What to Know About Deep Cycle Batteries for Solar Storage

What to consider when selecting a solar storage solution

Authors Note: This has been updated on Feb 17, 2022 with updated information, links, and resources.

So you’re excited about solar and have finally selected your solar panel kit. What you may not have yet considered is what kind of deep cycle battery you’ll need for your setup.

If you’re in an RV or van, you’ll need a way to store that energy produced from your panels. If you’re going solar at home, you may be interested in going off-grid or hybrid and utilizing battery storage. There’s a lot to take into consideration when deciding on a solar storage solution. Let’s break down everything you need to know about storing solar power with deep cycle batteries.


What are deep cycle batteries?

Deep cycle batteries look similar to car batteries, but are actually very different. In contrast to car batteries which only provide short bursts of energy, deep cycle batteries are designed to provide sustained period over a longer period of time. Deep cycle batteries can be discharged up to 80%, but most manufacturers recommend not discharging below 45%. Regularly going beyond that point will shorten the life of the battery.



What is the difference between a deep cycle and a regular battery?

We often get asked if solar deep cycle batteries are different from regular batteries. Regular batteries like those used in cars produce a shorter burst of electricity. But deep cycle batteries can produce ongoing, lower yet consistent, levels of power.

Deep-cycle batteries are popular for off-grid or hybrid solar systems because they can be completely discharged and don’t aren’t damaged as quickly as normal batteries can be. For example an acid lead-acid battery, can only be discharged at a maximum of 50% to extend its useful life.

When using batteries for solar panels as part of a home solar system, you’re able to store the excess electricity your panels produce instead of sending that energy back into the grid. Electricity will be sent to the grid if your batteries are fully charged and your panels are still producing energy.

Do I need battery storage?

Not every solar power setup needs battery storage. If you’re grid-tied, there’s no requirement to add a battery, however hybrid solar solutions are increasingly popular. But if you want to live an off-grid lifestyle or live and travel to locations where grid power is unavailable, you will need a solar battery. Here are some situations where you may need a deep cycle battery.

If you live in an RV, camper, or motorhome:

Yes, if you live in a van conversion, RV or motorhome you will need solar storage.
We highly recommend battery storage like a Renogy deep cycle battery in your RV. By adding solar storage to your RV solar set up, your solar panels, and batteries can take the place of a gas-powered generator. You’ll be able to keep things running even when your panels aren’t producing energy.

If you need solar on a boat:

Yes, if you have solar on your boat it's important to purchase the right solar battery for your system.
While it’s often tempting to just purchase any marine battery you shouldn’t assume that it’s safe. In reality many marine batteries aren't suitable for solar installations. Instead, a solar deep cycle battery will be more reliable so you have energy when you’re on the water.

If you live in a home with access to the electrical grid: /strong>

If you have access to the electrical power grid, then solar storage is not a requirement but is useful in case of outages.
Home battery storage can be very beneficial for homeowners, but it is not a requirement. However, it’s increasingly popular to have a hybrid solar system that can access both the grid and solar battery storage. If you’d like to go off-grid or be able to run your solar panels in the case of grid outages, battery storage is a great way to ensure continued energy access.

What are the different types of batteries?

There’s a range of deep cycle battery options.The most common ones used for solar installations are flooded lead acid, sealed lead acid, and lithium iron batteries. Flooded lead acid batteries are the most inexpensive option and are available at most big-box and auto stores. Sealed lead acid batteries store 10 to 15 percent more energy than lead acid batteries and charge up to four times faster. Lithium iron batteries are the most expensive options, but also last four times longer than lead-acid batteries and weigh much less.

Flooded lead-acid batteries

Flooded lead-acid batteries are common and the most inexpensive battery option. These batteries are available at most big-box and auto stores.

In a flooded lead-acid battery, lead plates get submerged into an electrolyte mix of water and sulfuric acid. A chemical reaction then occurs during charging and discharging which produces gases that get vented from the battery. This venting process creates a drop in the electrolyte levels, which then need to be periodically topped up. This means that the usable capacity (how much battery power can be used before it needs to be recharged) of a solar flooded lead-acid battery falls around 30-50%.

Flooded batteries are affordable, reliable, and fairly tolerant of overcharging. However, they do require proper ventilation to release gases and must always be stored upright. Upright storage is necessary to avoid electrolyte leakage which makes these batteries impractical to store in some settings. This battery option also needs the most maintenance and has a shorter lifespan compared to other types.

Sealed lead-acid batteries

A valve regulated lead–acid (VRLA) battery is commonly called a sealed lead–acid battery (SLA). Lead-acid batteries are further categorized as either flooded lead-acid batteries or sealed lead-acid batteries.These Sealed lead-acid batteries store 10 to 15 percent more energy than lead-acid batteries and charge up to four times faster.

One of the benefits of lead-acid batteries is that they cost much less up front than some other battery options. However, on the downside they also have a shorter lifespan and do require much more regular maintenance to keep them running properly.

Lithium-ion batteries

Lithium-ion batteries are the most expensive solar battery option, but also last four times longer than lead-acid batteries and weigh much less. Because they are lightweight these often appeal to boat, van, or RV owners.

Lithium batteries are relatively new options when compared to lead-acid battery varieties. The newer types of lithium batteries are called Lithium Iron Phosphate (LiFePO4). These LiFePO4 batteries are frequently used in deep cycle battery applications — such as backup power systems and solar energy banks.

These batteries are 30% lighter in weight than flooded cell batteries and have a good usable capacity of between 80-100%. Lithium-ion batteries also have the fastest recharge rate of these three deep cycle options and have an extremely long cycle life.

A lithium-ion battery also offers a better and more constant voltage over any rate of discharge. This means if you have lithium ion-powered lights that they won’t dim slowly as the battery loses charge over time. Instead, the lights will just go out when there’s no more power.

The lithium deep cycle battery is considered by many to be the best battery option because it’s lightweight, compact, and maintenance-free. It also has an excellent usable capacity, a fast recharge rate, and reliable constant voltage. Despite having many benefits, the downside of lithium deep cycle batteries is that they’re often much more expensive than other options like lead-acid batteries. They also typically need a battery maintenance system (BMS) that monitors the battery’s safety and state. A BMS is usually equipped internally within deep cycle applications.



What should I consider when deciding on a deep cycle battery for my solar panels?

When shopping for deep cycle batteries for your solar installation, there’s some different factors to consider: price, capacity, voltage, and cycle life.

Price: Batteries can vary from around $100 for the cheapest lead acid battery to more than $1,500 for a lithium iron battery. Be sure to consider the ultimate lifetime and not just upfront costs, as you will have to replace lead acid batteries before you will need to replace a lithium iron battery. You’ll also need to do more maintenance on a flooded lead acid battery, and we all know time means money.

Capacity: Battery capacity is important because it measures the amount of energy you can store. If you need to power certain appliances for long periods of time, you'll need more batteries to carry a bigger load. Capacity is measured in total amp hours.

Voltage: Be sure to check the voltage of the battery bank to ensure it is compatible with your panels and the rest of the system, particularly your solar panels. Panels typically come in either 12V and 24V options. Most RV’s and boats typically use 12V battery banks, so people usually stick with the 12V panels. The advantage of using a higher voltage battery bank is that is saves you money in the long run as you need less charge controllers and can use thinner cables for the same amount of power. If your energy needs are over 3KW, go for 48 volt system. Large off-grid houses often use 48V.

Cycle Life: This specifies the number of discharge and charge cycles a battery can provide before the capacity drops below the rated capacity. This varies sharply from technology to technology and is measured in number of cycles.

Which deep cycle battery is best for me?


Flooded Lead Acid: Cheap, but high maintenance
Cost: Around $100

Flooded lead acid batteries are the cheapest, but also require the most maintenance. You have to check water levels with a hydrometer and add water to keep them topped off each month. Lead batteries must be housed in a ventilated room since they emit gases. This is not necessary with lithium-ion batteries.



Sealed Lead Acid (Absorbed Glass Matt and Gel): Little maintenance, lower charge rates
Cost: $239-$449
Shop on Renogy for Gel and AGM Batteries

What is an agm battery and Gel battery? They are absorbent glass matt (AGM) and gel batteries, the two types of sealed lead acid batteries. Contrary to flooded lead acid batteries, sealed lead acid batteries require little to no maintenance and are spill-proof. They are more expensive than flooded lead acid batteries, but also have a much longer cycle life.

Gel batteries, which use silica to stiffen the electrolyte solution in the battery, tend to have lower charger rates and output than absorbed glass matt batteries. They also can’t handle as much current, meaning they take longer to recharge. However, gel batteries have a greater lifespan than AGM batteries and can be mounted in any orientation. Absorbed glass matt batteries offer a better temperature range and are a bit cheaper than gel batteries.



Lithium Iron Phosphate: Expensive, but zero maintenance and long lifespan
Cost: $499-$1499
Shop on Renogy
12 volt lithium battery

Lithium iron phosphate batteries are the most expensive battery option, but they have an extremely long cycle life, high discharge and recharge rates, and are incredibly compact and lightweight. They also require little to no maintenance.

Lithium batteries typically have a lifespan of at least 10 years. Lithium iron phosphate batteries also lose less capacity when idle. This is especially useful in cases where solar energy is only used occasionally. They also have the best cycle life of deep cycle batteries, offering approximately 2000 cycles at 100% DoD (depth of discharge.)



Do lithium batteries charge faster than flooded lead acid batteries?

Yes! As mentioned above, lithium iron phosphate batteries are more efficient and have a faster rate of charge. This is because they can typically handle a higher amperage, which means they can be recharged much faster than sealed and flooded lead acid batteries. Lead acid batteries are limited in how much charge current they can handle, mainly because they will overheat if you charge them too quickly.

How many deep cycle batteries will I need?

The amount of battery storage you need is based on your energy usage. Energy usage is measured in kilowatt hours. For example, if you need 1,000 watts for 8 hours per day, then your energy usage is 8kWh per day. A battery capacity of 4 to 8 kWh is usually sufficient for an average four-person home.

To size a system that will best fit your needs, we recommend making a list of all the devices you plan on running. Get the wattage information, or the amps and volts of the product, and provide an average run time per device. The Renogy solar panel calculator is a great tool that makes it a quick and easy process to help determine your specific needs.



Can I wire different deep cycle battery types and sizes together?

We recommend wiring batteries of the same type and amp hour rating, i.e. if you go with absorbed glass matt batteries, all the batteries in your battery bank should be AGM batteries. This will limit any efficiency loss due to having different batteries.



Are solar batteries safe?

Yes! In general, solar batteries are very safe. Issues that may arise come if they are installed incorrectly or the battery quality is low. Because of that, it is important to ensure batteries are properly installed and are purchased from a reputable manufacturer.

Lithium iron phosphate batteries have a slightly higher risk of overheating than others if installed incorrectly or supplied from an unreliable manufacturer. However, in most cases lithium iron batteries rarely pose a risk to homeowners. Renogy deep cycle solar batteries have a BMS, which stands for Battery Management System. The BMS safely protects the battery from being used/charged during incorrect conditions. Also, a battery charger 12v can enhanced the safety and efficiency of a solar battery.  


How long will it take to charge a deep cycle battery?

Total charging time depends on the weather, as well as state and type of battery. If a battery is completely drained, a panel can typically charge the battery within five to eight hours.

The total charging time will vary depending on the state of a battery. If a battery is totally drained, a solar panel can energize the cells within five to eight hours. The position of the sun in the sky can impact a panel’s charging speed. When sunlight shines directly on a panel in the middle of summer, the charging speed will be faster. Charging cycles are slower on cloudy days.



What is the lifespan of deep cycle batteries?

Battery life varies a bit from technology to technology. For example, many gel batteries typically last 1,100 cycles, absorbed glass batteries 600 cycles, and lithium iron phosphate batteries 7,000 cycles. It’s fair to assume the general range for a solar battery’s lifespan is between 5 and 15 years. Therefore, you may need to replace your batteries at least once within the lifespan of the rest of your solar system.

Although many people focus on the performance of solar panels when planning a solar installation, one cannot afford to forget to take the time to weigh the different battery options available to you. After all, a poor functioning or under performing battery can reduce the overall solar system performance and end up costing you thousands in the long run.



Conclusion

Deciding on the right solar storage solution can be challenging with all of the deep cycle battery options available. Flooded lead acid, sealed lead acid, and lithium iron phosphate all have their own advantages, from maintenance levels, to cycle lives, and charge rates. There’s not a one-sized-fits-all solution, but by weighing those costs and benefits you’ll be able to build a battery bank that works best for you. Now it's time to select your own solar storage system. Whether you want a 12 volt deep cycle battery, 48v battery, marine battery, or other type of batteries, you can find a suitable one at Renogy store!