:

Given that a battery’s capacity is expressed in Ah, and the voltage of your battery bank must be equal to the voltage of your trolling motor, it’s easier to calculate the size of your battery bank using the amperage that your trolling motor draws.

We’ll provide an example of calculating boat battery size using Ah and Wh, and you’ll see that we’ll achieve the same results.

Let’s say you have a 24V trolling system that draws 57 amps at maximum speed. If you use it, on average, at 80% throttle level, you’ll be drawing around 40 amps. (Here’s the chart used in this example.

Of course, there will be times when you’ll draw more than that, but there will also be times when you draw very few amps, so let’s consider this 40 amps average.

Using Amp-hours (Ah)

If you use lead-acid batteries, like AGM or Gel, you must remember the 50% recommended DoD. This way, if you want to run your 24V trolling motor for 2 hours, you’ll have a 24V battery bank with a capacity of:

Capacity (Ah) = amperage (A) x runtime (h) = 40A x 2h = 80Ah

Because you should only discharge 50% of your battery bank’s capacity (for lead-acid batteries), you’ll need a battery bank with at least 160Ah of capacity. This way, you’ll be able to safely use 80Ah of your battery bank capacity to power your 24V trolling motor for 2h.

If you use Lithium-ion batteries, like LiFePO4 batteries, you’d need to consider an 80% (0.8) DoD. This way, you would need a 100Ah battery bank: 80Ah/0.8 = 100Ah.

Using Watt-Hours (Wh)

Let’s consider the same average amperage draw of 40A. If your trolling motor requires 24V, then the power it consumes on average is:

Power (W) = Voltage (V) x Amperage (A) = 24V x 40A = 960W

In 2h, you’ll consume:

Energy (Wh) = Power (W) x runtime (h) = 960W x 2h = 1920Wh

With a 24V battery system, the required capacity to run this trolling motor for 2h would be:

Capacity (Ah) = Energy (Wh) / Voltage (V) = 1920Wh / 24V = 80Ah

(Next, you’d need to consider your battery’s recommended DoD to determine the size of your battery bank).

80 Amp-hours is the same result we’ve reached previously, in only one step. This shows that using Ah to calculate your boat battery size is better in this case.

Now you can use the average amperage your trolling motor draws, how long you wish to power it for, as well as the recommended DoD for your battery chemistry, and you’ll be able to calculate the boat battery size you require.

Best Battery Size For A Boat With A 36-volt Trolling Motor?

Similarly to the 24V trolling motor, to power a 36V trolling motor, you need to wire three 12V batteries in series, as shown below:

The best battery size to power your 36V trolling motor once again depends on:

• Your 36V trolling motor’s average amperage draw
• The amount of time you wish to run your trolling motor
• The recommended DoD for your battery chemistry

Calculating How Many Amp-hours You Need For Your 36V Trolling Motor

Let’s say your 36V trolling motor consumes 54A when running at maximum speed, and let’s consider the average amperage draw as being 29A (amperage drawn at 80% throttle level).

Suppose you want to run your trolling motor for 2 hours; you’d need a battery bank with:

Capacity (Ah) = Amperage (A) x Runtime (h) = 29A x 2h = 58Ah

Now, consider the recommended DoD for your battery:

Lead-acid: 50% DoD. Therefore, you would need a battery bank with a little over 116Ah (58Ah/0.5 = 116Ah) of capacity to efficiently run this trolling motor for 2h.

Lithium-ion: 80% DoD. You would need a battery bank with a little over 72.5Ah (58Ah/0.8 = 72.5Ah) capacity to run this trolling motor for 2h.

Difference Between A 24-Volt And A 36-Volt Battery Bank

Note that a 36V trolling motor that draws 50A consumes more power than a 24V trolling motor that draws 50A, despite drawing the same amperage. Here’s why:

Power (W) = Voltage (V) x Amperage (A)

So, for a 24V trolling motor, the power consumed when drawing 50A is:

Power (W) = 24V x 50A = 1200W

Meanwhile, the same amperage on a 36V trolling motor will draw:

Power (W) = 36V x 50A = 1800W

So even if the amperage drawn by your 36V trolling is the same or lower as the amperage drawn by a 24V trolling motor, the power consumed is greater due to the greater voltage.

The same logic is applied to battery banks: a 200Ah 36V battery system can provide more power/energy than a 200Ah 24V battery system, despite having the same rated capacity of 200Ah.

That happens because power is the product of voltage and amperage (and in this case, energy is the product of voltage and battery capacity). Therefore, the greater the voltage, the greater the power/energy provided by a battery bank.

So, the main reason to choose a higher voltage battery is that it allows you to power a higher load in a safer a manner, reducing the risk of anything over heating and potentially causing a fire.

Calculating Trolling Motor Runtime

What if you want to know how long you can run a trolling motor with a particular battery?

Suppose you have a 24V 250Ah AGM battery system to run your 24V trolling motor, which draws an average of 25A. In this case, your battery’s usable capacity is only 125Ah, due to the 50% DoD recommended for AGM batteries.

Now you just need to divide the usable capacity of your battery system by the average amperage your trolling motor draws:

Runtime (h) = Usable Capacity (Ah) / Average Amperage draw (A) = 125Ah / 25A = 5h

Therefore, with this battery system, you’ll be able to run your 24V trolling motor for 5 hours.

Final Thoughts

Determining your boat battery size is crucial to avoid running out of power during your fishing trips or boat rides.

To size your boat battery bank, you first need to estimate your energy demand according to the rated power of the appliances and devices you wish to run on your boat.

Once you’ve done that, you need to consider the recommended DoD of your battery according to its chemistry.

Following the sizing guide presented in this article, you should be able to size your boat battery bank without any problems. It might seem a bit complicated at first, but it’s actually a pretty straightforward process, and it’ll pay off.

Trust us, knowing you’ve packed your boat with enough battery capacity to run all your devices will make your boat rides much more enjoyable!