Can I Use A Deep Cycle Battery In My Car? – Replacing The Starter Battery

Can I Use A Deep Cycle Battery In My Car?

Have you been thinking about using your car battery more with the engine switched off? So can I use a deep cycle battery in my car? It seems like a great idea replacing the car starter battery with a deep cycle version. 

You would then be getting the best of both worlds. Being able to start your car and have some proper battery power available. To run some appliances and charge my smartphone and music player while you’re out and about.

Well let’s take a look and see if it’s possible!

Can I Use A Deep Cycle Battery In My Car?

“If the deep cycle battery you want to use in your car is 12 volt then the answer is yes. It must also meet the minimum cold cranking amps (CCA) requirements to start the car. You will also need to consider the battery size for your battery compartment. And if there will be a difference with the battery terminal ends”

What Is The Difference Between A Deep Cycle and A Car Battery?

A video from Battery Systems Inc. A Simple Explanation of “Starting” vs. “Deep Cycle” Batteries

Deep Cycle and car starter batteries are designed to perform different tasks. Let’s take a look at what they are. 

A Car Battery

A car battery is designed to produce a very high current in short bursts to start the engine. Once the car has started the alternator takes over. 

The alternator generates alternating current (AC) which the rectifier converts to direct current (DC). Its job is to run most of the electrical systems and keep the battery charged while the car is running.

Can I Use A Deep Cycle Battery In My Car? - Replacing The Starter Battery
A Deep Cycle Battery

A deep cycle battery is designed to produce a lower current over a longer time period. So not really that well suited for producing short high bursts of energy 

It is usually recharged with solar panels, an onboard generator or a shore power hook-up. To charge the battery with a solar panel all you need to do is add a solar charge controller. Then run cables with a fuse and alligator clips from the charge controller to the battery. 

If there are several solar panels used the installation takes care of the charging process. A rectifier is not required as the solar panels produce their current in DC. 

Charging from an onboard generator or shore power hook up needs a conversion from AC to DC. So a rectifier is needed just like with the alternator used to charge a car battery.

What is A Car Batteries Allowable Depth of Discharge (DOD)?

Well this is a question and I won’t bore you with the details. But a car battery is considered fully discharged at 80% of its capacity. For example a 100Ah car battery can be discharged to 80Ah and no more. 

Car batteries are generally lead-acid flooded wet cell types.

Lead Acid Flooded Wet Cell Car Starter Battery

Lifewire says;

The basic technology is incredibly simple. Lead plates are suspended in pairs in a bath of sulfuric acid, which acts as an electrolyte.
Each pair of plates has one that is coated in lead dioxide, and when a voltage is applied, a chemical reaction occurs.
When a lead-acid battery discharges, which happens any time it provides power to start an engine, illuminate headlights or run your fancy car stereo, the plates are slowly coated in lead sulfate. (Source; lifewire.com)

This lead sulphate coating is what ends up killing the battery in the end. But the process is really accelerated if you discharge the battery below 80% of its capacity.

What Is A Deep Cycle Batteries Allowable Depth Of Discharge (DOD)?

Deep cycle batteries come in 2 varieties Lead-Acid and Lithium

A deep cycle lead-acid battery can be discharged to 50% of its capacity. So if it were a 100Ah battery it can be discharged by 50Ah. An AGM battery can be discharged to 80% of its capacity but not on a regular basis.

Lead- Acid deep cycle batteries have fewer and thicker lead plates than a car battery. This means they are better able to withstand the lead sulphication enabling a deeper depth of discharge. 

Renogy Smart LiFePO4 Battery

Lithium Iron Phosphate or LiFePO4 batteries can be theoretically discharged to 0%. I wouldn’t recommend it though. But they can be discharged to 70% to 80% of their capacity. This can be done on a regular basis without damaging it.

LiFePO4 battery chemistry is different to lead-acid. In that it is based on lithium-ion and Iron Phosphate rather than lead plates and acid.

BatteryGuy says;

A lithium battery is made up of an Anode (Negative) and a Cathode (Positive) immersed in electrolyte. When connected to an outside device, chemical reactions take place between the plates. These plates cannot touch or they would immediately short out the battery, so a porous separator is placed between them allowing the electrolyte to move, while keeping the plates separate. (Source; batteryguy.com)

It is a build up of Lithium Oxide and Lithium Carbonate known as Stable Electrolyte Interface (SEI). That ultimately kills a LiFePO4 deep cycle battery.

If you would like to take a look at a LiFePO4 deep cycle battery review you can do so here;

Related == >>>>>>>>>> Renogy Smart Lithium Iron Phosphate Battery Review 

What Do You Need To Consider Before Using A Deep Cycle Battery In Your Car?

I can see why using a deep cycle battery in your car might appear attractive. Listening to the radio or running electronic devices and appliances with the engine switched off. Having a picnic or a tailgate party or waiting for the other half to finish shopping. Without killing the battery does seem like a great idea.

But honestly it is like trying to fit a square peg into a round whole. You can do it but the battery isn’t designed to produce the initial umph to start the car. In fact this kind of usage will most certainly lead to it’s early death.

Cold Cranking Amps (CCA)

Cranking amps are what the battery industry uses to measure how many amps are needed to start a car battery. Under normal temperatures this doesn’t pose a problem. 

But in cold and subzero temperatures 32°F/0°C or lower. A 12V car battery needs to be able to start the car. So the industry has a requirement that the batteries can deliver at these low temperatures.

The batteries are tested to 0°F/-17.8°C. The test requires that the battery delivers the correct amps for 30 seconds at 7.2 volts. The cold cranking amps should be between 600 to 800 plus. The higher the CCA the better the battery will perform.

Cold Cranking Amps (CCA) In Sub Zero Temperatures

It all means that if you were to use a deep cycle battery to start a car. it would need to conform to these specifications in the wintertime. Unless of course you live in a place where it never gets cold. 

Charging A Deep Cycle Battery With The Car’s Alternator

The car’s alternator will charge a deep cycle battery but not as effectively as you might think. Think about it, if you have a deeply discharged battery and your journeys are very short. The battery will never be fully recharged this will lead to its early demise

Also the car’s alternator is not designed to charge it and will probably end up being damaged. This may eventually lead you down the path to a mechanic to replace it, sounds costly to me!

The Difference In Size Between A Starter And Deep Cycle Battery

There is also the sticky problem of fitting it into your car’s existing battery compartment. Will it be the right height, width and depth that might take some research. Matching the right deep cycle battery size may pose a problem if you are looking for higher capacity. 

For example if your current car starter battery size is currently group 35. With the dimensions of 9.05 x 6.89 x 8.86inch/230 x 175 x 225mm. You would have to fit a deep cycle battery of equivalent size. 

This might pose a problem if you are looking to choose the battery because of amp hour capacity. In the end you might have to compromise to achieve the right fit.

The Difference In Battery Terminal Ends

A difference in battery terminal ends might prove to be a headache. If you find the correct sized deep cycle battery. You will then have to make sure the terminal wires ends will fit the battery terminals. 

If they don’t you will have to either put your DIY skills to use. I wouldn’t recommend this option unless you know what you are doing. Or it’s an appointment with the electrical guy at your local car shop. 

 Difference In Battery Terminal Ends
The Price Difference Between A Starter And Deep Cycle Battery

Then there is the difference in cost. A deep cycle can be significantly more expensive than a car battery.

For instance the Renogy 12V 100Ah LiFePO4 costs $759.99, the AGM $233.99 and the Gel $249.99. At the time of writing you can pick up a starter battery at Walmart much cheaper. 

Well having established that the two battery types are meant to be used for different applications. I think if you want to have a deep cycle battery in your car it would be better kept in the trunk. Hook it up to an inverter and run some devices and appliances from it.

Even better to keep it charged why not use a solar panel and charge controller. Sounds like a great way to picnic. This combo can even be used to keep your car battery topped off.

If you would like to know a little more about inverters follow this link;

Related == >>>>>>>>>> DC To AC Pure Sine Wave Inverter Review (500w to 3000W)

My Closing Thoughts

Using a deep cycle battery to start your car and use it with the engine switched off  isn’t a great solution. This is because it doesn’t really produce the necessary high amp output required to start the car. Unless the cold cranking amps are high enough.

Some will and yes the battery will have the depth of discharge capability you are looking for. But ultimately as I explained earlier the battery will have a premature death using it for this purpose.

It does seem like a good idea because you want to use the battery more with the engine switched off. But the reality could prove very expensive. A normal car battery can’t be used because of its limited depth of discharge capabilities. So what can you do?

You could think about keeping a portable solar generator in your trunk. It can be charged with your car cig port as you are driving along or with a solar panel or mains socket. Because it has a built-in inverter you can charge electronics and run appliances from it. Saving your poor car battery from an early grave.

If you would like to take a look at a small portable solar generator you can do so here;

I hope you enjoyed my post and have found it helpful. If you have any questions about using a deep cycle battery in a car. Or want to leave your own personal review, please feel free to leave a comment below.

Spread The Solar Love

2 Comments on “Can I Use A Deep Cycle Battery In My Car? – Replacing The Starter Battery”

  1. Hi Steve. I recently bought a used car and have been wanting to replace its battery with a deep cycle battery, that is why I went online to do more research. Based on your detailed explanation, I think I am going to pass the idea for now and just buy a battery that looks exactly like the one it had. I am going to do some more consideration about going with the deep cycle battery in the near future. Thanks for the well-detailed explanation here. It went a long way. 🙂

    1. Hi Dave,

      I am pleased you decided to stick with a normal battery for your car it makes sense. Yes, there are people out there saying that they have run their cars using a deep cycle battery for ages and they haven’t had any problems. 

      And some deep cycle marine batteries do produce the amount of cold cranking amps required to start the car without harming the battery. But that is for use with a boat which isn’t being started anywhere near the same amount of times your average car is. 

      The fact remains if you use a deep cycle to both start a car and draw from it to power devices,  eventually it will die. And sooner rather than later. 

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