How To Charge A Deep-Cycle Battery

How To Charge A Deep-Cycle Battery

Do you need information on how to charge a deep-cycle battery? Then, read through this article for the details you need.

This article details how to charge a deep-cycle battery and the various types of deep-cycle batteries, their depth of discharge (DoD), their charging efficiency, and other relevant information.

However, this article is presented in sections, beginning with an overview of batteries.

In this section, I will also briefly explain primary and secondary cells and starter batteries and then focus on deep-cycle batteries.

Furthermore, the next section will contain details on how batteries work.

After this, I will explain the types of deep-cycle batteries and state their pros and cons.

Subsequently, the next section will contain the factors you should consider while buying a deep-cycle battery.

This will be followed by the precautions you must take when you want to charge a deep-cycle battery.

Then, you will learn the step-by-step procedures you should follow to properly charge your deep-cycle battery.

After this, I will provide answers to some frequently asked questions on how to charge a deep-cycle battery.

In conclusion, I will give my final thoughts. I urge you to open the links in this article for more explanations.

Sit back; let’s do this.

Overview – How To Charge A Deep-Cycle Battery

A battery is an electrochemical device that stores electrical charges as chemical energy and uses them to produce electricity.

It delivers direct current (DC) electricity when you connect a load to its terminals.

Also, it consists of one or more electrochemical cells with external connections that power electrical devices.

Batteries could either be rechargeable or non-rechargeable.

Furthermore, rechargeable batteries are composed of secondary cells and can be recharged when discharged.

On the other hand, non-rechargeable batteries are made of primary cells and can not be recharged when they are fully discharged.

However, rechargeable batteries apply to different areas of generating, storing, and using electricity.

More so, most automobile devices obtain electrical power from rechargeable batteries.

Even so, batteries play a vital role in generating electricity from solar energy.

This is because it gives room for the energy generated during the day (when there is enough sunlight) to be used at night ( when there is no sunlight).

In addition, rechargeable batteries are used in automobile devices to either start or operate the device.

A starter battery does the starting, while a deep-cycle battery is used to operate the automobile device.

This article concentrates on deep-cycle batteries.

Hence, a deep-cycle battery is a rechargeable battery that deeply discharges, using almost all of its capacity, at which point it needs to be recharged.

Unlike starter batteries, which discharge a very small percentage of about 2 to 5% each time they are in use, deep-cycle batteries discharge about 80%.

However, manufacturers recommend that your deep-cycle battery discharge below 45% to extend the battery’s lifespan.

How Do Batteries Work – How To Charge A Deep-Cycle Battery

A battery consists of two or more electrochemical cells, and each cell has two electrodes (anode and cathode) and an electrolyte.

The anode is the negatively charged electrode, while the cathode is the positively charged electrode.

However, these electrodes are metals, with one reacting with the electrolyte to create electrons while the other reacts with the electrolyte to receive the electrons.

The anode produces the electrons, while the cathode receives them.

In addition, you can use any two conducting materials with different standard potentials to form an electrochemical cell.

This is because the stronger conducting material will be able to take electrons from the weaker one.

Ideally, the choice of material for an anode should produce a reaction with a significantly lower standard potential than the cathode.

In other words, it is more negative than the cathode.

As a result, the cathode will attract more electrons from the anode. Thus, when you connect a wire, you can harness its energy to provide electrical power.

Particularly, the difference in potential between the electrodes almost equates to the force with which the electrons will travel.

This is the electrochemical potential of the cell, and it determines the voltage of the cell.

On the other hand, the electrolyte is a liquid, gel, or solid substance that allows the movement of charged ions.

When the anode produces electrons (which are negatively charged) that flow through the circuit, the electrolyte provides a medium for charge-balancing positive ions to flow.

Also, a semi-permeable barrier within the electrolyte prevents the electrons from flowing to the cathode through the electrolyte.

Thus, the only part the electrons could flow through is the external circuit.

This occurs while discharging the battery.

During charging, the electrons move from the cathode to the anode.

Types Of Deep-cycle Batteries – How To Charge A Deep-Cycle Battery

There are three types of deep-cycle batteries. These are:

Flooded Lead-acid Batteries

The flooded lead-acid (FLA) batteries, sometimes called “wet cells,” consist of lead plates in a container containing a liquid electrolyte.

Usually, the electrolyte is concentrated sulfuric acid mixed with water.

Due to the chemical reaction that occurs during the charging and discharging of the FLA batteries, the electrolyte produces gases that vent out of the battery.

This reduces the electrolyte level, which requires you to top it up periodically.

However, flooded lead-acid batteries have a 30 to 50% depth of discharge (DoD) rate. Also, its charging efficiency is about 70 to 85%.

The depth of discharge rate of a battery is the usable capacity of that battery.

Therefore, it indicates how much of the battery capacity you can use.

Pros Of FLA Batteries

  • Flooded lead-acid batteries are the cheapest type of deep-cycle batteries.
  • They have low internal impedance.
  • Also, they can deliver very high currents.
  • Even so, it has an infinite shelf life if you store it without electrolytes.
  • It can leave it on float charge or trickle for a prolonged period.

Cons Of FLA Batteries

  • It is very heavy and bulky.
  • Also, it has a low DoD rate of 50%.
  • Even so, it has a high self-discharge rate of 5% per month.
  • In addition, it can overheat while charging.
  • You need to maintain it regularly by topping up electrolytes.
  • Its life cycle is very short.
  • While charging, it generates poisonous gas.

Valve Regulated Lead-acid (VRLA) Batteries

The value-regulated lead-acid battery is a generation of battery that was created to overcome the setbacks associated with flooded lead-acid batteries (FLA).

Rather than a free-flowing liquid electrolyte, the valve-regulated lead-acid (VRLA) batteries have a thick electrolyte.

However, there are two varieties of valve-regulated lead-acid batteries: absorbent glass mat (AGM) deep-cycle batteries and gel deep-cycle batteries.

Absorbent Glass Mat (AGM) Deep-Cycle Batteries

The absorbent glass mat (AGM) deep cycle consists of a thin fiberglass mat between the lead plates.

This glass mat absorbs the electrolyte, keeps it from spilling, and acts like a damper between the plates.

As a result of the damping action of AGM deep-cycle batteries, they are shock- and vibration resistant and can withstand cold temperatures.

However, the AGM batteries have low internal resistance and charge faster than the gel or FLA batteries.

Its depth of discharge and charge efficiency are 80% and 95%, respectively.

Pros Of AGM Batteries

  • It has a high charge efficiency of about 95%.
  • Also, it can charge at a higher rate than lead-acid or gel batteries.
  • Even so, it has a depth of discharge close to 80%.
  • Electrolytes in AGM batteries do not spill.
  • Freezing temperatures do not affect AGM batteries.
  • It requires no maintenance.
  • In addition, it has a low self-discharge rate of 1 to 3% per month.

Cons Of AGM Batteries

  • They are more expensive than FLA batteries.
  • Overcharging can damage it.

Gel Cell Batteries

Gel batteries are another type of valve-regulated lead-acid battery.

It uses a gelled electrolyte made from sulfuric acid and water suspended in a silica agent.

However, the gel batteries have no off-gassing effect and excellent heat tolerance. Its charge efficiency is about 85 to 90%.

Even so, gel batteries do not tolerate fast charging and are very sensitive to overcharging. This could cause it to be irreparably damaged if it happens.

Pros Of Gel Cell Batteries

  • It requires no maintenance.
  • Also, it has a low self-discharge of 1 to 3%.
  • Its charge efficiency is very high.
  • Even so, it tolerates more heat than AGM batteries.
  • Its gelled electrolyte does not spill.
  • There is no risk of leakage or spillage since the gel cells are sealed.

Cons Of Gel Cell Batteries

  • Overheating can damage them.
  • Also, they cannot tolerate fast charging.
  • They are too expensive.
  • Even so, they require their own specific charging profile.

Lithium-ion Batteries

Sometimes referred to as a “lithium battery,” the lithium-ion battery consists of cathode and anode plates or cylinders in a solution of lithium salt.

It is the safest battery for use in recreational vehicles (RVs).

Despite being the most expensive among deep-cycle batteries, they last much longer than lead-acid batteries.

In addition, they are about 30% lighter than flooded cell batteries.

However, its depth of discharge is about 80 to 100%.

Therefore, lithium-ion batteries have the fastest discharge rate and a very long cycle life of about 2000 to 5000 cycles.

In addition, it offers a constant voltage over any discharge rate.

Thus, your lithium-ion powered lights will not dim as the battery loses charge.

Lithium Iron Phosphate (LiFePO4) is the most recent type of lithium battery frequently used in deep-cycle applications.

Pros Of Lithium-ion Batteries

  • They are very compact.
  • Also, they have a usable capacity of 80 to 100%.
  • Even so, they have a very low self-discharge rate.
  • It has a charge efficiency of 99%.
  • In addition, its charge rate is about 5 times that of AGM batteries.
  • It can last for 2000 to 5000 cycles with a high energy density.
  • Lithium batteries require no maintenance.
  • You can store it in a low state of charge with no damage.
  • It loses very little power at low temperatures.

Cons Of Lithium-ion Batteries

  • Lithium-ion batteries are the most expensive of all deep-cycle batteries.
  • The chemistry of some lithium-ion batteries is thermally unstable, which causes overheating and the risk of fire or explosions.
  • You can not charge it at low temperatures, near or below freezing.
  • It requires a battery maintenance system for protection.

Important Factors To Note When Buying Deep-Cycle Batteries – How To Charge A Deep-Cycle Battery

Before buying a deep-cycle battery that will serve you, these are the factors you need to consider:


The voltage of your battery should match your purpose for buying it. Particularly, a 12V deep-cycle battery is standard for RVs, boats, and portable applications.

However, you will need a higher voltage capability of 24 volts and 48 volts for home energy storage. Hence, the fewer solar panels, the lower the voltage rating your battery should have.


The battery capacity is the ampere-hour rating of the battery. Therefore, if the ampere-hour rating of the battery is high, the battery will deliver more energy.

Most deep-cycle batteries denote a charge rate within 20 hours of measurement. Thus, a 300Ah battery can discharge at a rate of 15 amps for 20 hours until it discharges fully.

Depth of Discharge (DoD)

The depth of discharge of a battery describes the capacity of the battery you can safely use without harming it. Also, the DoD of deep-cycle batteries varies depending on their type.

Flooded lead-acid batteries have a DoD of 50%, whereas gel and AGM batteries have 80 to 100% DoD.

Hence, draining them will not cause long-term damage but might limit their usage for optimized performance.

Battery Life

The cycle life of batteries describes the number of charge cycles the cell supports. Often, gel batteries last 1,000 cycles at 50% DoD and 450 cycles at 100% DoD.

However, AGM batteries can last 500 cycles at 50% usage and 200 cycles at 100% usage.

Thus, battery life has much to do with DoD, and its useful life ends when you deplete its capacity.


The cost of the battery is another important point you should consider for any deep-cycle battery you intend to buy.

However, the prices of deep-cycle batteries differ according to the type and model.

Lithium-ion deep-cycle batteries are the most expensive, even though they are highly efficient.

Precautions – How To Charge A Deep-Cycle Battery

When charging a deep-cycle battery, these are the precautions you need to take for safety.

  • While charging your battery, keep the battery charger as far as possible from the battery.
  • Also, do not connect heavy loads to your battery while charging. If possible, do not connect anything.
  • If you are charging a flooded lead-acid battery, ensure that the liquid electrolyte does not spill or go below its normal level while charging.
  • Ensure that the voltage rating of the charger equates to that of the battery.
  • Do not use traditional battery chargers. Always use a deep-cycle battery charger.

Procedures – How To Charge A Deep-Cycle Battery

Deep-cycle batteries are rechargeable batteries. Thus, it allows you to recharge and use for several times, depending on its cycle life when it discharges.

However, the steps below will guide you through charging a deep-cycle battery.

Step 1

The first thing you should do before charging a deep-cycle battery is to check the electrolyte.

This is because the process of charging and discharging a deep-cycle battery causes evaporation, which reduces the level of the electrolyte.

To do this, open the cover at the top of the battery and check the level of liquid inside it.

Top it up with distilled water if it goes below its normal level.

However, this is only applicable to flooded lead-acid batteries. You do not need this step to charge a gel, AGM, or lithium battery.

Step 2

The next thing you should do is connect the battery charger to the battery.

Always connect the negative terminal with the black wire first, then connect the battery’s positive terminal with the red wire.

However, there are various types of battery chargers, and these chargers have different features with which they work.

Thus, you need to understand how your charger works.

Step 3

Put on the charger and allow the battery to charge. However, you need to check it hourly while charging.

Some chargers have displays and indicators that help you identify when the battery is full.

Even so, smart chargers make monitoring easier, as they communicate with the battery management system to control and monitor the charging process.

In addition, some chargers have current modes that allow fast charging. Thus, it reduces the charging time.

Step 4

When the battery is fully charged, switch off the power supply and disconnect the charger from it. Then, remove the wires from the battery terminals.

Allow the battery some time before you begin to connect it to a load.

Nevertheless, in solar applications, the battery is charged by the energy from the sun through the solar charge controller.

Click here for a visual explanation of how to charge an FLA deep-cycle battery. Also, watch a video on how to charge VRLA and lithium batteries here.

Frequently Asked Questions – How To Charge A Deep-Cycle Battery

What Is A Solar Battery: Frequently Asked Questions
1. Can you charge a deep-cycle battery with a regular charger?

A traditional battery charger produces excess heat whenever a battery is charging. However, a deep-cycle battery is not designed to handle the extra heat and rapid charging.
Hence, using a traditional battery to charge your deep-cycle battery will not give the best possible results

2. Do I need a special charger for the deep-cycle battery?

It should be a deep-cycle battery charger.

3. Can a completely dead deep-cycle battery be recharged?

If your battery has completed its cycle life and it is completely dead, you need to find a replacement

4. What amp should I charge my deep-cycle battery?

You need to charge your deep-cycle battery at a current rating between 5 and 15amps per hour.

5. How do you tell if a deep-cycle battery is fully charged?

You can tell if your battery is fully charged by reading its voltage with a voltmeter. When your battery is fully charged, the voltage of each cell will read 2.12 to 2.15 volts, or 12.7 to 13 volts for a 12-volt battery.

6. How many years do deep-cycle batteries last?

With proper care and charging, most deep-cycle batteries last up to six years.

7. What ruins a deep-cycle battery?

The factors that ruin deep-cycle batteries are deep discharges, vibration, heat, undercharging, overcharging, and non-usage.

8. What happens if a deep-cycle battery goes flat?

When a deep-cycle battery goes flat and is left for an extended period, irreversible internal damage gradually begins to occur. Hence, always keep your battery fully charged.

9. How many times can a deep-cycle battery be recharged?

About 200 to 3000 times, depending on the type of battery.

10. 10. Can a dead battery charge a dead battery?


My Final Thoughts – How To Charge A Deep-Cycle Battery

The deep-cycle battery is a rechargeable battery that can discharge a good percentage of its capacity, at which point it should be recharged.

Unlike the starter battery, which discharges only 5% of its capacity each time it is in use, the deep-cycle battery discharges up to 80% of its capacity.

Depending on the type of deep-cycle battery, the battery’s depth of discharge (DoD) differs.

For example, flooded lead-acid batteries have a low DoD of about 50%, valve-regulated lead-acid batteries have a DoD of about 80 to 90%, and lithium-ion batteries have a DoD of almost 100%.

Recharging your deep-cycle battery when it discharges is one thing you should not take for granted, as it contributes to the durability of your battery.

Always use the right charger while charging to get good results, and do not allow your battery to discharge 50% before recharging.

I hope you found this article helpful.

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References And Further Reading

What Is A Solar Battery

Best 12-Volts RV Battery

5 Best Battery-Operated Air Conditioners