By Brian Fischer
How to charge a deep cycle battery properly

Whether for marine or auto use, sometimes your deep cycle battery drops below its needed voltage to run your boat, golf cart, or electronics. If this happens, you’ll need a special charger to get your battery back up to its recommended voltage. 

Not sure if your battery is good? We’ll show you how to test a deep cycle battery at the end of this article. 

Re-charging your deep cycle battery is a matter of 4 tasks.

  1. Purchasing the correct charger
  2. Clean the battery
  3. Properly discharging the battery while in use
  4. Maintaining the battery while not in use

Cleaning the Terminals

If your battery is free of rust or corrosion, feel free to skip the cleaning section. 

First things first, you need to get the proper supplies and materials. If the battery has been sitting for a while, it may have corrosion and acid buildup on or around the terminals. If this is the case, you’ll need:

  • Latex gloves
  • Safety glasses
  • Paper towels
  • Wire brush
  • Fine grit sandpaper

You’ll also need a deep-cycle battery charger, which is the most essential piece.  

You can tell if there’s corrosion if there’s a white, powdery substance that tends to blow away in the wind. If you don’t clean this away, you’ll get a poor connection between the battery and the charger. This will result in longer charge times with less available power in the battery.

  1. Put on the latex gloves and safety glasses
  2. Wipe away excess corrosion with the brush
  3. Create a baking soda and water solution: one tablespoon of baking soda for every cup of water.
  4. Spray the solution onto heavily corroded areas and wipe away.
  5. Use sandpaper to thoroughly clean off the 12V battery’s terminal posts of any remaining corrosion or rust. 

Choosing the Correct Charger

After the terminals are clean, it’s a matter of hooking up the positive lead to the positive terminal and the negative lead to the negative terminal. 

More importantly, you need to buy the right charger. A deep-cycle battery charger will provide the appropriate amperage and voltage for your battery to reach full capacity in a timely fashion. 

Purchasing the wrong type of charger can damage your battery or not provide enough power for the battery to charge fully. 

Black Decker
12v deep cycle battery charger

  • 15A of charging power
  • Automatic switching between phases
  • 40A starting ability
CTEK PRofessional
CTEK professional 12v deep cycle battery charger
  • Delivers 25A to large batteries
  • Ideal for professional applications or multi-unit battery banks
  • Includes wall hanger and an 18' cable

Trickle Chargers vs. Deep-Cycle Chargers

Trickle chargers are meant for keeping regular batteries (not deep-cycle), topped off. A more appropriate name may be a ‘maintainer,’ as they maintain the charged status of the battery, rather than actively ‘charging’ the battery. These are also called ‘float’ chargers. 

Thus a trickle ‘maintainer’ will not do anything if hooked up to a depleted deep-cycle battery. 

The main difference between a trickle charger and deep-cycle specific charger is the number of amps each provides. 

For example, a trickle charger that can be picked up from harbor freight for $10 will provide enough power to keep an ATV battery, or a smaller car battery topped off while not in use. 

While these chargers can, technically, bring a regular lead-acid battery back from the dead, it sometimes can take days or weeks depending on the charger and battery. Deep-cycle batteries can’t be brought back with these chargers. 

Why Use a Deep Cycle Specific Charger

A charger specifically for deep-cycle batteries can provide increased amperage to the battery and vary the amps sent. With a built-in monitoring system, these chargers typically charge a battery in phases and protect against overcharging, which is optimal for the battery’s lifespan.

PRO-TIP - Consistent overcharging from cheaper no-name manufacturers can be catastrophic to the battery and your boat. The electrolytes are boiled out of the cells, which causes thermal runway - aka, the battery becomes hotter and hotter.

For example, a battery starting from 20% capacity, which is essentially dead, will be charged up to 80% capacity with, say, a 15 amps load. After the charger detects the battery is at 80% capacity, it will drop the load to say 5-10 amps. 

As it approaches 90% or more of capacity, the load may drop even more.

The benefit with a specialized charger is the charge time is significantly shorter. You can spend more time out on the water with power, rather than waiting for the battery to charge fully. 

Best Deep Cycle Batteries – An Informative Guide
Expert Power
Best Marine Boat Battery ExpertPower Boat Lift Deep Cycle

Economically priced, value option

  • 33Ah of continuous use
  • Rugged construction, durable materials
  • AGM sealed - no maintenance
REnogy 200Ah
Best Boat Battery Renogy Deep Cycle Gel

Large capacity battery at a great price

  • 200Ah rating gives you all-day power
  • Low monthly self-discharge rate
  • Leak-proof, spill-proof, and maintenance free

Why can’t the charger continually charge a high amp load?

There are three main reasons:

  • You run the risk of overcharging the battery
  • A lot of heat is generated and bad for the health of the battery
  • Too much power too fast can cause explosions or a fire

The reason is you don’t want to overcharge the battery or generate too much heat. The longer it charges, the more energy is built up, and more heat is created as a result. 

Too much heat is not good for the longevity of the battery. Lowering the amps being sent allows it to charge while reducing the amount of heat generated. 

After the battery reaches a near full capacity, the charger will revert to what’s known as a pulse-charge. This is essentially what a float or trickle charger does. The charger turns on and off, depending on the battery’s level to keep it topped off. 

All batteries lose some energy overtime when they sit around, not in use. Some lose energy quicker than others, which is the purpose of a trickle charger. 

Deep-cycle batteries do not need to be at 100% to be used, although fully charging the battery after use extends its lifespan. 

Tips to Maintain a Deep Cycle Battery

  • A shallower depth of discharge will help to extend battery life. Turn off the battery when not in use, and try not to run the power down farther than needed.
  • Keep terminals free of debris and corrosion
  • Don’t leave the battery deeply discharged for an extended period of time. Charge it as soon as your back to shore.
  • If connecting multiple batteries, do not connect new to old batteries
  • When not in use, disconnect the terminals from the battery
  • Do not store partially charged. Connect a trickle charger to the battery if storing for extended periods of time. 
  • If your battery is not sealed, be sure to check the water levels regularly and refill with distilled water when needed.

Onboard Portable Marine Chargers

Similar to the alternator in a car, an onboard charger can help replenish your battery while not connected to shore power. A solar-powered option is the perfect example of this. 

If you’re out cruising, or fishing, a solar panel can help charge the battery back up while it’s not in use.

Additionally, keeping a portable charger handy can help you if you decide to land where your home charger is not available. 

If you are powering multiple electronics on your boat, you may need to connect wire multiple batteries together and upgrade your alternator to meet energy demands. 

How to Test a Deep Cycle Battery

Here are a few ways you can test whether or not your battery has gone bad or needs to be replaced.

Load Testing

If you have a load tester, fantastic! You probably know how to use it. Otherwise, you’ll need to take the battery to a store like Autozone, or your local boat, RV, or car mechanic may have one. 

A load tester will simulate a load by attempting to draw power from the battery. From there, it will measure what it was able to draw and at what rate. Ultimately letting you know whether or not the battery can continue to function. 

Multi-Meter Approach

A multi meter is an inexpensive device that will tell you the voltage of the battery. They’re easy to use, and the video below explains how. 

Most deep-cycle batteries are rated 12 volts. When fully charged and being freshly taken off the charger, the battery will likely read in the range of 12.5V to 13.5V. This is normal. 

When a deep-cycle battery reaches 80% depth of discharge, the rating will be close 11.6V or lower. Any lower than that and you risk damaging the battery. If the reading is less than 10V, the battery is likely unusable, especially if it’s older or been used frequently. 

Attempt to charge the battery fully and then take a reading of the voltage every couple of hours. 

If the voltage drops from its ‘full’ reading (12-13V normally) to the 8-10V range in less than a day, the battery has probably gone bad. 

What you’re determining is whether or not the battery has the ability to hold a charge. If the battery can’t hold a charge, then it won’t be able even to provide power to a load tester. 

Check the Age and Usage

Under normal conditions, a deep-cycle battery can last around 2-5  years. Take a step back and gauge how much or how little you’ve used the battery over the years. 

If it’s an older battery, but will still turn on lights or hold some of a charge, it’s still probably gone bad. What it ultimately comes down to is whether or not it can provide the required load. 

Frequently Asked Questions

What is a deep-cycle battery?

A deep cycle battery is specifically designed to be used to a low depth of discharge. Most batteries have a depth of discharge of 20%, meaning the battery can only be drained to 20% before it is unable to provide adequate power. These are the batteries you’ll typically find in a car.

Deep-cycle batteries have a depth of discharge of around 70-80%, allowing more energy to be used by whatever it’s connected to. Thus the battery can be discharged ‘deeper’ per charge ‘cycle.’ 

A regular lead-acid battery can’t go that low, or it becomes unusable. More on this is explained in this article. 

How do I charge a deep-cycle battery fast?

The short answer is to purchase a reputable deep-cycle specific battery charger. A higher amp rating will generally charge the battery quicker than a lower amp rating. However, the caveat is not all chargers meet the specs they advertise. 

Additionally, without the proper shutoff mechanisms and monitoring, an unregulated and cheap charger is a fire hazard. 

How long do deep-cycle batteries usually last?

Under normal conditions, most deep cycle batteries will last in the range of 2 to 5 years on average. This will vary massively on the environment, use, and maintenance. 

How to test a deep-cycle battery?

The best way to test any battery is to have it load tested. An auto parts store or mechanic will place the battery under a simulated load to determine if the battery can provide the energy needed. 

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

Charge times vary on the size of the battery, temperature, and the charger used. A standard 100 amp hour battery on a 15 amp charger at room temperature, will typically take around 5.33 hours to reach 80% capacity. 

Simply take the battery’s capacity, and divide it by the rating of the charger and you’ll have a rough idea of how long it will take to charge. Colder temperatures and dirty terminals will slow down charge times. 

Are AGM batteries deep cycle?

No. Deep-cycle batteries can be AGM, but not all AGM batteries are deep-cycle.

How much does a deep-cycle battery weigh?

The Yellow-Top Deep-Cycle battery offered by Optima weighs 36.4. This is typical of most deep-cycle batteries because of the heavy lead plates in the battery.

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