Solar lights are leading the go-green trend in the household lighting industry. No doubt they have provided an efficient, environmentally-friendly solution to the security issue facing every property owner.
The rechargeable battery is the core element in this wonderful invention as they can supply power for the whole system’s operation. Because batteries are selling on the market with different types and capacities, consumers may run into the trouble of getting the wrong type. Go on reading to learn about the most common types and their specific uses.
Four most major types of batteries for solar lights, which we will discuss in this article, include NiCd, NiMH, lead-acid and lithium-ion.
1. NiCd Battery
NiCd, or nickel-cadmium, battery is a combination of cadmium and nickel, alkaline and separator. It is available in various capacities and sizes ranging from AAA to D. At first, NiCd batteries were designed with the aim of replacing older non-rechargeable alkaline batteries, so they can work well in the same portable devices that use the latter. On average, the cell voltage of a NiCd battery is measured at 1.2V.
Unlike lead-acid batteries, those of NiCd can keep a steady voltage of 1.2V during the discharging process, even when they are nearly exhausted. Also, NiCd batteries are well-known for low internal resistance, which means they can discharge and charge power within a very short period of time, just approximately 1-2 hours. At the same time, they only need replacing after about 400-500 times of charging. Due to their low cost, they are normally used in low-end portable lighting, which needs only “some sort of illumination.”
When being unused, a NiCd battery tends to self-discharge about 10% of energy each month under the condition of normal temperatures. If there is an increase in the temperature of the surrounding environment, that discharge rate can rise by twofold. Therefore, users should completely discharge the battery before storing it in a cool place.
2. NiMH Battery
NiMH, known as nickel-metal hydride, battery belongs to the rechargeable category. It has many things in common with its NiCd counterparts like form factor and components. But rather than using toxic cadmium, it contains hydrogen absorbing alloy (links) that serves as a negative electrode or anode. With this feature, NiMH batteries boast their environmental friendliness and ability to recycle.
Moreover, they have twice as high energy density as NiCd batteries. That means they can store more energy in smaller and lighter battery. In solar flood lights, the larger capacity is equal to the longer period of operation with one charge. To be more specific, a NiMH battery can maintain a solid 1.2V for almost 70-80% of its total discharge time.
The lifespan of such type of battery varies according to their capacity, but in general, it ranges from 500 cycles as for high capacity batteries to nearly 3000 cycles for lower capacity ones. They can be rapidly charged within an hour. Not developing a memory effect, NiMH batteries enable solar panels to refill power at the same rate for various times without doing any harm to the battery capacity. Because of long discharge duration, NiMH batteries are widely used in all range of solar flood lights, from low to high-end ones. But they are mostly found in low and mid-range lights, for example, front door lights, garden lights, security lights, decoration lights, shed lights and so on.
However, they run a high risk of self-discharge, up to three times greater than that of NiCd batteries. If being left on the shelf for a month, a fully-charged battery will lose about a third of its power, which can increase in line with the rise in the surrounding temperature.
3. Lead Acid Battery
As the oldest type of rechargeable battery, lead acid batteries are often used in high power portable floodlights. They include two sub-categories: starting batteries, suitable for use in places needing short and powerful energy outputs, and deep cycle batteries. Capable of handling discharge cycles with long-term usage.
Lead acid batteries are the combination of multiple single cells, whose approximate voltage is measured at 2.1 volts. They are available on the market with different sizes and capacities with common voltages falling into the range of 2, 4, 6, 8, and 12. To increase the lead-acid battery’s capacity, it is feasible to connect multiple batteries in parallel.
Unlike NiCd and NiMH ones, lead-acid batteries self-discharge only between 2 and 5% a month so that they can be stored unused for a long time. Cheap cost, good low-temperature tolerance, and no need for frequent maintenance make such type of battery ideal for solar flood lights that demand a great deal of power. However, they have a relatively short life cycle, which goes to about 300-500 cycles. Therefore, after a few years of usage, you have to change the battery of solar flood lights.
4. Lithium-Ion Battery
Known as Li-ion, Lithium-ion batteries have just entered the solar rechargeable battery market for a short time. Their fast charging rate and weight savings make them suitable for portable devices’ use, but not so much in solar light applications, especially mid-range solar lamps.
Like NiMH, lithium-ion batteries are environmentally-friendly with no use of toxic materials such as cadmium and mercury. They are available in different sizes and shapes with voltages fluctuating between 3.2V and 3.8V. They can be as small as coin-shaped cells and as large as battery packs found in electric vehicles. On the whole, they are much lighter and more compact than other rechargeable batteries while providing the same capacity.
Their charging and self-discharge rates greatly vary among different types of lithium-ion battery. But in general, it takes about an hour to charge fully and the maximum self-discharge rate is 8 percent per month. When it comes to average lifespan, a lithium-ion battery can be used for 500-2000 times. In particular, the Li-phosphate sub-category is the most used battery type for solar lighting in the sense that its life cycle reaches over 2000 cycles and it can stand the test of both lower and higher temperatures.
However, the high manufacturing cost and low-temperature intolerance can make customers hesitant over buying Lithium-ion batteries.
In short, depending on the size and energy requirements of each solar light, there are corresponding types of battery. Therefore, remember to use the proper type of batteries for your solar lights to exploit their operation to the maximum.
It is also essential to replace batteries when they have gone to the end of life cycle and remove them from the device when you intend to store garden lights unused for a long time. And make sure to keep batteries away metal objects and in a dry and cool area to minimize chances of an electrical short. Once you get the hang of each battery type and its maintenance tips, you are likely to enjoy the wonderful applications for solar lights for many years.