At the risk of stating the obvious, solar lights rely on the sun to generate power. When the sun shines on a solar panel, energy from the sunlight is absorbed and converted into electricity thanks to some magic science called the photovoltaic effect. This electricity can then be used to power your light—so long as you need light during the day.
The thing is, we don’t generally need light during the day when the sun is out. We need it at nigh (when it’s not). This challenge is commonly known as “intermittency,” and it would be a significant one were it not for batteries.
Rechargeable batteries like the ones Sol uses allow solar lighting systems to store energy collected during the day so that they can operate at night. They also allow the system to outlast periods of low solar irradiation when new electricity generation isn’t possible.
However, not all batteries are created equal, and not all batteries are right for every project. Whereas some manufacturers offer a single battery and affirm its versatility, Sol offers three—and understands when and where each works best. Here, we’ll cover some basic battery concepts that are helpful for solar lighting customers to know, as well as explain the ins and outs of our battery technologies.
Even if you don’t know much about batteries, you’re probably familiar with ‘capacity,’ which refers to the total amount of energy a battery can store. It’s usually measured in watt-hours (Wh) units and is easily understood as how many watts a battery can expend in an hour.
Depth of discharge (DOD) is the percentage of a battery’s capacity that is consumed relative to its total capacity. For example, if you were to discharge 500Wh from a 1500Wh battery, the DOD would be 30% (500Wh/1500Wh); if you were to discharge the same battery by 1200Wh, the DOD would be 80%.
Some battery types have relatively shallow DODs (~25%), and some are very deep (80% or more). Regardless of the number, what’s important to understand is that if you repeatedly discharge a battery past the recommended level, you will greatly reduce its lifespan.
Along with recommended DOD, manufacturers also estimate cycle life, which represents the number of discharge/charge cycles the battery can experience before performance starts to degrade. Every time a battery goes from 100% to its DOD and back to 100% again is a cycle.
Cycle life varies greatly between manufacturers, chemistries, and how (and where) the battery is used. The deeper a battery is discharged, the harder it has to work to return to a fully charged state, and the sooner it will reach the end of its cycle life. Batteries exposed to higher temperatures will also experience reduced lifespans: for every 18° over 77°F (the standard temperature batteries are rated at), battery life is cut in half.
However, it’s important to note that batteries don’t simply die on the last day of their estimated lifespan. Cycle life is usually defined as the number of cycles a battery can support before capacity falls below 80% of its initial value. Most batteries will continue to operate with only marginal declines past this.
As the name suggests, backup power refers to how long a battery can function without recharging. We sometimes call it ‘autonomy,’ and it’s important because even if a system is sized correctly, weather is never 100% predictable. What if you get an unusually dark and stormy December? What happens if it snows… in Arizona?
If you want your light to outlast these types of weather events, you need backup power. How much you need depends on your location (and your comfort level!), but we say two full days should be the minimum.
You can calculate a battery’s backup power by dividing total available capacity (that’s the total capacity minus derating factors such as temperature and state of charge) by load (the amount of energy required to power a system). For example, if you take your 1500Wh battery from earlier and divide it by a hypothetical 550Wh load, you get 2.7—meaning your system can run 2.7 days without any solar.
Now that you understand the key metrics that can be used to compare batteries, let’s… compare batteries! Sol offers three battery types: Lead-acid, nickel-metal hydride (NiMH), and Lithium-ion (LiFePO4).
Lead-acid batteries are a proven technology that has been used in a variety of applications for decades. They have a relatively shallow depth of discharge (~25%); however, they have significantly more backup power. A well-maintained lead-acid battery can function for around 4 years and 1,600 charge cycles.
Compared to lead- acid, both LiFePO4 and NiMH batteries have a much deeper DOD and can expend more energy daily (70% and 90%, respectively). They also store energy more densely, meaning they’re more compact for a given compacity, translating to systems that are lighter, more contained, and aesthetically more pleasing. The latest LiFePO4 batteries have a cycle life of about 2,000 charge cycles; NiMH batteries can last more than 5,000.
Not all batteries perform well in all climates. While NiMH batteries can tolerate a wide range of temperatures (from -40°F to 158°F), both lead-acid and LiFePO4 have a narrower range, making them a sustainable solution for certain locations but not others.
Lead-acid batteries are highly tolerant of the cold (-40°F), making them ideal for projects in northern areas (they also come with more backup power, which, as we’ve said, is handy in places where you can’t always rely on sunshine). LiFePO4 batteries perform best between -4°F to 140°F, meaning they aren’t the best choice for colder environments like the northern U.S. and Canada but perform well in temperate zones.
Looking at cost, lead-acid batteries are by far the most affordable—about one fifth the cost of an NiMH battery. However, they also have the shortest lifespan and must be replaced more frequently. LiFePO4 batteries are generally less expensive than NiMH batteries but still more than lead acid.
Lead-acid batteries also have an edge when it comes to recyclability. According to Battery Council International, 99% of lead-acid batteries in the U.S. are recycled since lead can be infinitely recycled without notable losses in quality or performance. The recycling rates of NiMH and LiFePO4 batteries are considerably lower—50-60% for NiMH and less than 5% for LiFePO4.
Since batteries are what allow solar lighting systems to do what they do—provide light when it’s dark—it’s important to get them right. When batteries fail prematurely, it’s not because of poor design or unreliable technology but because they’re either discharged too often, too deeply or because their chemistry is incompatible with the climate.
By educating yourself on the characteristics of various technologies—and working with an experienced manufacturer like Sol!—you’ll ensure that you get a battery capable of supporting your system now and 10 years from now.
Have a question about batteries or solar technology in general? Need some advice on a project? Drop us a line; we’d love to hear from you!
