Selecting a Battery Charger for Your Boat

By Tom Burden, Last updated: 1/29/2024

At the heart of our boat's electrical systems are one or more batteries that store the electrical energy needed for the critical task of engine starting, powering cabin lights, electronics, and other loads. Given the central role that batteries play on a boat, selecting the right type of battery charger, one that has the "smarts" to keep your batteries charged and in tip-top shape, is crucial.

Do you need a portable, waterproof or vented charger?
Portable Chargers
Waterproof Onboard Chargers
Vented Onboard Chargers
Output Voltage
Charger Capacity: How many amps do you need?
Advanced Recommendation
Input Voltage: Do you need a dual-input charger?
Number of Outputs
Benefits of Smart Chargers
What Batteries Want: The “Ideal Charge Curve”
Charging Lithium Batteries
Multi-Stage Smart Charging While Underway
Tips for Battery Longevity

Do you need a portable, waterproof or vented charger?

Noco Genius 10-Amp Portable Automatic Battery Charger

The process of selecting a battery charger begins with choosing a model that is appropriate for your boat. West Marine offers three types of battery chargers: portable chargers, and two types of onboard mounted battery chargers, which include onboard waterproof chargers and onboard vented chargers.

Portable Chargers

Application: Suitable for use on smaller boats, including dinghies, kayaks, small open fishing boats, personal watercraft, or other boat types where installing an onboard (fixed-mount) charger is not practical or possible.
Considerations: West Marine offers several portable chargers, including 1- and 2-amp battery maintainers and more powerful portable chargers with output ranging from 5 to 30 amps. Some portable chargers can also be used to jump charge a boat's starting battery.

Waterproof Onboard Chargers

ProSportHD20 Plus 20-Amp 3-Bank Onboard Marine Battery Charger

Application: Originally designed for bass boats and other open boats which require a battery charger capable of surviving wet conditions, waterproof chargers are now found on all types of boats. Since waterproof chargers are designed to charge each battery individually — regardless of whether the batteries are wired in series or parallel — they are ideal for multi-voltage applications such as bass boats that typically have a 12V starting battery and a 24-, 36- or 48-volt trolling motor powered by 12-volt batteries wired in series. For example, bass boats with a 12-volt starting battery and a 24-volt trolling motor powered by two 12-volt batteries wired in series.
Considerations: One drawback of waterproof chargers is that their sealed design inhibits their ability to dissipate heat, which makes them a poor choice for powering continuous loads and meeting the charging requirements of larger boats. Given this limitation, waterproof chargers should not be used to charge battery group sizes larger than Group 31.

Vented Onboard Chargers

ProNautic 1230P, 30-Amp, 3-Bank Onboard Marine Battery Charger

Application: Suitable for powering continuous loads at the dock and meeting the charging needs of cruisers, yachts, and larger boats.
Considerations: Dry-mount, vented chargers are for boats with a shore power connection and AC distribution/breaker panels. They need to be mounted in a dry area where they will not get dripped on or sprayed with water. They can handle heavier capacity batteries and/or charge batteries while they are under load. Their vented design enables them to dissipate the heat generated during charging. In addition to their vented design, some include cooling fans to further dissipate heat.

Output Voltage

West Marine offers 12V, 24V,36V, and 48V battery chargers. The charger you select should output the same voltage as your boat’s electrical system’s voltage.

What about boats with trolling motors?

West Marine offers excellent options for boats equipped with trolling motors. As an example, for a typical bass boat with a 12V engine cranking battery and a 24V trolling motor powered by two 12V batteries connected in series, we recommend a three-output 12/24V Guest ChargePro™ or a ProMariner™ ProSportHD series charger. For other configurations and more information about these chargers, see our product descriptions and/or the respective owner’s manuals.

Charger Capacity: How many amps do you need?

Our conservative recommendation is that you select a battery charger that outputs 10–20 percent of your total battery capacity in amp hours. (Note: In the succeeding discussion, total battery capacity in amp hours is referred to as "C".) For example, for two separate 100Ah batteries, where C = 200, we suggest you choose a dual-output battery charger with output in the 20– to 40-amp range.

Mastervolt Charge Master battery chargers

If you don't know the total amp hour capacity of your batteries this might be indicated on the battery case, or online at the manufacturer's website. You can also determine the amp hour capacity of your batteries by their group size. Here is a listing of battery group sizes and the approximate amp hour capacity for each:

Group 24: 85 Amp Hours
Group 27: 105 Amp Hours
Group 31: 125 Amp Hours
Group 4-D: 215 Amp Hours
Group 8-D: 255 Amp Hours

Advanced Recommendation

While the conservative recommendation that we make above satisfies the needs of most boaters, if you want to charge your batteries as quickly as possible, we suggest you select a battery charger with output amperage equal to or slightly greater than what your batteries can accept. Maximum amperage that batteries can accept during the Bulk Phase of the charging cycle varies with battery chemistry: flooded batteries can accept a charge rate of up to 25 percent of C; gel batteries have a higher acceptance rate of as much as 30 percent of C; AGM batteries accept as much as 40 percent of C, and Lithium Iron Phosphate batteries can accept the most, at 50–80% (sometimes 100%) of C.

Input Voltage: Do you need a dual-input charger?

While electrical service at most marinas is 110–120V AC, shore power at many marinas also includes 240V AC. If you are concerned that you might find yourself in a situation where 240V AC might be the only service available, we suggest you choose a battery charger that accepts both.

Number of Outputs

The charger’s number of outputs should equal the number of batteries or battery banks you need to charge. Because the battery chargers we offer differ in allowable wiring configurations, in order to ensure that the charger you select has the capabilities you want, we suggest you carefully read our product descriptions and view the wiring diagrams found in the charger’s owner’s manual.

Benefits of Smart Battery Chargers

Unlike archaic (some would say “dumb”) ferroresonant chargers that charge batteries at a constant voltage — risking overcharging and damage — the majority of marine battery chargers that West Marine offers are "Smart" chargers that include customized charging profiles tailored to meet the unique needs of different battery chemistries, including flooded, gel, AGM, and in many cases, Lithium. They adjust voltage and current dynamically, with multi-stage charging that includes temperature compensation, and maintenance modes. This ensures optimal charging, prolongs battery life, and enhances safety. Here are a few of the benefits common to many of the smart chargers offered by West Marine:

Automatic Charging: Smart chargers can automatically adjust the charging voltage and current based on the state of the battery. This ensures that the battery is charged optimally without overcharging, which can extend the battery life.
Multi-Stage Charging: Smart chargers typically employ a multi-stage charging process, including bulk, absorption, and float stages. Each stage serves a specific purpose, such as quickly charging the battery, reaching maximum capacity, and maintaining a steady charge without overloading the battery.
Temperature Compensation: This feature is particularly important in climates of extreme temperature fluctuation. For example, charging a battery in Maine in winter is dramatically different than Maine in the summer. Chargers with temperature compensation monitor the temperature of the battery and adjust the charging voltage accordingly. This prevents overcharging or undercharging and helps to prolong the life of the battery.
Maintenance Mode: Smart chargers often have a maintenance or float mode. Once the battery is fully charged, the charger switches to a lower voltage to keep the battery topped off without overcharging. This feature is particularly useful for long-term storage or when the boat is not in use.
Diagnostic Functions: Many smart chargers have diagnostic capabilities, providing information about the battery's condition and performance. This can include voltage levels, charge status, and potential issues with the battery, allowing users to address problems before they become more significant.
User-Friendly Interface: Look for a user-friendly interface, such as a digital display or indicator lights that makes it easy to monitor and understand the status of your batteries. Some chargers can be used with a remote display that enables you to view battery status from a convenient location.
Safety Features: Many smart chargers incorporate safety features such as reverse polarity protection, short circuit protection, and overcurrent protection. These features help prevent damage to the charger, the battery, and the surrounding equipment.

In summary, “smart” battery chargers offer efficient, convenient, and safe charging solutions for marine batteries, promoting longer battery life and reliable performance.

What Batteries Want: The “Ideal Charge Curve”

Marine deep-cycle batteries last the longest and charge the fastest when they are charged in distinct phases, which comprise what we refer to as the “Ideal Charge Curve,” the charging schedule recommended by virtually all manufacturers of marine batteries. For AGM, Flooded and Gel batteries, this three-phase charging process is depicted in the graphic below. Two caveats apply to the accompanying diagram and discussion:

Optimum charging voltages vary with battery chemistry; and
Optimum charging voltages are temperature-dependent. Higher temperatures require lower voltages, and lower temperatures require higher voltages. (Note: Regarding the unique charging needs of Lithium batteries, see “Charging Lithium Batteries below.)

Chart showing charging curves for flooded, Gel and AGM batteries

Bulk Phase: In the initial phase, bulk charging, the charger delivers relatively high amperage to rapidly increase the battery's state of charge. This stage efficiently recharges the battery to around 75-90% of its capacity./p>

Absorption Phase: Following bulk charging, the absorption (also referred to as the Acceptance phase as in the chart above) phase begins. Here, the voltage is held steady while the battery continues to recharge at a declining rate.

Float Phase: When the battery’s acceptance declines to two to four percent of C, the voltage is reduced and the maintenance phase begins, wherein the battery is maintained in a fully-charged condition. Float charging is particularly beneficial during periods of inactivity.

Equalization: For flooded lead-acid batteries only, this optional, usually omitted phase is employed after the acceptance phase, wherein a controlled higher voltage is applied for a set duration. This intentional overcharge promotes mixing within the electrolyte, preventing the stratification of acid and aiding in the dissolution of lead sulfate crystals that may have accumulated on the battery plates. This controlled “boiling” action helps maintain the battery's optimal state of charge and overall health. Caution: This procedure should never be performed on sealed AGM, gel or lithium batteries. It is for flooded lead-acid batteries only and should be performed by a professional.

Charging Lithium Batteries

Unlike the three-phase charging routine employed for AGM, Gel and Flooded batteries (described above), Lithium Iron Phosphate Batteries (LiFePO4) batteries can be charged with either a one-stage profile (constant current “CC” aka Bulk Phase) or a two-stage profile (constant current, constant voltage “CC-CV” profile aka Bulk and Absorption Stages). The one-stage profile will charge the battery to about 97% and the two-stage profile will charge the battery to 100%. Interestingly, unlike AGM, Gel and Flooded batteries whose lifespan is reduced by repeated undercharging, the one-stage charging profile is sufficient for LiFePO4 batteries, since they do not need to be fully charged. Given these differences, if your boat is equipped with Lithium batteries, it is critical to select a battery charger with a charging profile for Lithium. In most cases, this will be indicated on the charger’s packaging or in the charger’s online description. Charging lithium batteries without a lithium profile could create situations where batteries are deeply discharged but the charger views them as charged, leading to major issues on the water.

Multi-Stage Smart Charging While Underway

While this article has focused on AC powered battery chargers for use at the dock, when it comes to efficient battery charging, you also need a “smart” charging system for after you cast off. Depending on how much charging capacity you require, options to consider include upgrading to a high-output alternator and a “smart” voltage regulator to meet your batteries’ unique charging needs. Owners of cruising sailboats should also consider installing marine solar panels and/or a wind generator for battery charging away from the dock.

Tips for Battery Longevity

The following tips apply to AGM, Gel and Flooded marine batteries. For the unique charging needs of Lithium batteries, see Are Lithium Batteries a Good Choice for You?

Shallow discharges lead to longer battery life.
Discharge flooded and AGM batteries to a maximum of 50% and up to 80% for GEL and Lithium.
Don't leave batteries deeply discharged for any length of time.
Charge batteries after each period of use.
Don't mix old batteries with new ones.
Don’t mix batteries of different chemistries.

How to Choose a Battery Charger for Your Boat