Guide to deep cycle batteries from RVs to solar
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Whether you’re looking for an energy storage solution to pair with home solar panels or a way to run your trolling motor, a deep cycle battery is the answer. These batteries are specially designed to charge and discharge many times during their useful life, providing battery power to devices and appliances for many years.
There are important things to learn about deep cycle batteries, including the various types, how they differ, and the applications for which each works well. So let’s jump right into the deep end and get to it!
A deep cycle battery is a battery that is meant to be charged and discharged regularly and can offer a steady stream of power while it discharges. Although they come in many sizes, they mostly look like a standard 12-volt car battery, housed in heavy-duty rectangular boxes with positive and negative terminals on the top. Like car batteries, deep cycle batteries often use lead-acid technology, with lead plates connected to the terminals and suspended in an electrolyte solution containing sulfuric acid, but lithium batteries are becoming more popular.
Gasoline-powered cars use their 12-volt battery for three things: providing power to the ignition switch, activating and powering the starter motor, and running the lights. That’s why a car battery is also called a starting, lighting, and ignition (SLI) battery.
In order to provide the very powerful burst of energy the starter motor needs, an SLI battery has to draw lots of electrons from its electrolyte solution. To do this, an SLI battery uses many thin metal plates connected to its terminals. This increases the surface area that’s in contact with the electrolyte, allowing the battery to draw lots of electricity at once.
Some of Trojan’s deep cycle lead acid batteries for solar storage. Image source: Trojan
In contrast, a lead acid deep cycle battery has thicker metal plates. This means the battery can’t draw a very high current through its terminals like the SLI battery, but instead can provide a steady stream of power, and its thicker plates help it avoid corrosion even when the battery is discharged. In addition, deep cycle lead acid batteries come in many sizes, because they don’t have to fit in the standard auto battery enclosure.
All lithium batteries qualify as deep cycle batteries because they can be fully discharged and recharged. Lithium batteries are built a little differently, though. They use cylindrical cells with the electrolyte between two electrodes that control the flow of electrons. Many of these cells are wired together to produce a battery pack, which is then encased in some sort of enclosure with positive and negative terminals on it. Unless you’re buying raw cells to make your own battery, lithium batteries almost always come with an onboard battery management system (BMS).
A lithium battery pack before being installed in an enclosure. Image source: AA Portable Power
As mentioned above, there are two basic kinds of deep cycle batteries that are commonly used today: lead acid and lithium batteries. Lead acid batteries can be further broken down into the categories of flooded lead acid (FLA) batteries, which require regular watering and maintenance, and sealed lead acid (SLA) batteries, which are maintenance-free. Within the SLA sub-category, there are absorbent glass mat (AGM) and gel batteries. You can read about these different types in our full article on lead acid deep cycle batteries.
Deep cycle Lithium-ion batteries are mostly very similar, but there are two specific types that differ in the materials used to make their cathodes (positive terminals). One type uses Nickel, Magnesium, and Cobalt, and is referred to as a lithium NMC battery, while the other uses lithium iron phosphate, with a chemical composition of LiFePO4. These are called LFP batteries for short. In general, they perform very similarly. LFP batteries have the advantage of being safer, less toxic, and more robust than NMC batteries, but also tend to be heavier and more expensive.
There are a few important concepts to understand in order to be informed about the benefits of different types of battery technology. These are:
Voltage is related to how much power the battery can push out at one time. Common voltages are 6V, 12V, and 24V. A 12V battery is often used by itself for mobile uses, while battery banks for home energy storage can be made of several low-voltage batteries wired in series to increase the total voltage to 24V or 48V.
Capacity, or amp-hours, tells you how much energy a battery can deliver. For deep cycle batteries, this discharge rate is measured over 20 hours. For example, a 100ah battery can discharge at a rate of 5 amps for 20 hours until it is no longer able to produce power.
Depth of discharge (DoD) describes the amount of capacity that can be safely used without potentially damaging the battery. Yes, it's true that some batteries tease you with a larger capacity than you can safely use.
The guideline for FLA batteries is to discharge them to no more than 50% of their capacity for maximum lifespan. A very deep discharge of an FLA battery can cause irreparable damage to the plates inside. It's recommended to never deplete these batteries below 20% of their capacity.
Gel and AGM batteries can be discharged to 100% of their rated capacity, but most experts still advise a DoD of no more than 50% for maximum battery life. As you know if you have a cell phone, lithium batteries can be fully discharged and fully charged back up (100% DoD), but an 80% DoD is often recommended by manufacturers to ensure they last as long as possible.
That brings us to battery life, which is measured in charge cycles. This is related to DoD, in that the number of discharge cycles the battery can handle goes down when DoD goes up. For example, a gel battery can survive for 1,000 cycles at 50% DoD, but only 450 cycles at 100%. In contrast, an AGM battery can typically go through 500 cycles at 50% DoD, but only 200 at 100%.
It's important to note that the rated battery life does not mean the battery is expected to immediately become unusable afterward. Instead, a battery's useful life "ends" when it is reduced to 80% of its original capacity.
Here’s a quick guide to the different kinds of batteries and their cycle life and recommended DoD:
|Battery type||Recommended DoD||Average Cycle life @ recommended DoD|
The various kinds of deep cycle batteries have their own best uses.
Flooded lead acid batteries are best used for stationary applications. They have the advantage of being relatively inexpensive and are easy to wire into battery banks for off-grid solar energy storage and backup power applications. Their disadvantages include the need for regular maintenance and the fact that their electrolyte can spill unless the batteries are held stationary.
AGM batteries are spill-proof and vibration-resistant, so they can be used in vehicles like golf carts, wheelchairs, and scooters. They are also commonly used as UPS backup batteries and in solar backup power applications. They are somewhat more expensive than FLA batteries, must be charged carefully to prevent overcharging, and do not work in very cold environments.
Gel cell batteries, as their name implies, have their electrolyte suspended in a silica-based gel, This makes them very resistant to shocks and very safe for use for things like motorcycles and trolling motors. Because the electrolyte is already a thick paste, they are less susceptible to freezing in low temperatures. Cons of gel batteries include relatively slow charging speeds and high initial cost.
Lithium batteries have been used in electronic devices like cell phones for a long time and are quickly becoming the best choice for large applications like cars, home solar backup, and portable solar chargers. They can be discharged to 100% without too much damage, aren’t susceptible to vibration shocks, and have long cycle lives. The disadvantages of lithium batteries are their high initial cost (although that’s coming down) and the possibility of NMC batteries exhibiting thermal runaway (catching fire) when they’re damaged. As far as ease of use, capacity, and lifespan, they are the best choice for home solar backup.
There is no one “best” deep cycle battery. Instead, there are great deep cycle batteries for many different applications. If you’re looking for the cheapest option and can handle the maintenance, get an FLA battery. If you need a simple, vibration-resistant battery with medium up-front cost, a gel or AGM battery would work best for you. If money is no option and you want the easiest, most powerful option, go lithium.
Here’s a rundown of some of the best batteries of each type on the market:
Trojan has been the #1 name in flooded lead acid deep cycle batteries for solar energy storage for a long time. They're so popular, in fact, that they currently have a 4-5 month lead time on new orders. If you can't wait that long, batteries from Rolls and Crown are also very good, and currently available.
When it comes to stationary solar power storage, Concorde Sun Xtender batteries are second to none. They can be a little heavy and bulky but are excellent, long-lasting batteries. If you're looking for something that can be installed in an RV for solar storage, the Weize 12V 100AH battery on Amazon is a better choice.
Gel batteries are not generally recommended for solar storage in residential applications. The batteries we’ve listed here from Mighty Max and Weize are good for smaller applications where quick recharge isn’t necessary, such as powering a trolling motor or mobility scooter.
For durable, robust home energy storage, it's hard to find a better battery than those made by SimpliPhi. Using LiFePO4 chemistry and with a lifespan of up to 10,000 cycles at 80% DoD, these batteries are built to perform for many, many years. They can be quite expensive up-front, but that high cycle life and DoD means the ultimate cost per kWh of throughput is much lower than other batteries on the market.
For more portable solutions like small storage in an RV, BattleBorn batteries receive high marks from people in the DIY and off-grid communities. But for the most part, these batteries are drop-in solutions for an established storage system. In the case that you'd prefer the simplicity of an all-in-one solution, Bluetti portable solar battery banks are an excellent choice. Bluetti's solar batteries all include an MPPT solar charge controller, battery management system, pure sine wave inverter, AC and DC outlets, and multiple ways to charge.
Whether you live a life on the go or prefer to stay home, deep cycle batteries are essential if you need to store energy for later use. They are the perfect match for a home solar system, or even just a portable solar panel. As our world becomes more reliant on devices powered by electricity, it will be essential for people to have knowledge of deep cycle batteries and all they can do.