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How To Use Boats Alternator to Charge Lithium Trolling Motor Batteries

Mike Smith
Mike Smith
—
18/09/2024

Charging your lithium trolling motor batteries on the water can be a game-changer for boaters who need continuous power for extended trips. One efficient way to do this is by using your boat's alternator. In this guide, we’ll cover how you can safely and effectively charge your lithium trolling motor batteries using the boat's alternator system.


Understanding the Boat’s Alternator and Lithium Batteries

When it comes to powering your boat, it’s essential to understand the role of the alternator and how it interacts with modern lithium batteries. While lead-acid batteries have long been the standard for powering boat electronics and starting engines, lithium batteries are increasingly being used for their superior performance, lifespan, and efficiency.

What is a Boat’s Alternator?

An alternator is a critical component in a boat’s electrical system. It is essentially a small generator driven by the engine that charges the boat's battery system while the engine runs. It converts mechanical energy from the engine into electrical energy to recharge the batteries. Typically, alternators are designed to charge the boat's lead-acid starting battery, which powers the engine and electronics.

Key Features of a Boat’s Alternator:

  • Voltage Output: Alternators typically produce around 13.8 to 14.2 volts, which is suitable for charging 12V lead-acid batteries.
  • Power Source: The alternator only produces power when the boat's engine is running.
  • Multiple Functions: It charges the starting battery and can be used to charge additional batteries, like house batteries or trolling motor batteries.

How Lithium Batteries Differ from Lead-Acid Batteries

Lithium batteries have unique charging requirements compared to lead-acid batteries. While lead-acid batteries can tolerate inconsistent voltage and overcharging to some degree, lithium batteries are much more sensitive to both. Therefore, simply connecting lithium batteries directly to an alternator could result in inefficient charging, or worse, potential damage to both the alternator and the battery.

Key Differences:

  • Charging Profile: Lithium batteries require a stable and specific voltage to charge efficiently. They need around 14.2V to 14.6V for optimal charging.
  • Battery Management System (BMS): Most lithium batteries come equipped with a Battery Management System (BMS) that regulates charging and discharging to safeguard the battery from overvoltage, undervoltage, and overheating. Fortunately, LiTime lithium trolling motor batteries are designed with a built-in BMS for enhanced protection. In addition to the standard safeguards—such as overvoltage, undervoltage, short circuit, and overheating protection—LiTime lithium batteries offer extra protection against dust, water, and salt spray, making them more resilient in harsh marine environments.

LiTime lithium trolling motor battery BMS

Can You Use an Alternator to Charge Lithium Trolling Motor Batteries?

Yes, you can connect the device to your 12V starter battery on one side and to your 24V or 36V trolling motor batteries on the other. When the boat’s engine is running, power is smoothly transferred, providing a steady current and an extra boost to your trolling batteries. This smart system, with constant voltage control, helps keep your trolling motor powered longer, allowing you to stay in your favorite fishing spot.

Read on How to Charge Trolling Motor Batteries for more charging ways.

With a simple installation process, you can:

  • Boost the capacity of your 24V or 36V trolling system with additional charging.
  • Extend your trolling time.
  • Maintain steady performance in windy or strong current conditions.
  • Prolong your trip without needing to disconnect or recharge your trolling batteries separately.

However, charging lithium batteries with an alternator presents unique challenges, including compatibility with the alternator's output and ensuring the proper equipment is in place to regulate the voltage and current.

The next part will dive into these challenges.

Common Concerns about Charging Lithium Batteries with a Boat’s Alternator

When considering using a boat’s alternator to charge lithium trolling motor batteries, a few common concerns often arise:

  • Voltage Compatibility: 12V Lithium batteries require a stable charging voltage, typically around 14.4V to 14.6V, which is higher than what most lead-acid batteries need. Standard alternators are often set up to charge at around 13.8V to 14.2V, which can be too low for lithium batteries. If the alternator's voltage output isn’t high enough, the lithium battery may not charge fully, reducing its performance.
  • Alternator Load and Strain: Lithium batteries have a much lower internal resistance, allowing them to draw higher current more quickly than lead-acid batteries. This can strain the alternator, causing it to overheat or wear out prematurely if it’s not designed to handle this higher load.
  • Battery Management: Without the proper equipment in place, there's a risk of draining the boat's starting battery or putting additional strain on the alternator. This can affect both the performance of the starting battery and the lithium trolling motor battery.

Equipment Needed

To safely and efficiently charge lithium trolling motor batteries using a boat’s alternator, you’ll need specific equipment to regulate the process. Here’s a breakdown of the essential components:

1. DC to DC Charger

A DC-DC charger is the most critical piece of equipment in this setup. This device is specifically designed to regulate the alternator's output to match the charging requirements of your lithium battery. It takes the variable voltage from the alternator and converts it to the precise voltage and current that lithium batteries require. The benefits of a DC-DC charger include:

  • Voltage Regulation: Ensures the lithium battery gets the correct voltage (14.4V to 14.6V) for optimal charging.
  • Current Control: Prevents the lithium battery from drawing too much current too quickly, protecting both the battery and the alternator.
  • Battery Protection: The charger helps to prevent overcharging and protects the lithium battery from voltage spikes.

2. Battery Isolator

A battery isolator ensures that your boat’s starting battery and lithium trolling motor battery are kept separate during the charging process. This prevents your lithium battery from draining power from the starting battery when the engine is off and ensures that your starting battery is always charged and ready.

Key advantages of using a battery isolator include:

  • Protection: Keeps the starting battery from discharging, so your boat is always ready to start.
  • Dual Charging: Allows the alternator to charge both the starting and trolling motor batteries simultaneously without interference.

3. Wiring Kit

Having the right wiring kit is crucial for setting up your alternator-to-lithium battery charging system. This kit should include:

  • Proper Gauge Wires: Lithium batteries can draw significant current, so you’ll need appropriately sized wires. Using undersized wires can lead to overheating and inefficiency.
  • Fuses: To protect the system from electrical faults, it’s important to install fuses between the alternator, DC-DC charger, and the lithium battery.
  • Connectors: Quality connectors ensure a secure and efficient electrical connection between all components in the system.

Visit LiTime battery accessories to meet your needs.

4. Additional Equipment

Depending on your boat’s setup and specific needs, you may need additional equipment for optimal performance and safety:

  • Alternator Temperature Sensor: Since charging lithium batteries can strain your alternator, installing a temperature sensor can monitor the alternator’s heat output. If it gets too hot, the system can cut off the charging process to prevent damage.
  • Charge Controller: If you’re using multiple charging sources (such as solar panels in addition to your alternator), a charge controller can help manage these inputs to ensure the lithium battery is charged efficiently without overloading any one source. Or you can just utilize the DC-DC charger with MPPT function.

dc-dc with mppt function

LiTime 12V 40A DC to DC Battery Charger with MPPT

Steps to Charge a Lithium Trolling Motor Battery with a Boat’s Alternator

Here’s a simple step-by-step process to set up your boat’s alternator to charge your lithium trolling motor battery:

1. Install a DC-DC Charger

The most important step is installing a DC-DC charger between your alternator and the lithium battery. This charger ensures that the voltage delivered to the lithium battery is within a safe range, typically around 14.4V for most lithium-ion batteries. Most alternators are set up to charge lead-acid batteries at higher voltages, which can damage lithium batteries if not regulated.

how to install dc-dc charger

2. Connect a Battery Isolator or Combiner

To prevent the starter battery from drawing power from your trolling motor batteries, install a battery isolator or combiner. This will keep the two systems separate, ensuring your engine starts reliably while allowing your alternator to charge both batteries when the engine runs.

3. Run the Boat Engine

Once the system is set up, running your boat’s engine will charge both the starting battery and the lithium trolling motor battery. The alternator generates electricity, and the DC-DC charger ensures the lithium battery receives the correct voltage and current for safe charging.

4. Regularly Inspect the System

Check your charging system regularly to ensure it’s working properly. This includes inspecting the DC-DC charger, isolator, and the battery management system to ensure optimal charging and battery health.

Lithium Batteries & DC to DC Chargers for Trolling Motors

Pairing lithium batteries with DC to DC chargers creates an optimal setup for charging boating. LiFePO4 lithium batteries, renowned for their exceptional performance and longevity, are the perfect power source for your trolling motors. They offer extended usage times before requiring a recharge and have a longer lifespan overall, reducing the need for frequent replacements. The benefits of lithium batteries extend far beyond these basics.

benefits of lithium trolling motor battery

LiTime 24V 50Ah Bluetooth Lithium Trolling Motor Battery

The real magic happens when lithium batteries are combined with DC to DC chargers. These chargers transform the voltage output from your boat’s alternator into the ideal level for charging your trolling batteries. This ensures that your batteries remain at peak power while you’re out on the water, allowing for longer and more enjoyable boating adventures.

While your trolling motor batteries remain the primary power source for your trolling motor, the charging system doesn’t replace the need for regular charging. Instead, it enhances their capacity and usability by maintaining their charge during operation. This ensures your trolling motors and batteries are always ready for extended use, offering a more reliable and convenient fishing experience.

In summary, the combination of lithium batteries and DC to DC chargers provides a range of benefits that make them the preferred choice for serious anglers. Their superior efficiency, longevity, and seamless compatibility make them the best option for powering your trolling motor. 

Common Mistakes to Avoid

When setting up your boat’s alternator to charge lithium trolling motor batteries, it's crucial to avoid common mistakes that can damage your system or reduce the efficiency of your battery setup. Below are some key errors to watch out for:

1. Charging Lithium Batteries Directly from the Alternator Without a DC to DC Charger

One of the most critical mistakes is connecting a lithium battery directly to an alternator without using a DC to DC charger. Lithium batteries require specific voltage and current profiles for safe charging. Alternators, designed for lead-acid batteries, may provide inconsistent or inappropriate voltage, which can damage the lithium battery or lead to overcharging. A DC to DC charger regulates the alternator’s output, ensuring safe and efficient charging for your lithium battery.

2. Using Improper Wiring or Not Installing a Battery Isolator

Another common issue is using the wrong wiring size or neglecting to install a battery isolator. Lithium batteries can draw a significant amount of current, requiring the use of proper gauge wires to handle the load. Using undersized wires can lead to overheating, inefficiency, and potential electrical failures.

Additionally, skipping the installation of a battery isolator can lead to the boat’s starting battery being drained when the engine is off. This could leave you stranded without power to restart your boat. A battery isolator ensures that the starting and trolling motor batteries are charged separately and won’t interfere with each other.

3. Failing to Monitor Battery Health and Alternator Output

It’s essential to regularly monitor both your lithium battery’s health and the alternator’s output. Failing to do so can lead to system inefficiencies or even damage over time. Many lithium batteries come with a Battery Management System (BMS) that helps monitor the battery's state of charge, but you still need to check that your alternator is not being overworked or overheating. Over time, neglecting this can result in poor charging performance or even alternator failure.

FAQs about Charging Lithium Trolling Motor Batteries with Alternator

1. Can I Charge My Lithium Trolling Motor Battery with Any Alternator?

While most boat alternators can be used to charge lithium trolling motor batteries, not all are ideal without additional equipment. Many alternators are designed for lead-acid batteries, which have different voltage requirements than lithium batteries. To safely charge lithium batteries, you’ll need a DC to DC charger to regulate the voltage and current. Without this, your alternator could undercharge or overcharge the lithium battery, reducing its lifespan.

2. What Size DC to DC Charger Do I Need for My Setup?

The size of the DC to DC charger depends on the capacity of your lithium battery and the output of your alternator. Most DC to DC chargers are rated by amperage (e.g., 20A, 40A). For example, a 40A DC to DC charger is suitable for larger lithium batteries or setups where you need faster charging. If you’re unsure about sizing, consult the battery manufacturer or a marine electrical professional to ensure compatibility with your alternator.

3. How Long Will It Take to Charge the Battery Using an Alternator?

The charging time depends on several factors:

  • Battery capacity: Larger batteries will take longer to charge.
  • Alternator output: A more powerful alternator will charge the battery faster.
  • DC to DC charger size: Higher amperage chargers can speed up charging. For example, if you have a 100Ah lithium battery and a 40A DC to DC charger, it will take approximately 2.5 hours to charge the battery from 50% to full. Keep in mind that this calculation assumes continuous operation of the alternator, which may vary based on engine runtime.

Conclusion

Using your boat’s alternator to charge lithium trolling motor batteries can be an efficient solution, but only if done correctly. The right equipment—including a DC to DC charger, battery isolator, and proper wiring—is essential to ensure safe and effective charging. Failing to install these components can lead to system damage, poor battery performance, or even dangerous electrical failures.

By following the steps outlined in this guide, you’ll not only extend the life of your lithium battery but also improve the efficiency and reliability of your boat’s electrical system. Always consult with a marine electrical professional if you’re unsure about any aspect of the setup to ensure everything is installed safely and efficiently.

Mike Smith
Mike Smith
Mike Smith is a marine energy expert with 15+ years of experience and a Master’s in Electrical Engineering from Stanford. Passionate about lithium battery integration, he also enjoys sailing and exploring coastal waters in his free time.

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