Solar street lights are very convenient since they provide light during night hours without requiring access to the electrical grid. These lighting systems include a solar module and a battery, allowing the equipment to generate power during the day, store it at the battery, and use it during night hours.
It is always important to pick the right battery for each solar street light since this will define its autonomy, aesthetics, required maintenance, and more. If you are looking to install a battery on a solar street light, this article is for you.
What types of batteries are being used in the solar industry?
There are many solar battery technologies available for solar street lights, each one delivering different benefits but also including some cons to it. In this section, we explain each of these technologies:
Nickel-Cadmium (Ni-Cd) batteries are durable, resistant to high temperatures, and do not require regular maintenance. Manufactured with an oxide-hydroxide electrode for the anode and cadmium for the cathode, Ni-Cd batteries have been widely used in civil and military aviation, but have also become quite popular for solar applications.
Ni-Cd batteries are excellent for street lights in remote locations, since they are highly reliable, and require low maintenance. These batteries are cheaper than Li-Ion and can be discharged to a 60% Depth of Discharge (DOD) while delivering 2,500 cycles, making them excellent for solar applications.
Lead-Acid batteries became the norm for many years since they are robust and cheap. They include a cathode made out of lead oxide, and a metallic lead anode, both immersed in a sulphuric acid electrolyte.
While they are cheap, lead-acid batteries might require regular maintenance, which is why we do not recommend them for underground installations on solar street lights. These batteries provide between 500 cycles at a 50% DOD to 1,200 cycles at a 30% DOD. AGM and Gel batteries are the most commonly used Lead-Acid batteries for solar street lights.
Lithium-Ion (Li-Ion) batteries are among the most popular batteries for solar street lights, but also the most expensive ones. They use a lithium metal oxide cathode and a lithium-carbon anode, immersed in a lithium salt electrolyte.
Li-Ion batteries are widely popular due to their higher energy density, resulting in a higher capacity with a compact design. These batteries can be discharged to an 80% DOD while delivering 2,000-3,000 cycles for the street light.
Lithium Iron Phosphate
Lithium Iron Phosphate (LiFePO4) batteries are another great lithium battery technology, but for a lower price. These batteries have high energy density and can be discharged to an 80% DOD while delivering around 4,500 cycles.
LiFePO4 batteries are manufactured with graphite anodes and lithium iron phosphate cathodes, immersed in a lithium salt electrolyte. This is the preferred lithium technology for solar street lights since it is cheaper than Li-Ion but still provides a maintenance-free option with high energy density.
Flow redox batteries are a peculiar case encasing two electrolytes, they convey two half-cells separated by a membrane, one holding the anode and first electrolyte, and the second one the cathode and second electrolyte. The electrolyte pair is usually comprised of vanadium/vanadium (different states), iron/chromium, or zinc/bromine.
This is an emerging technology, which is why they are not as popular for street lights or the solar industry. Flow batteries provide more cycles than most batteries, can operate at low temperatures, and are easily scalable for most applications.
What to consider when choosing the battery for a solar street light?
After learning about different battery technologies, we should learn what aspects to consider when picking a solar street light since these will help you choose the right battery.
Capacity and Size
The capacity of a solar battery defines how long a solar street light can operate, depending on the consumption of the street light fixture. When picking up a battery, consider how many Watts-hour (Wh) or Amps-hour (Ah) a battery can provide, since this will define how many hours the solar street light operates.
To size the capacity required for the battery, it is valuable to use the expression below:
As an example, we can take a 1,500-lumen fixture that consumes nearly 15W, while a 12,000-lumen solar street light consumes 120W. To power a 12V solar street light for 12 uninterrupted hours (19:00 to 07:00) considering losses due to an 80% round-trip efficiency, a DOD of 50%, and taking 2 days of autonomy, you would require a 75Ah@12V battery for the 1,500-lumen fixture and nearly 600Ah@12V battery bank for the 12,000-lumen street light.
Power Rating and Voltage
The voltage of a battery is the electric potential difference between terminals, this is measured in volts (V). The voltage of a battery should always match the required voltage of the fixture.
On a similar basis, the power rating is how much power a battery can deliver, this is measured in Kilowatts (kW). Since solar street light fixtures do not demand that much power, we measured it in Watts (W). A battery should always match or surpass the power requirement of a solar street light fixture.
Depth of Discharge (DoD)
The Depth of Discharge (DoD) is the maximum percentage (%) at which you can safely discharge a battery. This is related to the cycle life of a battery since batteries deliver a certain amount of cycles varying on the regularly used DoD.
To extend life cycles for solar streetlights, some manufacturers recommend a 20% DOD for lead-acid batteries, however, 30-40% values are also acceptable as a good balance between performance and life cycles. As for lithium batteries, the DOD can be larger up to 75%.
The Round-Trip Efficiency means the amount of DC power you can extract from the battery, regarding the DC power the solar panel injected into it. In other words, is the charge/discharge cycle efficiency of a battery. Most batteries deliver an 80% round-trip efficiency, but this varies between technologies and manufacturers.
Battery Lifespan (Life cycle)
The battery lifespan is usually measured in years, however, this is an estimate of how many cycles it delivers. The life cycle of a battery is the number of charge/discharge cycles a battery can provide at a certain DoD until the capacity (life) is reduced, this varies with the technology and DoD at which you discharge the battery.
For instance, most lead-acid batteries would last around 2000 cycles at a 50% DoD, but some advanced lead-acid battery models discharged to a 50% DoD can last around 4,250 cycles or nearly 11 years. Using an 80% DoD for this same battery would reduce its life to around 2,700 cycles or 7 years for solar street lights. Lithium batteries are a more advanced technology delivering around 4,000 cycles while operating at an 80%-100% DoD.
Safety and Environmental Impacts
Each battery has a different type of safety certification, regarding electrolyte chemicals and the manufacturing process. Solar street lights require a battery with UL-8750 certification or a safer one.
One major aspect to consider in safety measures is avoiding batteries falling under thermal runaway, this can rapidly heat the battery and cause it to explode or release hazardous gases. To avoid this, we recommend installing batteries resistant to high temperatures, temperature-triggered circuit breakers, or looking at batteries with an advanced Battery Management System (BMS).
Installing batteries underground provides lower temperature variations, which is beneficial to avoid thermal runaway, however, the battery must be able to endure the lack of ventilation (only applicable in AGM, GEL, or lithium batteries).
One unavoidable aspect to consider is the budget, being the biggest restriction. When picking a solar street light battery, consider picking a battery that delivers the most cycles and performance under budget limitations.
Considering batteries for the aforementioned case of 1,500-12,000 solar street light fixtures, you could find these estimated prices in both cases:
- Lead-Acid AGM batteries: $12-$96 ($0.80/Ah)
- Lead-Acid Gel batteries: $15-$120 ($1.00/Ah)
- LiFePO4 batteries: $18-$144.0 ($1.20/Ah)
- Li-Ion batteries: $23.7-$189.6 ($1.58/Ah)
Which battery type is the best? How to choose?
While knowing about the different aspects to consider when picking a battery is important, you should know how to relate them to each battery technology. Here we explain the best battery technology under different circumstances.
When to choose Nickel-cadmium batteries?
Ni-Cd batteries are robust, reliable, and resistant to high temperatures. They provide a decent energy density while delivering many cycles, but still maintaining a budget. These batteries are excellent for solar street lights exposed to regular high temperatures, where you require a moderate capacity, but are looking for a low-priced battery. However, some elements in Ni-Cd batteries are toxic and hard to dispose of.
When to choose AGM lead-acid batteries?
AGM lead-acid batteries are the most common choice under a budget, they have a lower energy density and should only be discharged to a limited DoD. We only recommend them for underground installations where you can install several batteries together to provide the same capacity as a lithium battery. They are perfect for underground installations since they do not expel gases as flooded batteries do, and require little to no maintenance.
When to choose lithium-ion batteries?
Lithium-ion batteries are the most expensive options, but they excel in every other aspect. Having a high energy density, Li-ion batteries are small and are barely noticeable, being able to power the fixture for a longer time compared to other batteries. However, we do not recommend them for solar street lights in extremely cold or hot weather, since they only charge well for 32ºF–113ºF temperatures.
When to choose lithium iron phosphate batteries?
LiFePO4 batteries are the best option in general. They are more expensive than Ni-Cd or lead-acid batteries, but cheaper than Li-ion options, and can operate at a -40ºF–158ºF temperature range, delivering the best performance for lithium options. These batteries can be installed on pole mounts or underground configurations for solar street lights since they have a high energy density and do not require maintenance.
There are different types of technologies used in the solar industry. Picking the right battery for solar street lights varies depending on several factors like the technical specifications of the fixture or the panel, the desired aesthetics for the street light, and the budget.
When looking for a battery for a specific solar street light, consider the aspects talked about during the article, while considering the recommendations in the previous section. Combining these regarding technical requirements of the fixture and desired aesthetics will let you pick the best battery, delivering a great performance, and resulting in the best investment in the long run.
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