Deep Cycle Battery vs Dual Purpose Battery

Not all batteries are the same. Batteries can be built differently to help them perform in certain conditions and scenarios. This is why we want to clear up the difference between a dual-purpose battery and a true deep cycle battery. Make sure you’re getting the right one when you go shopping.

Battery Basics – A Series to Learn About Batteries

Battery Basics – Types of RV Batteries
Battery Basics – How to Care for RV Batteries
Battery Basics – True Deep Cycle vs Dual Purpose
Battery Basics – How to Wire RV Batteries
Battery Basics – RV Battery Sizing

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The Heart of an RV Electrical System is the Battery

The battery is the heart of an RV’s electrical system. That electrical system contributes to your comfort and safety when you’re camping.

When dry-camping or boondocking (that is camping without power or water), the battery powers your devices. The battery powers the lights, fans, water pump, smoke detector, coffee maker, or any other device you bring.

What’s a Deep Cycle Battery?

A deep cycle battery is a type of battery that has the ability to be deeply discharged and still work to an effective level. They are great to use when you need good reliability in powering devices for long periods of time. Perfect for RVs and other low-power draw applications.

The term deep cycle is better known for a type of lead-acid battery. Deep cycle differentiates between batteries that deliver sustained power over a long time and batteries that deliver a large amount of power in short bursts. However, with the introduction of Lithium type batteries, the term deep cycle can be applied to them as well. Lithium batteries can be deeply discharged and perform better than lead-acid batteries.

But to understand the difference and why we call them deep cycle you need to learn about another common lead-acid battery, your car starting battery. These batteries give BIG, but short bursts of power. They won’t last long and if you drain them too deeply they will fail quickly.

Starting Battery = Big, short bursts of power but won’t last long if discharged deeply.
Deep Cycle Battery = Long, sustained power that can be deeply discharged but no BIG bursts.
Dual Purpose Battery = A blend of both short bursts and long sustained power. Not great at either.

Marine / RV Battery – Also Called Dual Purpose Battery

There is also a third battery introduced into the market that is an in-between battery as compared to starting batteries and deep cycle. A dual-purpose battery can deliver a good amount of power over a long period of time. This is good for trolling motors or power draws that aren’t for starting large car motors. But they can’t take a deep discharge well like a true deep cycle battery and they don’t have enough short bursts of power for starting.

Dual Purpose isn’t Perfect for Your RV

I have beef with the RV and battery industry marketing Marine / RV batteries as perfect for RVs. Technically they can be considered a deep cycle battery, but there is a better choice for your RV and that’s a true deep cycle battery.

When you get a battery from an RV dealership they will generally use a dual-purpose battery. If you have a choice I would make sure you get a true deep cycle lead acid or a lithium Iron Phosphate type battery.

Dual-purpose batteries are cheaper than a true deep cycle battery. Since battery technology isn’t well understood, and RV dealerships try to get away with cheap stuff, they sell you a dual-purpose battery. These types of batteries generally only last 1 to 3 years due to RVs deeply discharging the battery. This requires you to buy a new battery in a very short period of time.

Why Deep Cycle Battery, Starting Battery, and Dual-Purpose Battery Differ

Lead-Acid batteries are configured for various applications. The main ingredients at work are Lead and Sulfuric Acid. The lead inside the battery is in the form of ‘plates’. The acid flows around the plates. More plates mean more surface area and a faster reaction. This is how a starter battery works. Fewer plates mean thicker plates and less surface area, and a slower reaction. This is how a deep cycle battery works.

Starter Batteries – Thin lead plates = more plates = more surface area = faster chemical reaction.
Short burst of high power.
Deep Cycle Batteries – Thick lead plates = less plates = less surface area = slower reaction.
Long, steady level of power.

As the batteries age, or are in use, the lead plates get corroded by the acid. The deeper you discharge a battery the more corrosion happens. When you charge a battery a good amount of this corrosion gets re-suspended in the acid. But when you deeply discharge a battery the less and less effective re-suspension is when charging. This corrosion hinders performance, this is the main reason why you have to replace your battery.

Start batteries have thinner plates that are weaker. Deep discharges put extreme stress on the lead plates and the life of the battery is shortened. Deep cycle batteries have thick plates that can take deep discharges without much stress. In RVs, the batteries see deep discharges down to 12VDC or what’s considered flat frequently.

A dual-purpose battery is between a starter battery and a deep cycle battery. So you can expect when you deeply discharge a dual-purpose battery the re-suspension of the acid is less effective than a deep cycle battery.

Battery Lifespan is Dependent on Rates of Discharge

The level of discharge to the battery will have a big effect on the life of the battery. Most RVers need to change out their batteries every 1 to 3 years. If you can reduce the level of discharge and keep the battery charged up you can extend the life of your batteries by years.

This is the main reason why dual-purpose batteries don’t last as long as deep cycle batteries in RV applications. The thinner plates and the frequency of deep discharges weaken the battery rather quickly. If you can make sure never to deeply discharge a battery you could successfully use any type of battery in your RV.

True Deep Cycle Batteries

You might have heard many people recommending 6-volt batteries over 12-volt batteries.

Almost all large 6-volt batteries have large lead plates making them a true deep cycle battery. People will recommend 2, 6-volt batteries over 2, 12-volt batteries all day long. The reality is that if you have true deep cycle batteries for both 6-volt and 12-volt you will have similar performance. It’s just hard to identify a true deep cycle 12-volts battery. It’s easier to recommend a 6-volt because you know it’s will be a deep cycle battery.

Common True Deep Cycle Batteries

Here are some true deep cycle batteries to keep an eye out for if you’re going to be using them for your RV or a renewable energy project.

12-volt batteries designated for Solar or Renewable Energy
4D Battery – 180-215 Amp-hours a piece
8D Battery – 225-255 Amp-hours a piece

6-Volt Batteries – You’ll need 2 batteries for a 12-volt system.
Golf Cart Batteries (various brands) – 180-225 Amp-Hours
L16 Battery – 340-415 Amp-Hours a piece

ProTip – When using 2 12-volt batteries the amperage increases. Thus, if both batteries are 100 Amp-hours you have a total of 200 Amp-hours. When using 2 6-volt batteries the voltage increases. This, if both batteries are 100 Amp-hours you have a total of 100 Amp-hours, but an increase of 12 volts.

Get a True Deep Cycle For Your RV – It will Last Longer

I hope this helps clear up the differences between battery types. Don’t get sold a dual-purpose battery if you can get yourself a true deep cycle one. It’s common to get 6-volt deep cycle batteries. But if you have a smaller RV you can easily get a deep cycle 12-volt and be just as happy. But if you do need to use a dual-purpose battery, get an RV solar system where it will charge the battery as you use it. By replacing the power you’ll help the battery last longer.

RV batteries are a big topic. We have years of experience and knowledge when it comes to them. We hope this article helped. We’ll be creating more when we get a chance. If you want to be notified when we have more info, sign up below.

And for this reason, if you’re still confused you can reach out to us. We can do an energy audit for you and calculate what size battery, type, and voltage might work for you. We can also size your solar system. We’ll need to hop on a call and get some info and ask some questions so we can get you rolling with some good recommendations.

For this consultation, we do have a fee of $49.95 per consult. Just reach out to us – if interested.

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