​Is a Solar Battery Necessary for my Solar Energy System?

The short answer is…YES. But, where to start your journey to find the right battery for your system is another question altogether. There are so many options available, all designed for different types and sizes of systems, that if you are new to solar technology, it can make your head spin. That is where we come in. As educators, our goal is to create a resource for the average person (without a degree in Physics, Electrical Engineering, or Math) to be able to compare the pros and cons of the different battery types and their long-term operating cost.

What are the 4 Types of Solar Batteries?

Solar batteries are deep cycle batteries, which are designed to cycle from a full charge to a safe depth of discharge and back to full charge…over and over again. They are designed to be charged and recharged in a variety of ways including solar panels, DC to DC chargers, AC to DC chargers, inverter chargers, AC converters, and more. The energy stored in these batteries can be used to directly power 12V appliances and/or 120V appliances with the assistance of an inverter.

It is important to note that deep cycle batteries have a different chemistry than the average car battery – which is also referred to as a CCA (Cold Cranking Amps) battery. A CCA battery is designed to deliver a lot of battery current (amps) quickly to get your car started and is not appropriate for a solar installation.

Renogy manufactures 3 of the different types of batteries in discussion here, except for the flooded lead acid battery. For sake of comparison, we chose a popular selling model on the market so that we could compare all 4 types of solar deep cycle batteries.

Lead Acid Battery – These flooded batteries were the industry standard for many years, and recent improvements have made them last longer. They are the least expensive batteries on the market, but will only survive a few years of regular use. These batteries require some maintenance, should not be housed inside your living space due to potential off-gassing, and fluid levels should be checked regularly.

AGM Battery – Absorbent Glass Mat is a type of sealed lead acid battery which uses a fiberglass mat between the battery plates. This allows the electrolyte solution to exist in a “dry” or suspended state, rather than in a free, liquid form. AGM does not require any additional fluid, off-gasses minimally, and is maintenance free. With a wider temperature range, they can be a popular choice for folks in colder climates.

Gel Battery – Another type of sealed lead acid battery, these use a silica compound to thicken the electrolyte solution in the battery. They offer more flexibility with installation options because they can be installed on their side. With no maintenance or off-gassing as well as a long-life span, they can be a great investment over the traditional flooded version. They do have a higher internal resistance than AGM which leads to longer charging times.

Lithium Iron Phosphate Battery (aka LFP) – This is a newer technology for deep cycle batteries, and differs from the Lithium Ion batteries that are common to hand-held electronics. The LFP chemistry is much more stable, not prone to overheating, and maintenance free. Because they do not off-gas, the options for installation can include the living space of your RV, van, or boat. With a long cycle life and higher charge and discharge rates, these batteries represent an upfront investment that pays off within just a few years. Below you will find a side-by-side comparison of many of Renogy’s leading LFP batteries, including 12V, 24V, and 48V options. For an in-depth blog which breaks down the differences between these system configurations and helps you choose the right one for your situation, click here.

Which Solar Battery is Best for My Lifestyle, Budget, etc.?

A couple of years ago, we wrote a blog which included a direct side-by-side comparison of the 4 leading solar batteries at the time. This resource helped the average solar user compare and contrast capacity, performance, lifespan, specifications, and provided a cost breakdown of deep cycle batteries for off-grid applications. Just 2 years later, Lithium Iron Phosphate (LFP) battery technology has changed so significantly that it is high time we create a new chart which includes all of the new Renogy LFP batteries, price drops, new features, voltages options, and more!

In this blog, you will find 3 side-by-side comparison charts for the following battery types:

1. Four major solar battery types (Lead Acid, AGM, Gel, LFP)
2. Five 12V & 24V LFP batteries* – specifically highlighting new LFP features
3. Three 12V & 48V LFP batteries* – highlighting Renogy’s higher-end batteries

*Both of the LFP charts compare all the bells and whistles that have come on board with Lithium Iron Phosphate batteries over the past 2 years so you can easily see which battery may be best for you!

Be sure to read the discussion below the charts for more information about each topic/category.

CHART 1: COMPARISON OF THE 4 DIFFERENT SOLAR BATTERY TYPES

Specific batteries in chart above: EverStart, Renogy AGM 12V-100Ah, Renogy Gel 12V-100Ah, Smart LFP 12V-100Ah

CHART 2: COMPARISON OF 12V & 24V LITHIUM IRON PHOSPHATE BATTERIES

Specific batteries in chart above: Renogy Smart LFP 12V-100Ah, Renogy Self-Heating LFP 12V-100Ah, Renogy LFP 12V-100Ah with Bluetooth, Renogy 24V-50Ah, Renogy 24V-25Ah

CHART 3: COMPARISON OF HIGHER END 12V & 28V LITHIUM IRON PHOSPHATE BATTERIES

Specific batteries in chart above: Renogy LFP 12V-200Ah with Bluetooth, REGO LFP 12V-400Ah, Renogy 48V-50Ah

Notes About the Comparison Charts: Performance and battery lifespan depend on many factors, including: temperature, percentage of discharge, frequency of use, availability of charging source, months in storage, etc.

Assumptions on data: Operating temperature of 25°C (77°F), Depth of Discharge (DoD) 50% for Lead Acid, AGM and Gel (12.0V) and 80% for Lithium Iron Phosphate (12.8V).

*Data from northeastbattery

**Data from simpliphipower

All other chart data from Renogy’s product manuals and product specifications available on the website at Renogy.

Below is a discussion of each category listed above…

For All Battery Types

Price Tag

When looking at a side-by-side comparison of price tags, it’s pretty easy to see which battery will make the smallest dent in your pocketbook. But, keep in mind, this is just the upfront cost and not the whole financial picture. In addition to the price tag, it’s important to look at amp hour capacity, watt hour capacity, depth of discharge, number of cycles, typical lifespan, and ultimately cost per watt hour in order to get the full picture of cost over the lifespan of the battery. You may also want to examine some lifestyle considerations such as charging time, maintenance, weight, and more. So, let’s take a deeper look (pun intended!) at these important aspects of each type of battery.

Nominal Voltage

In the world of energy, voltage is pressure, while amperage refers to current -- like water flowing through a hose or pipe. Nominal voltage is a value assigned to designate a battery’s voltage “class” (i.e., 12V, 24V, 48V or 120V). It is equal to the supply circuit system voltage to which the unit may be connected. You may consider it an “approximate” voltage and all systems have a “safe” range.

You’ll note that all 12V-100Ah lead acid batteries have a nominal voltage of 12.0V while 12V-100Ah LFP batteries can handle a bit more, usually in the 12.8V range. What this means is that you get a bit more bang for your buck because LFP are a higher capacity battery. And, when you are looking at higher voltage batteries, this number simply doubles or quadruples.

Capacity & Depth of Discharge

Deep cycle batteries are rated in amp hours (Ah). While this number is important, where the rubber hits the road is the amount of energy / work the battery can specifically deliver in each cycle. This number is expressed in watt hours (WH), which is equivalent to amp hours x volts (e.g. 100AH x 12V = 1200WH). While this seems simple and straightforward, what most people don’t realize is that different battery chemistries have different Depths of Discharge (DoD), which is the percentage of charge that you can safely use without doing damage to the battery. For instance, you can only use 50% of a lead acid battery’s total capacity in a given cycle. In other words, if the battery has a capacity of 1200WH, you can only use up to 600WH per cycle without damaging the battery. With LFP, the battery can be discharged by 80%, and occasionally up to 100%, without significantly shortening the lifespan of the battery; thus, if the battery capacity is 1280WH, you can routinely use 1024WH and sometimes use all 1280WH. LFP batteries are almost a 2-for-1 deal when you consider the energy they can store.

Cycles

Cycles are the number of times a battery can go through a discharge and recharge state; in other words, it’s the lifespan -- like dog years for a battery! By following the recommended degree of discharge (DoD), your battery will live longer and give you back more cycles. As we just discussed above, the recommended DoD for any lead acid battery is 50%, for LFP is 80%. With 4000 available cycles over its lifespan, the LFP battery offers up to 16 times more cycles than flooded lead acid (250 cycles). This means that the potential lifespan watt hours (or total energy/work) are far greater for LFP!

Cost Per Watt Hour

As mentioned above, the upfront cost of the batteries is pretty clear – LFP can be 4-5 times as much as flooded lead acid. However, AGM, gel, and LFP batteries offer many more cycles, making the cost per watt hour decrease significantly. If you are using one full cycle per day throughout the lifespan of a battery, you might go through 5 Gel batteries in the time it takes you to wear out just 1 LFP battery, which reduces the cost benefit, especially if you have more than one in your battery bank.

To reach our calculations for cost per watt hour, we multiplied each battery’s WH capacity by its DoD%. Then, multiplied that product by the average number of cycles. For instance, for AGM: 1200WH x .50 DoD x 450 cycles = 270,000 Potential Lifespan Watt Hours. Then, divide the upfront price tag of the battery ($270) by the Potential Lifespan Watt Hours (270,000). $270 ÷ 270,000 = $.001000 per watt hour.

Maximum Continuous Charging / Discharging Current

Lead acid batteries typically charge more slowly than LFP batteries because their maximum continuous charging currents are much lower. Typical AGM/Gel batteries have a maximum charging current of 30A while Renogy’s LFP batteries are all rated at 50A. So if the sun is shining brightly on your panels, LFP batteries will charge nearly twice as fast as their lead acid cousins. Discharging current refers to how much energy your batteries can deliver in one hour to power your appliances. Again, there is a significant difference between lead acid and LFP batteries, as noted in the first chart. If your system requires a large draw of power at any given time (i.e., you typically run many appliances at once), it is better to purchase a battery bank with a higher rate of discharge to meet your energy needs.

Size & Weight

Another consideration on saving money is size and weight. Since LFP has nearly double the WH capacity of lead acid, you can install a smaller battery for roughly the same energy storage at a fraction of the weight. Weight contributes to the overall fuel economy of your vehicle, which is something to keep in mind when thinking about the investment.

Storage Loss

If you store your batteries for a long period of time without using them, they will lose their stored energy to a greater or lesser degree depending on their chemistry type. Temperature plays a role in this percentage of loss. For example, AGM and gel batteries will lose about 3% per month, while the chemistry of LFP is more stable and will only lose <1% per month. To prevent this energy loss, many people remove their batteries during storage and bring them inside a heated (or warm enough) garage or basement to keep them safe during the winter. Others prefer to use a solar battery trickle charge maintainer on lead acid, AGM, and gel batteries during extended periods of storage. And, you’ll note that some LFP batteries are now self-heating!

Storage Temperature

Since all deep cycle batteries have a different chemistry, they can handle different temperatures. In order to keep a battery healthy while not in use, it is important to note the optimal storage temperature for that specific battery. Storage temperature comes into play if you only use your deep cycle batteries for a portion of the year, and of course, in what type of climate your RV or boat will be stored.

Temperature Range (In Use)

It took us a while to get our heads around this concept, as the terminology can be confusing. The temperature range for charging and discharging are different for each type of battery. Discharging a battery means that you are using it to run and/or charge up your household appliances. Charging a battery means that energy is coming in from your solar panel or other power source. For instance, let’s say it’s wintertime in Montana and you go to bed with plenty of charge in your battery. As long as the battery itself remains above -4° F, it will continue to discharge and run your appliances throughout the night, (it will only stop discharging if the battery’s core temperature drops below -4° F). Then when the sun comes out the next day and hits your panel, as long as the battery itself remains above 32° F, the battery will receive energy from your solar panel and charge normally. If the battery’s core temperature drops below 32° F, it will not allow the solar panel or other power source to charge the battery (unless you purchase a self-heating battery which automatically heats the internal core temperature of the battery when it drops below 41° F). The same is true on the upper end of the temperature range, but it is very unlikely that a battery will get warmer than 131° F. Since LFP batteries can occupy the same space that you do, these temperature ranges should not cause a problem. But, if you must store your batteries outside your RV or cabin, temperature could become an issue.

Connection Configuration

If you are using more than one battery to power your life, you may want to set them up in series (to increase voltage) or parallel (to increase amperage). With all types of lead acid batteries (flooded, AGM, and gel), you can configure them in either series or parallel, but with LFP, you are limited to parallel only.

Installation Considerations

All flooded lead acid batteries need to be mounted upright because of the potential for spilling the liquid solution inside. They also must be vented to the outside, most in fact are stored in a special compartment or box on the tongue of the trailer, because of the potential off-gassing which occurs during charging. AGM and gel batteries have minimal off-gassing, and just need to be in a vented area (no sealed boxes). These can be installed in a variety of orientations which increases installation options. LFP batteries do not need to be vented, can be stored inside, and offer the same installation flexibility as AGM and gel.

For Lithium Iron Phosphate Batteries Only

Self-Heating -OR- Smart Energy Saving Mode

One of the greatest leaps in battery technology over the past couple of years have been these added features, and they are complete game changers for many of us who rely on solar energy year-round, such as the scenario above. To compensate for the temperature limitations of LFP batteries, some have a self-heating feature which keeps the battery core temperature above 41° F, so it will work throughout the year no matter how cold it gets outside! This self-heating feature does use a bit of the battery’s stored energy, but it’s worth its weight in gold for those who live in colder climates. On the other hand, Smart Energy Saving Mode is a new feature available on Renogy’s 24V battery line, as it automatically switches into this mode within 30 seconds if the system finds that the battery is not charging or discharging.

Bluetooth: Built-In -OR- Capable

Bluetooth has changed the way solar users can monitor their equipment. Previously, a shunt connected to the battery’s negative terminal then wired to a battery monitor, was the most reliable method to monitor a battery. With Bluetooth, users can monitor batteries, a charge controller, inverter, etc. from afar with the Renogy DC Home app and/or the Renogy ONE. Built-in Bluetooth means that this feature is part of the battery itself. Bluetooth capable refers to a port which allows the user to add a BT-Module, an easy to install and inexpensive addition.

Battery Management System

BMS refers to a computer chip in the battery which runs a health check on the battery and prevents it from overcharging or over discharging – two things which can shorten the lifespan of a battery significantly. BMS helps you get the most of your battery purchase in the long run!

We hope these charts and discussion assist you in finding a battery that is best for your lifestyle, energy needs, climate, budget, and more. If you need further explanation or help with troubleshooting your solar installation, give us a shout! We are educators by trade and love to help people get started with solar. If we don’t know the answers, we’ll get in touch with our Renogy Engineer friends. Please contact us via our website or social media listed below.

Shari Galiardi & David Hutchison have turned their higher education backgrounds, desire for life-long learning, and thirst for adventure travel into writing, photography, video production, and public speaking tours from coast to coast. Known to their friends as simply Shari & Hutch, you can learn more about their full-time, solar powered adventures on their website at freedominacan.com. Or, follow them on Facebook, Instagram, and YouTube as “Freedom in a Can.”