Factors to Consider

Battery Voltage & Number of Batteries

Most importantly, you will want to buy a charger that provides enough voltage for the battery to charge. All chargers, marine, car, RV, etc. are rated a specific voltage that should match the voltage of the battery, or battery bank, you’re charging. 

The most common voltages are 12V and 24V. Most modern chargers have the ability to operate on both. Some require you to adjust a switch, while others will automatically detect the voltage and switch instantly. 

If you have a battery bank, you will want to know the voltage and amperage of each battery or battery pack you will be charging.

For example, in the image below, there are two separate banks of batteries. Two and three batteries are wired in parallel and then those separate packs could be wired together. Based on the wiring and from a charging perspective, this would be the same as charging two individual batteries. 

Charge Time

Your charge time is most directly affected by the amperage the charger can supply. This is typically noted by a rating such as “10A,” meaning 10 amps. The higher the amp rating, the faster your battery will charge. 

To provide an accurate estimate for charge times from each charger, we have to make a few assumptions.

• Your battery has reached a depth of discharge of 50%.
• The battery charging is newer and in a climate controlled environment.
• Most chargers operate at 85% efficiency, meaning 15% of energy is lost through heat and other normal functions.

Let’s assume that you drain the battery to a 20% depth of discharge. So now the charger must fill the battery 80% of the way to reach a full 100% charge. We’ll use some of the same assumptions as above. 

How long does it take to charge a 24V deep cycle battery?

For a 24V battery, the charge time is typically doubled. A charger that has the ability to charge both 12V and 24V will half the charge time for the 24V or double the 12V - whatever way you want to look at it. You amount of energy coming from your electrical outlet does not change, so the charge time cannot be increased. 

Physical Size / Mounting Location

If you plan on using an onboard charger, you’ll want to pay attention to the physical dimensions of it. On any boat, space is at a premium, so you need to be able to securely mount the charger as close to the batteries as possible while remaining as high on the boat as possible. 

If you’re new to boating, you might be asking, “How does an onboard charger work?” The name is slightly deceiving, since you don’t actually charge the batteries while you’re out on the water. The only exception is if you have an alternator or solar panels hooked up to those batteries. 

An onboard charger simply means the bulk of the charging equipment is contained within the boat. So whenever you go to dock or bring the boat back on shore and near power, you can plug a single cable into one spot that will charge all of the batteries. 

The benefit of an onboard charger is convenience. If you have one or more batteries, you do not want to be running several different wires across your garage or deck to charge the batteries. Nor do you want to move the batteries in and out of the boat all the time.  

Exposure to Water

Specific to onboard chargers, you’ll want to understand their submersible rating. Double check the charger is water-resistant or water-proof and verify the rating. Remember, water-proof is different from water-resistant.  

Smart Charging & Safety Features

You may see some chargers advertised as having, “smart charging capabilities.” While there isn’t a definitive term for what makes a charger ‘smart’ or not, it generally boils down to several features that are a combination of safety and convenience. 

Some of these features include: 

• Reverse polarity protection - Protects the battery in case the ends are hooked up incorrectly. 
• Over-charging protection - Some cheaper chargers do not know when to stop charging.
• Overcurrent protection - Too high of a current can damage the battery.
• Waterproof or water-resistant housing - Ideal for onboard or marine applications
• Recovery modes - Can bring back batteries that were discharged too low.
• Thermal Monitoring - Keeps batteries from overheating. 
• Phased charging - Necessary for deep-cycle batteries. 

Portable or On-Board Charger

While it’s a matter of preference, an on-board charger is by the easiest way to charge your batteries. The initial mounting and setup takes a little bit of effort, but after it is installed all you to do is run an extension cord into one spot on the boat to charge all batteries. 

Portable chargers have their benefits too. For one, if you ever need to use it for a car or truck you can. An on-board charger is physically mounted with the boat, so unless you want to re-mount it after you’re done, you’re out of luck. 

Also consider what size extension cord you’ll need. Be sure to buy a lower gauge cord if you will be running greater lengths. 

FAQ’s

What are the best charger brands?

NOCO and Dual Pro are two of the most well-known and most reputable brands when it comes to battery chargers. Technical specifications and customer support are always readily available and the build quality of these products is fantastic. 

Some other brands to consider include:

• MinnKota
• Promariner
• ProSport

Batteries are expensive, so you do not want to skimp on a cheap charger that could ruin them. Think of it like tires on a car. A good pair of tires is well worth the money if you’ve spent any significant amount of money on the car itself. You wouldn’t want to put the cheapest tires on a new Corvette. 

Avoid any no-name brands on Amazon. Many of these are inferior Chinese designs that could ruin your battery or start a fire. 

What type of battery charger do I need?

For a starting or SLI battery, almost any charger on the market will work. If you are charging a deep-cycle battery, you will need a charger that is a phased charger and provides enough amperage to charge in your desired time. Be sure to check the voltage matches your battery or battery bank connections. 

How long does a marine battery hold a charge?

A marine battery can hold a ‘full charge’ for several months before needing to be re-charged. This depends heavily on battery type and the environment, but most modern batteries less only 1-3% of the overall charge every month if not connected to anything. Lead-Acid batteries will lose their charge quickly if left unattended for several weeks. 

How does an onboard marine battery charger work?

An onboard marine battery charger allows you to leave the charger connected to the batteries while you’re out on the water. Then once you are back on shore, there is only connection point for you to plug in to charge all of the batteries. 

What amp to charge marine battery?

The higher the amperage of the charger, the faster your batteries will charge. A 10-amp charger will usually suffice for an overnight charge of a 100Ah battery that reaches a 20% depth of discharge. 

How long to charge a marine battery?

Charge time depends on the battery size and the amperage of the charger. For a 100Ah battery that reaches a 20% depth of discharge, it will take a 10-amp charger about 9.2 hours to bring the battery back to full-capacity. 

How to charge a marine battery at home?

To charge a marine battery at home, you need to purchase a battery charger that offers phased charging. This is ideal for deep-cycle batteries. For starting, or SLI batteries, you can use any charger to charge the battery. 

How to trickle charge a marine battery?

To trickle charge a marine battery you need a phased, trickle, or deep-cycle specific charger. These charges have the capability to detect the battery’s capacity and turn on or off based on its capacity. The charger will safely allow your battery to hover between ~90%-100%.