Battery capacity (Ah) = (Motor power in lb thrust * 26.4 * Desired runtime in hours * Efficiency factor) / Battery voltage.
This formula is specifically designed for trolling motors, where power is often given in terms of lbs of thrust. The number 26.4 is used to convert lbs of thrust to watts (a measure of power).
The desired runtime is based on your preference and the intended use, and the efficiency factor is an estimated value; however, we typically use 1.2 (120%) to account for energy loss.
This gives you a safety margin of 20%- meaning that you assume the actual battery capacity needed is 20% more than what you calculated without the safety margin.
Before I go through an example, here’s a snapshot of what you’ll learn in this article:
At a Glance:
1️⃣ Voltage and Battery Types: Electric trolling motors require 12, 24, or 36-volt systems based on thrust level. Deep-cycle marine batteries like Flooded Lead Acid, Gel, AGM, or Lithium batteries are typically used.
2️⃣ Battery Size and Capacity: Consider amp-hour rating, battery quantity, and group size for optimal runtime and performance.
3️⃣ Performance Optimization: Properly charge and maintain your batteries, prevent sulfation, and take battery longevity into account to improve your motor operation.
Understanding what size battery you need also ties in directly with knowing the different types of batteries suitable for electric trolling motors. There are other factors to account for, so let’s get into it!
💡 The Formula In Action
A 50 lb thrust motor would have a power of approximately 1320 watts (50 * 26.4). Let’s assume you want a runtime of 6 hours, and since most trolling motors use a 12V battery, we will use 12V for this example.
So, therefore: (1320 * 6) / 12 = 660 Ah. We would then multiply the answer by 1.2 to add the safety margin. This would make the adjusted battery capacity 660 * 1.2 = 792 Ah to account for lowered efficiency.
🎓 Understanding Electric Trolling Motor Basics
Electric trolling motors are widely used in recreational and fishing boats for slow speed control, precise maneuverability, and, in some cases, as a silent alternative or supplement to traditional gas/propane outboard engines.
The voltage requirements of an electric trolling motor depend on its thrust level. These motors are typically powered by 12, 24, or 36-volt battery systems, and the manufacturer usually indicates the specific voltage required. For instance, a 24-volt trolling motor would require a different battery size compared to other types.
A trolling motor with 55 lbs. of thrust or less typically requires a single 12-volt battery. Motors with 55-80 lbs of thrust will need two 12-volt batteries for a total of 24 volts. Finally, if you have a motor with more than 80 lbs of thrust, you will need three 12-volt batteries for a total of 36 volts.
It’s critical to use batteries suited for marine applications, like deep-cycle batteries, which have a high amp-hour rating to prolong the operating time of your motor.
A suggested starting point is a 12-volt deep cycle marine battery with at least a 110 amp-hour rating, usually a group 27-size battery. If you’d like to learn more, take a look at our article on 12-volt trolling motors.
Keep in mind that increasing the voltage of your trolling motor setup will not only provide more thrust but also improve overall efficiency. A higher voltage motor will often run longer on a single battery charge than a lower voltage motor with the same thrust rating.
In short, the proper battery size and voltage for your electric trolling motor are dictated by the motor’s thrust level and requirements set by the manufacturer.
Be sure to use a battery that is suitable for marine use, like deep-cycle batteries with a high amp-hour rating, to enhance your motor’s performance and overall run time.
🔋 Determining Battery Size and Capacity
The next step in the selection process adds layers to what I’ve mentioned above amp-hour rating, battery quantity, and group size.
A battery’s amp-hour rating determines the current it can deliver over a specific period. A higher amp-hour rating generally results in longer runtime for your trolling motor.
For example, a 100 amp-hour rated battery can provide 100 amp hours of current. When operating the motor at a low speed that consumes four amps, the battery could last approximately 25 hours (100 amp-hour rating / four amps = 25 hours). You need a battery with an adequate amp-hour rating for your desired usage.
Electric trolling motors require different voltages based on the motor’s power. A 12-volt battery is generally sufficient for motors with 55 lbs of thrust or less, while motors with more than 55 lbs of thrust up to 80 lbs. will need two 12-volt batteries, providing 24 volts.
Some trolling motor power systems require 36 volts and use three 12-volt batteries. Assessing your motor’s power requirements will help you determine the appropriate battery quantity.
Group size refers to the physical dimensions and capacity of the battery. A standard Group 27 size battery is recommended for electric trolling motors (ideally a 12-volt deep cycle marine battery) rated at 110 amp-hours or more.
However, if the battery does not have an amp-hour rating, I’d suggest looking for one with at least 180 minutes of reserve capacity.
✅ Battery Types and Features
The most common battery types for trolling motors are Flooded Lead Acid, Gel Batteries, AGM Batteries, and Lithium Batteries.
Flooded Lead Acid
Flooded Lead Acid batteries, also known as wet-cell batteries, are a popular choice for electric trolling motors because they’re affordable and widely available.
However, they require regular maintenance, such as checking and refilling water levels, and they can potentially spill if not properly handled or stored. They are also heavy!
Gel batteries are a solution to spillage of lead-acid batteries by using a gel-like electrolyte, making them a safer and more convenient option than flooded lead-acid batteries.
They’re also more resistant to vibration, temperature fluctuations, and overcharging. However, gel batteries tend to be more expensive and may have a slower charging rate compared to other battery types.
AGM (Absorbent Glass Mat) Batteries are a type of sealed lead-acid battery with a special glass mat separator that absorbs the electrolyte. This design makes them maintenance-free, leak-proof, and more resistant to vibration and temperature fluctuations.
AGM batteries typically have a faster charging rate and longer cycle life than flooded lead acid and gel batteries. Keep in mind, though, that AGM batteries can be more expensive.
Lithium batteries are relatively new to the world of electric trolling motors, but they offer several advantages over other battery types.
They’re lightweight, have a high energy density, and can handle more charging cycles, resulting in a longer overall lifespan.
Lithium trolling motor batteries also have a much faster charging rate and can maintain a consistent power output during the entire discharge cycle. But, they are the most expensive upfront option by some margin.
📝 Other Factors Influencing Battery Selection
Boat Size vs. Motor Size
When selecting a battery for an electric trolling motor, consider both the boat size and motor size. Larger boats typically require more power and thus, larger motors.
As this Minn Kota article suggests, if the motor is 55 lbs. of thrust or less, a single 12-volt battery will suffice. However, if the motor has over 55 lbs of thrust up to 80 lbs, you’ll need (2) 12-volt batteries, providing a 24-volt output.
Speed and Running Time
Another crucial factor is the speed and running time of your trolling motor. The size of the battery must be adequate to support the desired speed and duration of use.
A battery with a higher amp-hour rating will give you more run time. Minn Kota recommends a 12-volt deep cycle marine battery with at least 110 amp-hours, typically a group 27 size battery, for optimal performance.
To better understand the correlation between battery size and running time, consider this example:
If a 100 amp-hour rated battery is connected to a motor pulling 4 amps at low speed, the battery would last around 25 hours (100 amp-hour rating / 4 amps = 25 hours).
But, at top speed with a 40-amp draw, the battery life would be reduced significantly, lasting only 2.5 hours (100 amp hour rating / 40 amps = 2.5 hours).
Finally, your budget plays a vital role in determining the size and type of battery for your trolling motor.
Higher-quality batteries with greater amp-hour ratings tend to cost more but offer better performance and longevity.
Again, assess your needs, and balance them with your budget to find the most suitable battery option.
🛶 Special Considerations for Kayaks and Canoes
Smaller Battery Options
When looking for a battery for your electric trolling motor on a kayak or canoe, it is essential to select a smaller size that is suitable for your lightweight watercraft. A battery with a lower capacity, such as a group 27 rating, can provide adequate power for a trolling motor while maintaining a compact, manageable size.
Another important factor to consider for kayaks and canoes is the weight of the battery. A lighter battery is more suitable for these smaller, more maneuverable boats.
For example, lithium-ion batteries are an excellent option for kayaks and canoes as they are both lightweight and efficient, providing a more balanced ride with less drag. Additionally, their amp hours often last longer compared to lead-acid batteries, leading to extended battery life.
Trolling Motor Thrust
Striking a balance between motor thrust and battery size is key for small vessels. 30 to 40 pounds of thrust for a lightweight boat like a kayak is a ballpark figure, as this amount of power can provide sustained, manageable speeds without overly depleting the battery.
📈Optimizing Trolling Motor and Battery Performance
Getting the most out of your trolling motor and its battery involves several factors, such as proper charging, prevention of sulfation, and factors affecting battery longevity. In this section, we’ll discuss each of these aspects in detail.
Proper Battery Charging and Recharging
Properly charging and recharging your trolling motor battery is crucial to ensure optimal performance and prolong the battery’s lifespan.
Use a marine-grade charger specifically designed for the type of battery you have. This will ensure optimal charging and prevent overcharging, which can damage the battery.
Knowing when to charge your trolling motor battery also plays a role in optimizing its efficiency and longevity.
Try and ensure it is recharged after each use or at least once a month if not used regularly. This helps maintain the battery’s capacity and prevents sulfation.
Preventing Sulfation in Batteries
Sulfation is a common issue in lead-acid batteries, leading to decreased battery capacity and shorter runtime. It occurs when a battery is left partially or fully discharged for an extended period.
Sulfation forms crystals on the battery plates, reducing the battery’s ability to accept and hold a charge.
To prevent it, keep your battery charged as much as possible. As mentioned earlier, regular charging helps maintain optimum battery performance and prevents sulfation buildup.
If you are particularly concerned about sulfation and would like to avoid the risk altogether, lithium batteries may be a better option as they are less prone to sulfation compared to lead-acid batteries.
Considering Battery Longevity
Ultimately, the longevity of your trolling motor battery depends on several factors, including battery type, usage patterns, and maintenance.
Deciding between lead-acid or lithium batteries will impact your battery’s lifespan. Although lithium batteries have a higher upfront cost, they offer longer runtime, faster recharge rates, and increased longevity compared to traditional lead-acid batteries.
To ensure a longer battery life, follow proper charging and maintenance procedures. This includes using the right type of charger and keeping the battery charged.
Taking care of your battery maximizes its performance and lifespan, allowing you to enjoy more time on the water with your electric trolling motor.
🔑 Key Takeaways
Now, let’s summarize the key takeaways for your trolling motor battery selection and maintenance.
1️⃣ The formula for calculating battery capacity for a trolling motor is Battery capacity (Ah) = (Motor power in lb thrust * 26.4 * Desired runtime in hours * Efficiency factor) / Battery voltage, with a safety margin of 20% added to account for energy loss.
2️⃣ Voltage and Battery Types: Electric trolling motors require 12, 24, or 36-volt systems based on thrust level. Deep-cycle marine batteries like Flooded Lead Acid, Gel, AGM, or Lithium batteries are typically used.
3️⃣ Battery Size and Capacity: The amp-hour rating, battery quantity, and group size are key factors for optimal runtime and performance of an electric trolling motor.
Determining the correct battery size is crucial since trolling motors rely on a battery for operation. While this equation can be very helpful, there is much more to it. Let’s take a brief look at what we are going to cover in this article: