We break down what a deep cycle battery is, what it does, and why you need one . . . or why you don’t.
If you’re powering a boat, RV, or overlander, you’ve probably heard of deep cycle batteries, also known as marine batteries. But whatever you call them, they’re not the same as the battery that cranks your car’s engine. They might both look the same, and they might both provide 12 volts of power, but using the wrong one can put an early end to your day or even leave you stranded.
So, what exactly is a deep cycle battery? We’re here to tell you.
What is a Deep Cycle Battery?
A deep cycle battery is a battery designed to discharge more of its storage capacity without damage. You’re cycling the battery deep into its reserve of power—hence the name.
With starter batteries, running just a few deep cycles can destroy the battery. That’s why if your car battery has gone dead a couple of times, your NAPA Auto Parts specialist or NAPA AUTOPRO service center might tell you it needs to be replaced.
The starter battery that powers your car, is probably rated in cranking amps and cold cranking amps. That’s how much current it can put out in the few seconds it takes to run your starter. It can only discharge that much current for a very short time.
That starter battery has a total capacity of around 50 amp-hours, depending on its size, which is usually called the battery group. However, you can’t use all of those 50 aH. With normal vehicle use, your car’s battery will only discharge 5-10 aH. That low battery cycling, how much power it uses and recharges, lets the car battery live for years under your hood.
A deep cycle battery can discharge much more of its total capacity. A high depth of discharge (or DoD) means a deep cycle battery can be safely discharged all the way down to 20%, though 50% is better for battery life.
That 50% DoD lets you run electrics, like the electronics in your boat, a trolling motor, golf cart, or the TV in your RV for hours longer than any starter battery could. And they’ll do it without you worrying about damage.
On the flip side, a deep cycle battery doesn’t like the fast and short discharge that you need to start your car. Using one as a starting battery might not cause damage, but it also might not start your car. Especially if you have a high-displacement engine that is harder to turn over.
What makes a battery deep cycle?
What’s the difference? It’s all in how the batteries are built.
Traditional lead-acid batteries have many lead plates dipped in sulphuric acid. Magic happens inside and electricity goes in and out. This how most car and deep cycle batteries are made.
Starting batteries use thinner plates. They’re great for quick power, but discharge them too deeply and the plates start to corrode and become physically damaged inside the battery.
Deep cycle batteries use thicker plates. They can’t match the peak output of a starter battery, but they don’t warp or corrode when you start using more of their total power reserve.
NAPA Deep Cycle Battery
Starting at 285.19 $
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What About AGM?
Most 12V lead-acid batteries are what is called a flooded battery. The case is full of liquid sulphuric acid, flooding the plates.
The other common type is called AGM, for absorbent glass mat. In AGM batteries, the electrolyte acid liquid is stored in a mat made of tightly woven fibreglass. The liquid can’t spill or leak, because it’s contained inside the mat.
NAPA AGM Deep Cycle Battery
Starting at 295.49 $
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Is a Deep Cycle Battery Right for You?
If you need a new battery in your car, then a deep cycle battery is probably not for you.
If you need a new battery to power your RV, trolling motor, boat, or golf cart, then a deep cycle battery is the best choice. They’re also great for home emergency backup power, like if you’re running an emergency backup sump pump to your basement dry during a power outage.
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2 Comments 
I stumbled upon this blog while researching deep-cycle batteries, and I must say it provided me with valuable insights. After reading up I went with a lithium deep cycle battery as it best suited my vehicle from what I researched. I’m Australian so I have to buy locally but your blog helped me nevertheless. Cheers!
I appreciate you letting us know that a deep-cycle battery is specifically designed to discharge more of its storage capacity without damage, unlike starter batteries that get destroyed after running just a few deep cycles. My brother runs a welding shop downtown, and it seems like some of his machines need a new battery replacement soon, so he was hoping to get those with deep cycles. I’ll keep this in mind while I help him find an industrial supplier in the area to contact about AGM deep-cycle batteries soon.