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What is a deep cycle battery?
A deep cycle battery is a battery that is meant to be used as a longer-term power source typically for a small motor or other device. Compare a deep cycle battery to a starting battery, like the one in your car, and you’ll see a significant difference.
A starting battery is meant to provide a lot of energy in a short cycle, or amount of time. In other words, to start the engine of the car. A deep cycle battery is meant to provide a smaller amount of power over a longer period of time, similar to your cell phone battery.
Deep cycle batteries are most often used for boating, RV-ing, offgrid power systems, and any other application where you want to have a large portable power bank with you. Think of a deep cycle battery to a trolling motor, how you would think of rechargeable AA batteries to a toy car. They provide a set amount of power and can be recharged when drained.
A deep cycle battery’s capacity will vary depending on its size. Roughly speaking, you can expect the output times below for a 50% depth of discharge and 80% efficiency.
How to tell if a deep cycle battery is bad
There are 3 ways you can tell if a deep cycle battery has gone bad.
- Check its age
- Check its voltage
- Check its appearance
Oftentimes, you’ll need to use a combination of the above to determine if the battery has gone bad. The easiest, hands-off way to test a battery is to take it to your local AutoZone or auto repair shop to have them load test it.
You will need to charge you battery to 100% prior to it being load tested. Leave it on a recommended charger for at least 24 hours, to ensure it reaches a full charge
If you have several vehicles, think cars, jet ski’s, a boat, riding lawn mower, or other vehicles that require a battery, investing in an inexpensive load tester can save you a lot of time and hassle.
Otherwise, start by removing the battery from the vehicle. Do a visual inspection and use a wire brush to clean off any corrosion. Wear a pair of thick latex gloves and safety glasses to prevent any battery acid from getting on your skin or in your eyes.
Below are some common things you will want to check for.
- Damaged or loose terminals - If the battery’s two terminals show any signs of corrosion or are not securely seated, the battery needs to be replaced regardless of whether it holds a charge or not. Damaged or loose terminals can cause a short, which could lead to a fire or explosion.
- Broken case - If the battery’s body or any part of its case is broken, chipped, or otherwise damaged, it’s highly recommended to replace it right away. Some batteries can leak out its acidic contents which is not good for you or the environment.
- Purchase date - When did you purchase this battery and how long has it been in use? Most reputable deep cycle batteries will not last much longer than 5 or 6 years depending on use. Some cheaper models will only last a year or 2 before needing to be replaced.
If everything above checks out, charge your battery overnight using a recommended charger, and use a multimeter to check the voltage after 24 hours. For a 12V battery, which most are, it should read 12.5V or higher. For a 6V battery, it should read around 6.3V or higher.
Anything lower than the figures above and the battery likely needs to be replaced. If you decide to keep a battery that is right on the 12.5V or 6.3V mark, let the battery sit and check it again in another 24 hours. Afterwards, if the voltage drops at all, you will know for sure that the battery needs to be replaced.
How long do deep cycle batteries last?
Generally speaking, a reputable, well-made battery like a Renogy will last around 5 to 6 years with average use. A number of factors play into this like:
- Temperature and environment
- Frequency of use
- Depth of discharge
- Charge speed
- Intensity of use
Batteries that are exposed to higher temperatures, whether through environment, heavy use, ‘speed charging’, or deep depth of discharge, will normally wear out faster than batteries in less stressful conditions.
A general rule of thumb is to only use 50% of a deep cycle battery’s total capacity before allowing it to rest and then fully recharging. Some people opt to have 2 batteries on hand so they can switch between the 2. This allows one to slowly recharge while the other is in use.
What size wattage of a solar panel to charge a deep cycle battery?
For a deep cycle battery, we recommend Renogy’s 100 Watt, 12V starter kit. This 42” x 20” solar panel can be used on boats, RV’s, trucks, and more. For a 100Ah battery, this system can bring the battery to full charge from 50% in about 6 hours. A number of factors affect the time to full charge, such as:
- Sunlight availability
- Age of battery
- Ambient temperature
- Battery type
The great thing about Renogy’s kit is its ability to charge sealed lead-acid, gel, flooded cel, and lithium ion batteries. The kit comes with all the wiring you need, mounting hardware, and corrosion resistant materials.
Group 24 vs. Group 27 Deep Cycle Batteries
What’s the difference between a group 24 and group 27 marine battery? The group number refers to the physical dimensions of the battery. Sometimes there are additional letters added onto the end, like Group 24F, or Group 27H. Those are sub-groups of the larger battery group.
These groups were established by BCI, or Battery Council International - a trade association of manufacturers, as a way to standardize battery size across all platforms, not just boating.
Group 27 batteries are larger than group 24 batteries. Group 27 batteries typically measure close to 12” x 6.8” x 8.9”, while group 24 batteries measure around 10.3” x 6.8” x 8.9”. The table below shows each group’s exact measurements and the measurements of their subgroups.
L x W x H (Inches)
L x W x H (Centimeters)
10.25" x 6.8125" x 8.875"
26.05cm x 17.3cm x 22.54cm
10.75" x 6.8125" x 9"
27.31cm x 17.3cm x 22.86cm
10.25" x 6.8125" x 9.375"
26.04cm x 17.3cm x 23.81cm
10.25" x 6.8125" x 9"
26.04cm x 17.3cm x 22.86cm
10.25" x 6.8125" x 9.75"
26.04cm x 17.3cm x 24.77cm
L x W x H (Inches)
L x W x H (Centimeters)
12.0625" x 6.8125" x 8.875"
30.64cm x 17.3cm x 22.54cm
12.5" x 6.8125" x 8.9375"
31.75cm x 17.3cm x 22.7cm
11.75" x 6.8125" x 9.25"
29.85cm x 17.3cm x 23.5cm
Sub groups are somewhat interchangeable and sometimes you can upgrade from a group 24 to a group 27. This is mainly dependent on your space limitations. Your battery tray may need to be upgraded or wiring may need to be routed in a different direction.
Regardless, always ensure you are using the correct battery type, whether a cranking (aka starting) or deep-cycle. Those two types of batteries cannot be used interchangeably.
AGM vs. Deep Cycle Battery
AGM, or Absorbent Glass Mat, is a type of deep cycle battery. Deep cycle batteries can be broken into 2 main categories with additional sub-categories:
- Lead Acid
- Flooded (FLA)
- Lithium Ion
Generally speaking, when referring to a ‘flooded lead-acid battery’, they shorten it to ‘lead acid’. The terms ‘flooded’ and ‘valve regulated’ aren’t used as often. Instead, batteries are simply referred to as AGM, Gel, or Lithium Ion. ‘Valve regulated’ are sometimes referred to as ‘sealed’.
SLI vs. Deep Cycle Battery
SLI refers to Starting, Lighting, and Ignition. These batteries are typically used in automobiles and as the name implies, are used to start the vehicle. They serve an additional, but less heavy-duty purpose of supplying power to the lights and other accessories.
The emphasis with these batteries is on starting. An SLI battery is meant to supply a large surge of power in a short amount of time. This is 90% or 95% of the battery’s purpose, with the other 10% or 5% to keep the radio or lights on when the engine is off.
Best AGM Deep Cycle Battery
AGM stands for Absorbed Glass Mat. Unlike a flooded battery where you can remove the caps to add additional fluid, an AGM battery is sealed and cannot be opened. This means the battery can be mounted in any position, even upside down, without worrying about electrolyte or battery acid leaking out.
In these batteries, a glass mat made of fiberglass acts as like a sponge between the lead plates in the battery. The fiberglass holds the electrolyte and even if the case becomes damaged, won’t spill.
These batteries are often advertised as maintenance free, and for the most part, they are. You’ll never need to do anything special to keep these batteries running other than the usual cleaning of the terminals or checking the case as you would with any other battery.
Deep Cycle Battery Weights
Remember that in almost all deep cycle and starting batteries there are thick lead plates sitting in an electrolyte solution. These lead plates are essential to the battery’s function, but are far from lightweight. A standard deep cycle AGM battery rated at 100Ah will weigh in the range of 60 to 70lbs (27.2 to 31.8kgs).
The exception to this weight range are Lithium-Ion Phosphate batteries, or LiFePo batteries. These batteries use a different type of technology to store and release energy. Because of this difference, the batteries weigh significantly less than a typical deep cycle battery - around 25-30lbs for a battery at the same rating of 100Ah.
LiFePo batteries are usually slightly smaller in size, last significantly longer, however have a greater upfront cost.
*Expected lifespan is based on normal usage with proper battery care and average lifespans. Normal usage defined as DoD 50% or less per cycle with an average of 120 cycles per year. All batteries listed are rated at 100Ah.
How to Charge a Deep Cycle Battery
The short answer is you must purchase a 12V battery charger, ideally one that charges the battery in different phases if you plan on leaving the charger connected for extended periods of time. This article covers the topic more in-depth, but expect to pay around $100-$200 for a decent, reputable charger.
Purchasing a no-name brand charger could spell disaster for your battery. Otherwise, an underpowered charger may take a week or more to charge a battery and possibly not even to a full charge.
Read more about charging a deep cycle battery in the article below.
Battery Self Discharge
You may see some points listed descriptions like the one below:
This may bring up some questions like:
- What is battery self-discharge?
- Why do batteries self-discharge?
- Do lithium-ion batteries self-discharge?
- What are the best low self-discharge batteries?
What is battery self-discharge?
Battery self-discharge is the slow, gradual release of energy overtime when not connected to a power source or device. It happens to all batteries - starting, deep cycle, even AA’s and AAA’s that you may use on a daily basis.
For example, if you charge your 12V battery to a full 100%, it may lose 1% every day that it sits and is not in use. So by the end of the week your battery will actually be closer to 90% of its full capacity if left off the charger.
If you are looking for a quick reference on the rates of self-discharge for different batteries, see the chart below.
Typical Self-Discharge Rate
1-3% per month
1-5% per month
Flooded Lead Acid
~1% per day
1-3% per month
Why do batteries self-discharge?
Recall that batteries use a chemical reaction to generate electricity. The flow of electrons that results from the chemical reaction is used to generate electricity for whatever device the battery is connected to. The chemical reaction is never 100% ‘turned off.’
There is a minute chemical reaction that will take place where the battery will self discharge. This is common across all types of batteries, even lithium-ions.
Batteries connected to appliances, computers, etc. will draw electricity, even if not in use, and can drain a battery over logner periods of time.
How to reduce self-discharge?
Self-discharge rate tends to increase in higher temperatures, older batteries, and batteries that have a higher cycle count (aka more use). Generally speaking, keeping the battery out of direct sunlight and in a climate controlled environment will aid in keeping your battery topped off.
Using a ‘float charger’ is also recommended if you want to have your battery ready to go at a moment’s notice. A float charger monitors the battery’s capacity and allows it to self-discharge to a certain point, after which it will kick on and top the battery off to full capacity before repeating the process.