What Is a Deep-Cycle Battery?

From smartphones to electric vehicles, much of the modern world runs on batteries.

In South Africa, where load-shedding seems neverending, backup batteries can be a lifesaver.

There are millions of types of rechargeable batteries depending on the application.

But, regardless of chemistry or use case, virtually all rechargeable batteries use these terms in common.

• Charging, Discharging, Recharging: All rechargeable batteries can be charged and discharged. Discharging is the technical term for using the electricity stored in the battery. Charging and recharging are used interchangeably. Both occur when you plug a battery into a wall socket or another power source — like a solar panel.
• State of Charge: Batteries are only briefly fully charged or fully discharged (flat) when operating. As soon as you start consuming electricity from a battery that’s not connected to a power source, it will be at less than 100% storage capacity. State of charge (SoC) measures the amount of electricity left in a battery relative to its total storage capacity. Think of it like a petrol tank. 100% is a full tank, 50% is half full. As you close in on O% you’re running on empty — then you’re out of petrol.
• Depth of Discharge (DoD) measures a battery’s state of charge in reverse. The formula is DoD = 1 – SoC. In other words, if you only have 10% of the battery’s total storage capacity left, the DoD is 90%.
• Cycles and Cycle Life: A battery cycle is one full discharge and recharge. Manufacturers often use cycle life as a specification that measures a battery’s lifespan based on the number of times it can be charged and discharged before diminishing in storage capacity and performance.    

All rechargeable batteries “cycle,” but what is a deep-cycle battery?

What Are Deep-Cycle Batteries?

Traditional lead-acid batteries are inexpensive and deliver a burst of surge power — making them the battery of choice for starting motors in cars, bakkies, trucks, and other petrol-powered vehicles.

However, traditional lead acid batteries are highly ineffective for applications that require electricity output over extended periods.

Cycle life is short, and only a small fraction of their total storage capacity can be discharged safely.

Even though car batteries only output short bursts of high current, they must be replaced every few years.

Deep-cycle batteries offer a much higher depth of discharge (D0D) and longer cycle life — in addition to many other advantages over traditional lead-acid chemistry.

To be considered “deep-cycle,” a battery must have a minimum DoD of 50%, although newer technologies approach 100% DoD.

Less susceptibility to the negative impact of SoC variation — and other characteristics of deep-cycle battery chemistry — resulted in better performance, less routine maintenance, and greater longevity as measured by cycle life.

Different Types of Deep-Cycle Batteries

The original deep-cycle batteries use alternative types of lead-acid chemistry to provide a higher depth of discharge and increased longevity.

Today, lithium-ion (Li-ion) rechargeable battery chemistry is the industry standard for many applications, including:

• Smartphones, laptops, and other personal electronic devices
• Electric vehicles (EVs)
• Solar batteries

Let’s take a look at all the deep-cycle options on the market.

Flooded Lead-Acid (FLA)

FLA batteries are the oldest and most affordable type of deep-cycle batteries; however, they require routine maintenance and have the shortest cycle life. 

(Source: ResearchGate)

All lead batteries are comprised of variations on three essential components: 

• Lead plates thick enough to withstand strong chemical reactions and resist corrosion
• An electrolyte solution of water and sulfuric acid that reacts with the lead to produce an electrical current
• A case to isolate and protect the lead and electrolytic components

FLA batteries are frequently known as “wet batteries” and require regular refilling with distilled or deionised water. 

Because FLA batteries are filled with diluted sulfuric acid,  they must be installed upright to prevent hazardous leaks and spills.

Additionally, FLA batteries must be installed in a well-vented area to allow the dangerous gases produced by charging and discharging to escape safely. 

Sealed and Valve Regulated Lead Acid 

Sealed lead-acid (SLA) and valve-regulated lead-acid (VRLA) are interchangeable terms for the same battery type.

SLA batteries reduce the volatility of a free-flowing liquid electrolyte and are commonly referred to as “dry” or “maintenance-free” batteries in addition to being marketed as “deep cycle.”      

There are two forms of SLA battery technology:

• Gel batteries mix diluted sulfuric acid with silica dust, resulting in a semi-solid electrolyte instead of a liquid
• Absorbed Glass Mat (AGM) batteries use fibreglass mats to soak up the electrolyte solution

Although AGM and gel batteries differ significantly in composition, they have similar advantages over flooded lead acid batteries in terms of applications and performance.

AGM and Gel Battery Applications

Here are some of the most common applications for VRLA/SLA rechargeable batteries.

• Photovoltaic (solar) 
• Electrical systems for recreational vehicles (motorhomes) and boats
• Uninterruptible power supplies (UPS)
• Starting batteries for smaller fossil-fuel-powered engines in boats, motorcycles, and ATVs
• Emergency lighting systems
• Mobility devices like motorised wheelchairs
• Golf carts

AGM and Gel Batteries — Differences in Performance

Many consumers don’t distinguish between AGM and gel batteries.

They’re both VRLA and any differences in performance are subtle.

For most applications requiring SLA batteries, either AGM or gel will do.  

Here are the most measurable differences between gel and AGM batteries when it comes to performance.

• AGM batteries produce slightly more surge power, giving them a slight edge in starting motor applications
• Gel batteries have a slightly higher depth of discharge
• Frequent discharging has less negative impact on gel batteries
• Gel batteries require more care when charging. If the charging current or voltage is even slightly outside the recommended parameters, the battery may be permanently damaged
• Gel batteries are more sensitive to extreme cold
• AGM batteries are more sensitive to extreme heat
• AGM batteries have greater resistance to vibration 
• In most use cases, gel batteries outperform AGM in cycle life and longevity

Gel batteries also tend to be slightly more expensive.

Sealed Lead Acid vs Flooded Lead Acid  

AGM and gel SLA batteries negate or minimise many disadvantages of FLA batteries while bringing additional benefits.

Pros

• 80% depth of discharge instead of 50% for deeper cycles 
• Faster charging
• Holds charge longer when not in use
• Low self-discharge rate
• Longer cycle life
• Minimal maintenance
• Installation flexibility — no need to keep AGM or gel batteries upright
• Less sensitivity to extreme cold
• Resistant to vibration 
• No spills or leaks
• Minimal off-gassing — can be operated in a confined, unvented space
• Less hazardous if the case is broken or damaged

Cons

• More expensive
• Greater sensitivity to overcharging
• Lower surge power makes SLA poorly suited for some starting applications — such as automotive batteries in petrol-powered cars, bakkies, and trucks
• Less electrolyte (acid-starved)

(Source: UL Research)

Lithium-Ion

Although the term “deep-cycle” was coined to describe sealed lead-acid variants like AGM and gel, lithium-ion batteries outperform SLA batteries by nearly every metric — including depth of discharge and cycle life.   

The only edge traditional deep-cycle batteries regularly have over Li-ion batteries is price.

Lithium-ion (Li-ion) batteries are a relatively new innovation compared to lead acid batteries, which were invented in 1859.

Li-ion batteries first appeared on the market in the early 1990s and are now, by far, the most widely used rechargeable batteries.  

For applications that require a significant amount of storage capacity — for example, home backup power and solar generators—  newer subsets of Li-ion chemistry like lithium iron phosphate (LiFePO4) deliver even more advantages over traditional deep-cycle batteries.

Current variations of lithium-ion battery cells typically contain the following components:

• Anode (graphite)
• Cathode (nickel-magnesium-cobalt (NMC), nickel-cobalt-aluminum (NCA), or lithium iron phosphate (LiFePO4/LFP) 
• Electrolyte (Lithium salt solution such as lithium hexafluorophosphate (LiPF6)
• Separators (polyethylene (PE)
• Positive and negative current collectors (aluminium and copper foil)

The materials used for the cathode vary, and that’s what determines the Li-ion battery type.  

For high-wattage storage applications — like solar batteries — NMC and LiFePO4 are usually the best choice.

LFP batteries are also increasingly replacing traditional lithium-ion batteries in electric vehicles (EVs).

As well as increased performance, LFP batteries offer a humanitarian advantage over other Li-ion battery types.

Cobalt is an essential material in NMC and other Li-ion battery types.

Over 70% of the world’s cobalt is mined in the Democratic Republic of Congo (DRC) under appalling conditions, and it’s often referred to as blood cobalt.

Because the cathode in LiFePO4 is composed of lithium iron phosphate, no cobalt is required.

Regardless of cathode type, the five components are combined in a protective cell housing, and the Li-ion battery cell assembly process is complete.

Li-ion cells are then “aged” and tested thoroughly over a period of weeks.

For high-capacity storage applications, multiple Li-ion battery cells are connected to each other and housed in a protective case.

A battery management system is required to regulate charging and discharging for optimal performance.

In off-grid electricity devices like portable power stations, advanced battery management systems are built-in. 

Deep-Cycle Battery Applications

In high-capacity storage applications, lithium-ion batteries significantly outperform sealed lead acid (SLA) deep-cycle batteries, like absorbent glass mat (AGM) and gel.

The only drawback of Li-ion batteries is the sticker price.

We covered the applications of sealed lead acid AGM and gel batteries above.

Here, we’ll focus on applications where the additional upfront cost of Li-ion batteries compared to SLA batteries really pays off in the long run.  

Home Backup Power

With up to 3400W of AC output, EcoFlow DELTA 2 Max can power 99% of home appliances.

But where it really shines versus traditional deep-cycle batteries is the depth of discharge and cycle life.

SLA batteries like AGM and gel may claim to offer 80% DoD, but when you look at the fine print, manufacturers will recommend that you try not to discharge the battery below 50%.

The LiFePO4 battery cells in EcoFlow DELTA 2 Max provide 80-100% DoD on a daily basis without shortening cycle life or negatively impacting performance.

EcoFlow’s LFP batteries can last 3000 cycles before their storage capacity diminishes to ~80% — up to 10 years of daily use.

Deep-cycle SLA batteries have an average cycle life of 300-500 — up to 90% less than LFP.

Depending on how much electricity you consume and the particulars of the battery, AGM or gel deep-cycle batteries could start giving out in under a year of daily use. 

EcoFlow DELTA 2 Max has everything you need for essential home backup power built-in.

If you go for the EcoFlow DELTA 2 Max + Transfer Switch option, it’s an easy job for an electrician to wire it to your home circuit board, giving you an uninterruptible power supply (UPS) during blackouts.

Residential Solar Power

EcoFlow’s portable power stations offer a solar charging input so you can generate your own electricity by connecting solar panels.

Or you can purchase a solar generator that includes a RIVER or DELTA series solar generator plus a portable solar panel.

If you’re looking for a whole-home backup power solution that can keep the lights on indefinitely during a blackout, EcoFlow DELTA Pro is the answer.

With up to 25kWh of off-grid electricity storage and 7.2kW of AC output (14.4kW starting watts), EcoFlow’s Smart Home Ecosystem is a highly scalable residential solar power solution.

Connect up to 16 x EcoFlow 400W rigid solar panels and say goodbye to National Grid power forever!  

Calculate your total home electricity consumption to ensure you have enough power. 

Portable Off-Grid Power

A portable off-grid power supply is essential for going on expeditions in your motorhome or overlanding in a 4×4. 

EcoFlow’s RIVER 2 series of portable power stations (PPS) is ideal for a Cape to Namibia road trip or for exploring anywhere else.

EcoFlow RIVER 2 can fit in your backpack and keep all of your personal electronic devices and small appliances running. 

 But if you need more power for the road, check out EcoFlow RIVER 2 Pro.

It weighs in at only 7.8kg but can power 80% of high-wattage appliances.

It can double-duty at home when you’re not on the road.  

EcoFlow RIVER 2 Pro offers the fastest charging speeds ever — from 0-100% in 70 minutes when you plug into AC power.

That’s plenty of time to recharge between blackouts — no matter what stage of load-shedding you’re on.

SLA deep-cycle batteries like AGM and gel with similar capacities take 8-16 hours to recharge fully.

For overlanding expeditions, consider the EcoFlow RIVER 2 Pro solar generator.  

By bundling the PPS with the EcoFlow 220W bifacial portable solar panel, you can produce up to 1.8kWh of off-grid electricity daily while on the road. 

Frequently Asked Questions

Final Thoughts

When first introduced, sealed lead-acid deep cycle batteries like AGM and gel represented a significant upgrade on flooded lead acid (FLA) batteries for many applications.

However, lithium-ion battery cells like LiFePO4/LFP have eclipsed SLA/VRLA deep-cycle by every measure of performance — including depth of discharge and cycle life.

If you’re looking for home backup power or residential solar power, EcoFlow DELTA Pro could be the answer.

EcoFlow offers a wide range of off-grid power solutions. 

Check out our selection today.