Battery Equivalents and Replacements

How Long Does It Take To Charge A Boat Battery?

Boat batteries are essential components for recreational and professional watercraft, powering essential systems such as navigation, communication, and lighting.

Charging these batteries efficiently and understanding the time it takes to do so is critical for maintaining a healthy battery life and ensuring optimal performance.

Published: May 2, 2023.

outboard engine 1

Types Of Boat Batteries

Boat batteries differ in terms of size, voltage, capacity, battery use, and battery chemistry.

Boat Battery Type/Use

  • Starting Boat Batteries: These batteries are designed to provide a high burst of power to start the boat's engine. They feature a large number of thin plates that allow for a quick discharge of energy. Starting boat batteries are not suitable for long-term, continuous discharge, as this can damage their internal components and lead to a shorter lifespan.
  • Deep Cycle Boat Batteries: Designed for extended use, deep cycle boat batteries are capable of providing a steady flow of power over a longer period. They have thicker plates that allow for a slower discharge rate (they don't warp easily), making them ideal for running various electrical systems on the boat. Deep cycle batteries are commonly used in conjunction with starting batteries to power the boat's electrical systems.

Boat Battery Chemistries

  • Lead-Acid Batteries: These traditional batteries are widely used due to their affordability and availability. Lead-acid batteries are available in two types: flooded (wet cell) and sealed (AGM and gel). Flooded batteries require regular maintenance, while sealed batteries are maintenance-free but generally have a higher initial cost.
  • Lithium Batteries: Lithium boat batteries offer several advantages over lead-acid batteries, including a longer lifespan, lighter weight, and faster charging times. They also provide a consistent voltage output throughout the discharge cycle. However, lithium batteries tend to be more expensive than lead-acid batteries and may require specialized lithium battery chargers or advanced lead-acid battery chargers with dedicated lithium battery charging modes.

Boat Batteries Specifications

Understanding the specifications of a boat battery is crucial to determine its charging time.

  • Voltage: The voltage of a battery is the potential difference between its terminals, with 12V being the most common for boat batteries. Other systems are rare, but there are also 24V, 36V, and 48V DC systems.
  • Capacity: The capacity of a battery, measured in ampere-hours (Ah), indicates the amount of energy it can store. Most boats with a single battery feature a battery in the 44-120 Ah range, or they feature one 60-70Ah starting battery and one larger deep-cycle battery or battery pack (100+ Ah).
  • Reserve Capacity: The Reserve Capacity (RC) represents the number of minutes a battery can deliver a specified current (25A) before its voltage drops below a particular level (10.5V for lead-acid batteries).

How To Charge A Boat Battery

Boat batteries may be recharged using several methods including, with the most common being the following methods:

  • Battery Chargers
  • Engine Alternator
  • Solar Panels/Wind Turbines, etc

Charging A Marine Battery Using A Battery Charger

Charging a marine battery is an essential aspect of maintaining its performance and prolonging its lifespan. Proper charging practices help ensure that your boat's electrical systems function optimally while preventing battery damage. Here's a step-by-step guide on how to charge a marine battery:

  • Choose the right charger: Select a battery charger that is compatible with your marine battery's chemistry (lead-acid or lithium), voltage (typically 12V), and capacity. An advanced multi-mode battery charger is often recommended, as it can adjust the charging current and voltage based on the battery's state of charge, reducing the risk of overcharging.
  • Disconnect the battery: Before charging, turn off all electrical devices on the boat and disconnect the battery from the boat's electrical system. This prevents any potential electrical issues and ensures that the charging process is focused solely on the battery. In many situations, this is not an option, but in that case, keep in mind that the battery charger will also supply power for the onboard electronics - keep the load on the charger minimum. Also, if the battery charger must desulfate the battery, disconnect sensitive electronics since voltage spikes can damage such devices.
  • Clean the battery terminals: Inspect the battery terminals for any signs of corrosion or dirt. Clean the terminals using a wire brush and a mixture of baking soda and water if needed. This helps ensure a secure and efficient connection between the charger and the battery.
  • Connect the charger: Attach the charger's positive (red) clamp to the battery's positive terminal and the negative (black) clamp to the negative terminal. Ensure that the connections are secure and that the clamps are not touching each other or any other conductive surfaces.
  • Set the charging rate: Based on the charger's specifications and the battery's capacity, select the appropriate charging current (amps). A lower charging rate (e.g., 6-10 amps) is typically safer and more suitable for smaller batteries, while larger batteries may benefit from a higher charging rate (e.g., 15-25 amps).
  • Begin charging: Plug the charger into a grounded electrical outlet and switch it on. Most smart chargers will automatically begin the charging process and progress through the various charging stages (bulk, absorption, and float) as needed.
  • Monitor the charging process: Keep an eye on the charger's indicators or display to monitor the charging progress. If the charger detects any issues, such as overheating or overcharging, it should automatically adjust the charging parameters or terminate the process to protect the battery.
  • Disconnect the charger: Once the battery is fully charged, unplug the charger from the electrical outlet and carefully disconnect the clamps from the battery terminals. Reconnect the battery to the boat's electrical system and ensure that all connections are secure.
  • Perform regular maintenance: Regularly check your marine battery for signs of wear or damage, and maintain proper electrolyte levels in flooded lead-acid batteries. Consistent battery care will help maximize its performance and longevity.

Notes: a good battery charger is also a battery maintainer - after fully charging the lead-acid battery, the battery charger will continue to maintain (hence the name) the boat battery charged without overcharging it.

marine battery charging

How Long To Run Boat Engine To Charge Battery

The time required to charge a boat battery by running the engine depends on several factors, including the battery's capacity, its current state of charge, the alternator's output, and the boat's electrical load.

However, it's essential to remember that using the engine to charge the battery is generally less efficient than using a dedicated battery charger.

Alternators in most boats are designed to maintain the charge of a battery rather than fully charging it from a depleted state regularly. When the engine is running, the alternator generates electricity to power the boat's electrical systems and recharge the battery simultaneously.

Here's a general guideline to estimate the time required to charge a boat battery while running the engine:

  • Determine the battery's capacity (Ah) and its current state of charge.
  • Calculate the required charge by subtracting the current state of charge from the total capacity.
  • Determine the alternator's output (amps) at cruising RPM. Keep in mind that the actual output may be lower than the alternator's rated capacity due to factors such as engine speed, temperature, and the boat's electrical load.
  • Divide the required charge (Ah) by the alternator's output (Amps) to estimate the charging time in hours.

For example, if you have a 100Ah battery that is currently at 50% charge, you need to recharge it with 50Ah. If your alternator has an output of 20 amps at cruising RPM, it will take approximately 3-3.5 hours to charge the battery and not 2.5 hours, despite 50Ah ÷ 20A = 2.5h.

In reality, the charging process tends to slow down as the battery reaches full charge, and the alternator's output may vary depending on the engine speed and electrical load, thus in this example, one should expect the battery charger to recharge the battery in 3-3.5 hours fully and not in 2.5 hours.

To ensure optimal battery charging and maintenance, it's recommended to use a dedicated marine battery charger when possible, especially for deep-cycle batteries. This provides a more controlled and efficient charging process, prolonging the battery's lifespan and performance.

marine solar panels

Note: to charge the boat battery, other systems can be used, for example, solar panels, wind turbines, and similar.

Adding A Second Battery To The Boat

Some boats feature electric systems with two batteries, including:

  • Starting battery.
  • Deep Cycle battery.

These batteries cannot be connected directly since both batteries will be used for both starting and deep-cycle applications.

For such systems, so-called DC-to-DC battery chargers are used, for example, Renogy 12V 40A DC to DC On-Board Battery Charger (Amazon link, opens in the new window).

renogy 40a dc dc onboard battery charger

Such battery chargers allow the users to keep the starting and deep-cycle batteries separated and to use a boat engine with an alternator to charge both batteries.

Also, when the engine is not running, such a battery charger isolates and doesn't charge deep cycle battery, protecting the starting battery from being discharged.

How Long Does It Take To Charge A Boat Battery

Charging time depends on many details, with the battery capacity and charging current being the most important ones:

  • 6 Amps: Charging a boat battery at a 6-amp rate will generally take longer than using higher amperage chargers. For a 100Ah battery, it may take approximately 16-20 hours to fully charge, depending on its initial state of discharge and the battery chemistry.
  • 10 Amps: With a 10-amp charger, the same 100Ah battery may take around 10-12 hours to fully charge, again depending on the initial state of discharge and battery type.
  • 15 Amps: A 15-amp charger will reduce the charging time further, potentially taking about 6-8 hours to charge a 100Ah battery fully.
  • 25 Amps: At a 25-amp charging rate, you can expect a 100Ah battery to reach full charge in approximately 4-5 hours. However, it's essential to verify that the battery can handle this high charging rate to prevent potential damage - most lithium batteries accept up to 1.0C charging currents, with 0.3-0.5C being recommended.

Using too strong charging current is not recommended since it can cause the battery to overheat, which can shorten the battery operating lifetime significantly.

Few Final Words

inboard engine

The time it takes to charge a boat battery depends on several factors, including the battery's capacity, initial discharge state, chemistry, battery age, battery temperature, charging current, etc.

By understanding these variables and using an appropriate charger, you can optimize the charging process for your boat battery.

Keep in mind that faster charging times may not always be suitable for all battery types, and it is essential to follow the manufacturer's guidelines to ensure the longevity and performance of your battery.

Proper battery care and charging practices will help you maintain a reliable power source for your boat, allowing you to enjoy your time on the water without unnecessary interruptions.