What’s the best battery for my solar light?

 

 

Solar lights depend on the sun to function. When sunlight hits a solar panel, it's converted into electricity through a process called the photovoltaic effect. This electricity is then stored in batteries and used to power your light when the sun isn't shining.

 

But here's the catch: we don’t need light during the day—we need it at night. That’s where the challenge of "intermittency" comes in. Without storage, solar energy would be useless after dark. This is why batteries are essential for solar lighting systems.

 

Rechargeable batteries, like those used in Sol’s systems, store energy collected during the day so that lights can operate at night. They also help maintain performance during cloudy days or periods of low sunlight.

 

However, not all batteries are the same. While some manufacturers offer one-size-fits-all solutions, Sol provides three different battery types—each with unique advantages depending on the application. In this guide, we’ll break down the key factors to consider when choosing the right battery for your solar lighting system.

 

Understanding Depth of Discharge (DOD)

 

Capacity refers to how much energy a battery can hold, usually measured in watt-hours (Wh). But what matters more is how much of that capacity you can actually use. That’s where depth of discharge (DOD) comes in.

 

DOD is the percentage of a battery’s total capacity that can be safely used before it needs recharging. For example, if a 1500Wh battery is discharged by 500Wh, the DOD is 33%. If it’s discharged by 1200Wh, the DOD is 80%.

 

Some batteries have a shallow DOD (around 25%), while others can go deeper (up to 80% or more). However, discharging too deeply too often can significantly shorten a battery’s lifespan.

 

 

Charge Cycles and Cycle Life

 

Cycle life refers to how many times a battery can be charged and discharged before its performance starts to degrade. Each full charge and discharge cycle counts as one cycle.

 

The deeper you discharge a battery, the fewer cycles it will last. Temperature also plays a role—higher temperatures can reduce battery life by up to 50% for every 18°F above 77°F.

 

Battery life is typically defined as the number of cycles until the capacity drops below 80% of its original value. After that, performance declines gradually rather than failing suddenly.

 

Backup Power and Autonomy

 

Backup power, or autonomy, refers to how long a battery can power your system without recharging. It’s especially important in unpredictable weather conditions.

 

For example, if your battery has 1500Wh of usable energy and your system uses 550Wh per day, you can expect about 2.7 days of backup power. We recommend a minimum of two full days to account for unexpected weather.

 

Calculating backup power involves dividing available capacity by daily energy consumption. This helps determine how long your system can run independently.

 

Comparing Battery Performance and Longevity

 

Now that you understand key metrics like DOD and cycle life, let’s compare the three battery types Sol offers: Lead-acid, NiMH, and LiFePO4.

 

Lead-acid batteries are reliable and cost-effective, but they have a shallow DOD (~25%) and a shorter lifespan (around 1,600 cycles). They’re great for areas with cold climates and high backup needs.

 

NiMH batteries have a deeper DOD (up to 90%) and a longer cycle life (over 5,000 cycles), making them ideal for frequent use. They also perform well across a wide temperature range (-40°F to 158°F).

 

LiFePO4 batteries offer the best balance of performance and longevity, with a DOD of around 70% and a cycle life of about 2,000 cycles. They are compact, lightweight, and perform well in temperate climates, though they are less suitable for extreme cold.

 

Climate Considerations

 

Not all batteries work equally well in every climate. NiMH batteries are the most temperature-tolerant, while lead-acid and LiFePO4 have narrower ranges.

 

Lead-acid batteries excel in cold environments, making them ideal for northern regions. LiFePO4 performs best in moderate climates and may not be the best choice for very cold areas like Canada or Alaska.

 

 

Cost, Recyclability, and Environmental Impact

 

When it comes to cost, lead-acid batteries are the most affordable—about one-fifth the price of NiMH. However, they also have the shortest lifespan and require more frequent replacements.

 

Recyclability is another factor. Lead-acid batteries are highly recyclable, with over 99% being recycled in the U.S. NiMH batteries have a lower recycling rate (50-60%), and LiFePO4 batteries are the least recyclable, with less than 5% being recycled.

 

Key Takeaways

 

Batteries are crucial for solar lighting systems because they allow energy to be stored and used when needed. Choosing the right battery depends on several factors, including climate, usage patterns, and budget.

 

 

By understanding the differences between battery types and working with an experienced manufacturer like Sol, you can ensure your solar lighting system remains reliable for years to come.

 

Have questions about batteries or solar technology? Need help choosing the right system for your project? Contact us—we’d love to hear from you!